GNU poke Manual
1 Introduction
  1.1 Motivation
    1.1.1 Decode-Compute-Encode
    1.1.2 Describe-Compute
  1.2 Nomenclature
  1.3 Invoking poke
  1.4 Commanding poke
    1.4.1 The REPL
    1.4.2 Evaluation
    1.4.3 Commands and Dot-Commands
      1.4.3.1 Dot-Commands
      1.4.3.2 Commands
    1.4.4 Command Files
    1.4.5 Scripts
2 Setting Up
  2.1 Setting up Hyperlinks
    2.1.1 Make sure your poke speaks hyperlinks
    2.1.2 Use a terminal emulator that supports hyperlinks
    2.1.3 Get and install the app-client utility
    2.1.4 GNOME Terminal
  2.2 Simple Init File
3 Basic Editing
  3.1 Binary Files
  3.2 Files as IO Spaces
  3.3 Dumping File Contents
  3.4 Poking Bytes
  3.5 Values and Variables
  3.6 From Bytes to Integers
  3.7 Big and Little Endians
  3.8 Negative Integers
    3.8.1 Negative Encodings
    3.8.2 Signed Integers
    3.8.3 Mixing Signed and Unsigned Integers
  3.9 Weird Integers
    3.9.1 Incomplete Bytes
    3.9.2 Quantum Bytenics
    3.9.3 Signed Weird Numbers
  3.10 Unaligned Integers
  3.11 Integers of Different Sizes
  3.12 Offsets and Sizes
  3.13 Buffers as IO Spaces
  3.14 Copying Bytes
  3.15 Saving Buffers in Files
  3.16 Character Sets
  3.17 From Bytes to Characters
    3.17.1 Character Literals
    3.17.2 Classifying Characters
    3.17.3 Non-printable Characters
  3.18 ASCII Strings
    3.18.1 String Values
    3.18.2 Poking Strings
    3.18.3 From Characters to Strings
  3.19 From Strings to Characters
  3.20 Strings are not Arrays
4 Structuring Data
  4.1 The SBM Format
    4.1.1 Images as Maps of Pixels
    4.1.2 SBM header
    4.1.3 SBM data
  4.2 Poking a SBM Image
    4.2.1 P is for poke
    4.2.2 Preparing the Canvas
    4.2.3 Poking the Header
    4.2.4 Poking the Pixels
    4.2.5 Poking Lines
    4.2.6 Poking Images
    4.2.7 Saving the Image
  4.3 Modifying SBM Images
    4.3.1 Reading a SBM File
    4.3.2 Painting Pixels
    4.3.3 Cropping the R
    4.3.4 Shortening and Shifting Lines
    4.3.5 Updating the Header
    4.3.6 Saving the Result
  4.4 Defining Types
    4.4.1 Naming your Own Abstractions
    4.4.2 Abstracting the Structure of Entities
  4.5 Pickles
    4.5.1 poke Commands versus Poke Constructions
    4.5.2 Poke Files
    4.5.3 Pickling Abstractions
    4.5.4 Exploring Pickles
    4.5.5 Startup
  4.6 Poking Structs
    4.6.1 Heterogeneous Related Data
    4.6.2 Mapping Structs
    4.6.3 Modifying Mapped Structs
  4.7 How Structs are Built
  4.8 Variables in Structs
  4.9 Functions in Structs
  4.10 Struct Methods
  4.11 Padding and Alignment
    4.11.1 Esoteric and exoteric padding
    4.11.2 Reserved fields
    4.11.3 Payloads
    4.11.4 Aligning struct fields
    4.11.5 Padding array elements
  4.12 Dealing with Alternatives
    4.12.1 BSON
    4.12.2 Unions are Tagged
  4.13 Structured Integers
  4.14 Working with Incorrect Data
5 Maps and Map-files
  5.1 Editing using Variables
  5.2 poke Maps
  5.3 Loading Maps
  5.4 Multiple Maps
  5.5 Auto-map
  5.6 Constructing Maps
  5.7 Predefined Maps
6 Writing Pickles
  6.1 Pretty-printers
    6.1.1 Convention for pretty-printed Output
    6.1.2 Pretty Printing Optional Fields
  6.2 Setters and Getters
7 Writing Binary Utilities
  7.1 Poke Scripts
  7.2 Command-Line Arguments
  7.3 Exiting from Scripts
  7.4 Loading pickles as Modules
  7.5 elfextractor
  7.6 Filters
    7.6.1 Stream IO Spaces
    7.6.2 Reading from Streams
    7.6.3 Writing to Streams
    7.6.4 pk-strings
8 Configuration
  8.1 '.pokerc'
  8.2 Load Path
  8.3 Styling
9 Time
  9.1 POSIX Time
10 Colors
  10.1 The Color Registry
  10.2 RGB24 Encoding
11 Audio
  11.1 MP3
    11.1.1 ID3V1 Tags
      11.1.1.1 Song Genres
      11.1.1.2 The ID3V1_Tag Type
12 Object Formats
  12.1 ELF
  12.2 Dwarf
13 Programs
  13.1 argp
14 Programming Emacs Modes
  14.1 poke-mode
  14.2 poke-map-mode
  14.3 poke-ras-mode
15 Vim Syntax Highlighting
  15.1 'poke.vim'
16 Dot-Commands
  16.1 '.load'
  16.2 '.source'
  16.3 '.file'
  16.4 '.mem'
  16.5 '.nbd'
  16.6 '.proc'
  16.7 '.sub'
  16.8 '.ios'
  16.9 '.close'
  16.10 '.doc'
  16.11 '.editor'
  16.12 '.info'
  16.13 '.set'
  16.14 '.vm'
    16.14.1 '.vm disassemble'
    16.14.2 '.vm profile'
  16.15 '.exit'
  16.16 '.quit'
17 Commands
  17.1 'dump'
    17.1.1 Information 'dump' shows
    17.1.2 Presentation options for 'dump'
  17.2 'copy'
  17.3 'save'
  17.4 'extract'
  17.5 'scrabble'
18 The Poke Language
  18.1 Integers
    18.1.1 Integer Literals
      18.1.1.1 The digits separator '_'
      18.1.1.2 Types of integer literals
    18.1.2 Characters
    18.1.3 Booleans
    18.1.4 Integer Types
    18.1.5 Casting Integers
    18.1.6 Relational Operators
    18.1.7 Arithmetic Operators
    18.1.8 Bitwise Operators
    18.1.9 Boolean Operators
    18.1.10 Integer Attributes
  18.2 Offsets
    18.2.1 Offset Literals
    18.2.2 Offset Units
      18.2.2.1 Named Units
      18.2.2.2 Arbitrary Units
      18.2.2.3 Types as Units
    18.2.3 Offset Types
    18.2.4 Casting Offsets
    18.2.5 Offset Operations
      18.2.5.1 Addition and subtraction
      18.2.5.2 Multiplication by a scalar
      18.2.5.3 Division
      18.2.5.4 Division by an integer
      18.2.5.5 Modulus
    18.2.6 Offset Attributes
  18.3 Strings
    18.3.1 String Literals
    18.3.2 String Types
    18.3.3 String Indexing
    18.3.4 String Concatenation
    18.3.5 String Attributes
    18.3.6 String Formatting
  18.4 Arrays
    18.4.1 Array Literals
    18.4.2 Array Types
      18.4.2.1 Writing unbounded array literals
      18.4.2.2 Array boundaries and closures
    18.4.3 Casting Arrays
    18.4.4 Array Constructors
    18.4.5 Array Comparison
    18.4.6 Array Indexing
    18.4.7 Array Trimming
    18.4.8 Array Elements
    18.4.9 Array Concatenation
    18.4.10 Array Attributes
  18.5 Structs
    18.5.1 Struct Types
    18.5.2 Struct Constructors
    18.5.3 Struct Comparison
    18.5.4 Field Endianness
    18.5.5 Accessing Fields
    18.5.6 Field Constraints
    18.5.7 Field Initializers
    18.5.8 Field Labels
    18.5.9 Pinned Structs
    18.5.10 The OFFSET variable
    18.5.11 Integral Structs
    18.5.12 Unions
    18.5.13 Union Constructors
    18.5.14 Optional Fields
    18.5.15 Casting Structs
    18.5.16 Declarations in Structs
    18.5.17 Methods
    18.5.18 Struct Attributes
  18.6 Types
    18.6.1 'type'
    18.6.2 The 'any' Type
    18.6.3 The 'isa' Operator
  18.7 Assignments
  18.8 Compound Statements
  18.9 Conditionals
    18.9.1 'if-else'
    18.9.2 Conditional Expressions
  18.10 Loops
    18.10.1 'while'
    18.10.2 'for'
    18.10.3 'for-in'
  18.11 Expression Statements
  18.12 Functions
    18.12.1 Function Declarations
    18.12.2 Optional Arguments
    18.12.3 Variadic Functions
    18.12.4 Calling Functions
    18.12.5 Function Types
    18.12.6 Lambdas
    18.12.7 Function Comparison
    18.12.8 Function Attributes
  18.13 Endianness
    18.13.1 '.set endian'
    18.13.2 Endian in Fields
    18.13.3 Endian built-ins
  18.14 Mapping
    18.14.1 IO Spaces
      18.14.1.1 'open'
      18.14.1.2 'opensub'
      18.14.1.3 'openproc'
      18.14.1.4 'close'
      18.14.1.5 'flush'
      18.14.1.6 'get_ios'
      18.14.1.7 'set_ios'
      18.14.1.8 'iosize'
      18.14.1.9 'ioflags'
    18.14.2 The Map Operator
    18.14.3 Mapping Simple Types
    18.14.4 Mapping Structs
    18.14.5 Mapping Arrays
      18.14.5.1 Array maps bounded by number of elements
      18.14.5.2 Array maps bounded by size
      18.14.5.3 Unbounded array maps
      18.14.5.4 Mapped bounds in bounded arrays
    18.14.6 Mapping Functions
    18.14.7 Non-strict Mapping
    18.14.8 Unmapping
  18.15 Exception Handling
    18.15.1 Exceptions
    18.15.2 'try-catch'
    18.15.3 'try-until'
    18.15.4 'raise'
    18.15.5 exception-predicate
    18.15.6 'assert'
  18.16 Terminal
    18.16.1 Terminal Colors
    18.16.2 Terminal Styling
    18.16.3 Terminal Hyperlinks
  18.17 Printing
    18.17.1 'print'
    18.17.2 'printf'
  18.18 Comments
    18.18.1 Multi-line comments
    18.18.2 Single line comments
    18.18.3 Vertical separator
  18.19 Modules
  18.20 System
    18.20.1 getenv
    18.20.2 rand
  18.21 VM
    18.21.1 'vm_obase'
    18.21.2 'vm_set_obase'
    18.21.3 'vm_opprint'
    18.21.4 'vm_set_opprint'
    18.21.5 'vm_oacutoff'
    18.21.6 'vm_set_oacutoff'
    18.21.7 'vm_odepth'
    18.21.8 'vm_set_odepth'
    18.21.9 'vm_oindent'
    18.21.10 'vm_set_oindent'
    18.21.11 'vm_omaps'
    18.21.12 'vm_set_omaps'
    18.21.13 'vm_omode'
    18.21.14 'vm_set_omode'
  18.22 Debugging
    18.22.1 '__LINE__' and '__FILE__'
    18.22.2 strace
19 The Standard Library
  19.1 Standard Integral Types
  19.2 Standard Offset Types
  19.3 Standard Units
  19.4 Conversion Functions
    19.4.1 'catos'
    19.4.2 'stoca'
    19.4.3 'atoi'
    19.4.4 'ltos'
  19.5 Array Functions
    19.5.1 'reverse'
  19.6 String Functions
    19.6.1 'ltrim'
    19.6.2 'rtrim'
    19.6.3 'strchr'
  19.7 Sorting Functions
    19.7.1 'qsort'
  19.8 CRC Functions
  19.9 Dates and Times
  19.10 Offset Functions
    19.10.1 alignto
20 The Machine-Interface
  20.1 MI overview
  20.2 Running poke in MI mode
  20.3 MI transport
  20.4 MI protocol
    20.4.1 MI Requests
      20.4.1.1 Request EXIT
      20.4.1.2 Request PRINTV
    20.4.2 MI Responses
      20.4.2.1 Response EXIT
      20.4.2.2 Response PRINTV
    20.4.3 MI Events
      20.4.3.1 Event INITIALIZE
21 The Poke Virtual Machine
  21.1 PVM Instructions
    21.1.1 VM instructions
      21.1.1.1 Instruction canary
      21.1.1.2 Instruction exit
      21.1.1.3 Instruction pushend
      21.1.1.4 Instruction popend
      21.1.1.5 Instruction pushob
      21.1.1.6 Instruction popob
      21.1.1.7 Instruction pushom
      21.1.1.8 Instruction popom
      21.1.1.9 Instruction pushoo
      21.1.1.10 Instruction popoo
      21.1.1.11 Instruction pushoi
      21.1.1.12 Instruction popoi
      21.1.1.13 Instruction pushod
      21.1.1.14 Instruction popod
      21.1.1.15 Instruction pushoac
      21.1.1.16 Instruction popoac
      21.1.1.17 Instruction pushopp
      21.1.1.18 Instruction popopp
      21.1.1.19 Instruction pushoc
      21.1.1.20 Instruction popoc
      21.1.1.21 Instruction pushobc
      21.1.1.22 Instruction popobc
      21.1.1.23 Instruction sync
    21.1.2 IOS related instructions
      21.1.2.1 Instruction open
      21.1.2.2 Instruction close
      21.1.2.3 Instruction flush
      21.1.2.4 Instruction pushios
      21.1.2.5 Instruction popios
      21.1.2.6 Instruction ioflags
      21.1.2.7 Instruction iosize
      21.1.2.8 Instruction iogetb
      21.1.2.9 Instruction iosetb
    21.1.3 Function management instructions
      21.1.3.1 Instruction call
      21.1.3.2 Instruction prolog
      21.1.3.3 Instruction return
    21.1.4 Environment instructions
      21.1.4.1 Instruction pushf
      21.1.4.2 Instruction popf
      21.1.4.3 Instruction pushvar
      21.1.4.4 Instruction pushtopvar
      21.1.4.5 Instruction popvar
      21.1.4.6 Instruction regvar
      21.1.4.7 Instruction duc
      21.1.4.8 Instruction pec
    21.1.5 Printing Instructions
      21.1.5.1 Instruction indent
      21.1.5.2 Instruction printi
      21.1.5.3 Instruction printiu
      21.1.5.4 Instruction printl
      21.1.5.5 Instruction printlu
      21.1.5.6 Instruction prints
      21.1.5.7 Instruction beghl
      21.1.5.8 Instruction endhl
      21.1.5.9 Instruction begsc
      21.1.5.10 Instruction endsc
    21.1.6 Format Instructions
      21.1.6.1 Instruction formati
      21.1.6.2 Instruction formatiu
      21.1.6.3 Instruction formatl
      21.1.6.4 Instruction formatlu
    21.1.7 Main stack manipulation instructions
      21.1.7.1 Instruction push
      21.1.7.2 Instruction drop
      21.1.7.3 Instruction drop2
      21.1.7.4 Instruction drop3
      21.1.7.5 Instruction drop4
      21.1.7.6 Instruction swap
      21.1.7.7 Instruction nip
      21.1.7.8 Instruction nip2
      21.1.7.9 Instruction nip3
      21.1.7.10 Instruction dup
      21.1.7.11 Instruction over
      21.1.7.12 Instruction rot
      21.1.7.13 Instruction nrot
      21.1.7.14 Instruction tuck
      21.1.7.15 Instruction quake
      21.1.7.16 Instruction revn
      21.1.7.17 Instruction pushhi
      21.1.7.18 Instruction pushlo
      21.1.7.19 Instruction push32
    21.1.8 Registers manipulation instructions
      21.1.8.1 Instruction pushr
      21.1.8.2 Instruction popr
      21.1.8.3 Instruction setr
    21.1.9 Return stack manipulation instructions
      21.1.9.1 Instruction saver
      21.1.9.2 Instruction restorer
      21.1.9.3 Instruction tor
      21.1.9.4 Instruction fromr
      21.1.9.5 Instruction atr
    21.1.10 Arithmetic instructions
      21.1.10.1 Instruction addi
      21.1.10.2 Instruction addiu
      21.1.10.3 Instruction addlu
      21.1.10.4 Instruction subi
      21.1.10.5 Instruction subiu
      21.1.10.6 Instruction subl
      21.1.10.7 Instruction sublu
      21.1.10.8 Instruction muli
      21.1.10.9 Instruction muliu
      21.1.10.10 Instruction mull
      21.1.10.11 Instruction mullu
      21.1.10.12 Instruction divi
      21.1.10.13 Instruction diviu
      21.1.10.14 Instruction divl
      21.1.10.15 Instruction divlu
      21.1.10.16 Instruction modi
      21.1.10.17 Instruction modiu
      21.1.10.18 Instruction modl
      21.1.10.19 Instruction modlu
      21.1.10.20 Instruction negi
      21.1.10.21 Instruction negiu
      21.1.10.22 Instruction negl
      21.1.10.23 Instruction neglu
      21.1.10.24 Instruction powi
      21.1.10.25 Instruction powiu
      21.1.10.26 Instruction powl
      21.1.10.27 Instruction powlu
    21.1.11 Relational instructions
      21.1.11.1 Instruction eqi
      21.1.11.2 Instruction eqiu
      21.1.11.3 Instruction eql
      21.1.11.4 Instruction eqlu
      21.1.11.5 Instruction eqs
      21.1.11.6 Instruction nei
      21.1.11.7 Instruction neiu
      21.1.11.8 Instruction nel
      21.1.11.9 Instruction nelu
      21.1.11.10 Instruction nes
      21.1.11.11 Instruction nn
      21.1.11.12 Instruction nnn
      21.1.11.13 Instruction lti
      21.1.11.14 Instruction ltiu
      21.1.11.15 Instruction ltl
      21.1.11.16 Instruction ltlu
      21.1.11.17 Instruction lei
      21.1.11.18 Instruction leiu
      21.1.11.19 Instruction lel
      21.1.11.20 Instruction lelu
      21.1.11.21 Instruction gti
      21.1.11.22 Instruction gtiu
      21.1.11.23 Instruction gtl
      21.1.11.24 Instruction gtlu
      21.1.11.25 Instruction gei
      21.1.11.26 Instruction geiu
      21.1.11.27 Instruction gel
      21.1.11.28 Instruction gelu
      21.1.11.29 Instruction lts
      21.1.11.30 Instruction gts
      21.1.11.31 Instruction ges
      21.1.11.32 Instruction les
      21.1.11.33 Instruction eqc
      21.1.11.34 Instruction nec
    21.1.12 Concatenation instructions
      21.1.12.1 Instruction sconc
    21.1.13 Logical instructions
      21.1.13.1 Instruction and
      21.1.13.2 Instruction or
      21.1.13.3 Instruction not
    21.1.14 Bitwise instructions
      21.1.14.1 Instruction bxori
      21.1.14.2 Instruction bxoriu
      21.1.14.3 Instruction bxorl
      21.1.14.4 Instruction bxorlu
      21.1.14.5 Instruction bori
      21.1.14.6 Instruction boriu
      21.1.14.7 Instruction borl
      21.1.14.8 Instruction borlu
      21.1.14.9 Instruction bandi
      21.1.14.10 Instruction bandiu
      21.1.14.11 Instruction bandl
      21.1.14.12 Instruction bandlu
      21.1.14.13 Instruction bnoti
      21.1.14.14 Instruction bnotiu
      21.1.14.15 Instruction bnotl
      21.1.14.16 Instruction bnotlu
    21.1.15 Shift instructions
      21.1.15.1 Instruction bsli
      21.1.15.2 Instruction bsliu
      21.1.15.3 Instruction bsll
      21.1.15.4 Instruction bsllu
      21.1.15.5 Instruction bsri
      21.1.15.6 Instruction bsriu
      21.1.15.7 Instruction bsrl
      21.1.15.8 Instruction bsrlu
    21.1.16 Compare-and-swap instructions
      21.1.16.1 Instruction swapgti
      21.1.16.2 Instruction swapgtiu
      21.1.16.3 Instruction swapgtl
      21.1.16.4 Instruction swapgtlu
    21.1.17 Branch instructions
      21.1.17.1 Instruction ba
      21.1.17.2 Instruction bn
      21.1.17.3 Instruction bnn
      21.1.17.4 Instruction bzi
      21.1.17.5 Instruction bziu
      21.1.17.6 Instruction bzl
      21.1.17.7 Instruction bzlu
      21.1.17.8 Instruction bnzi
      21.1.17.9 Instruction bnziu
      21.1.17.10 Instruction bnzl
      21.1.17.11 Instruction bnzlu
    21.1.18 Conversion instructions
      21.1.18.1 Instruction ctos
      21.1.18.2 Instruction itoi
      21.1.18.3 Instruction itoiu
      21.1.18.4 Instruction itol
      21.1.18.5 Instruction itolu
      21.1.18.6 Instruction iutoi
      21.1.18.7 Instruction iutoiu
      21.1.18.8 Instruction iutol
      21.1.18.9 Instruction iutolu
      21.1.18.10 Instruction ltoi
      21.1.18.11 Instruction ltoiu
      21.1.18.12 Instruction ltol
      21.1.18.13 Instruction ltolu
      21.1.18.14 Instruction lutoi
      21.1.18.15 Instruction lutoiu
      21.1.18.16 Instruction lutol
      21.1.18.17 Instruction lutolu
    21.1.19 String instructions
      21.1.19.1 Instruction strref
      21.1.19.2 Instruction strset
      21.1.19.3 Instruction substr
      21.1.19.4 Instruction muls
      21.1.19.5 Instruction sprops
      21.1.19.6 Instruction sproph
      21.1.19.7 Instruction spropc
    21.1.20 Array instructions
      21.1.20.1 Instruction mka
      21.1.20.2 Instruction ains
      21.1.20.3 Instruction arem
      21.1.20.4 Instruction aset
      21.1.20.5 Instruction aref
      21.1.20.6 Instruction arefo
      21.1.20.7 Instruction asettb
    21.1.21 Struct instructions
      21.1.21.1 Instruction mksct
      21.1.21.2 Instruction sset
      21.1.21.3 Instruction sseti
      21.1.21.4 Instruction sref
      21.1.21.5 Instruction srefo
      21.1.21.6 Instruction srefmnt
      21.1.21.7 Instruction srefnt
      21.1.21.8 Instruction srefi
      21.1.21.9 Instruction srefia
      21.1.21.10 Instruction srefio
      21.1.21.11 Instruction smodi
    21.1.22 Offset Instructions
      21.1.22.1 Instruction mko
      21.1.22.2 Instruction ogetm
      21.1.22.3 Instruction osetm
      21.1.22.4 Instruction ogetu
      21.1.22.5 Instruction ogetbt
    21.1.23 Instructions to handle mapped values
      21.1.23.1 Instruction mm
      21.1.23.2 Instruction map
      21.1.23.3 Instruction unmap
      21.1.23.4 Instruction reloc
      21.1.23.5 Instruction ureloc
      21.1.23.6 Instruction mgets
      21.1.23.7 Instruction msets
      21.1.23.8 Instruction mgeto
      21.1.23.9 Instruction mseto
      21.1.23.10 Instruction mgetios
      21.1.23.11 Instruction msetios
      21.1.23.12 Instruction mgetm
      21.1.23.13 Instruction msetm
      21.1.23.14 Instruction mgetw
      21.1.23.15 Instruction msetw
      21.1.23.16 Instruction mgetsel
      21.1.23.17 Instruction msetsel
      21.1.23.18 Instruction mgetsiz
      21.1.23.19 Instruction msetsiz
    21.1.24 Type related instructions
      21.1.24.1 Instruction isa
      21.1.24.2 Instruction typof
      21.1.24.3 Instruction tyisc
      21.1.24.4 Instruction tyissct
      21.1.24.5 Instruction mktyv
      21.1.24.6 Instruction mktyany
      21.1.24.7 Instruction mktyi
      21.1.24.8 Instruction mktys
      21.1.24.9 Instruction mktyo
      21.1.24.10 Instruction mktya
      21.1.24.11 Instruction tyagett
      21.1.24.12 Instruction tyagetb
      21.1.24.13 Instruction mktyc
      21.1.24.14 Instruction mktysct
      21.1.24.15 Instruction tysctn
    21.1.25 IO instructions
      21.1.25.1 Instruction write
      21.1.25.2 Instruction peeki
      21.1.25.3 Instruction peekiu
      21.1.25.4 Instruction peekl
      21.1.25.5 Instruction peeklu
      21.1.25.6 Instruction peekdi
      21.1.25.7 Instruction peekdiu
      21.1.25.8 Instruction peekdl
      21.1.25.9 Instruction peekdlu
      21.1.25.10 Instruction pokei
      21.1.25.11 Instruction pokeiu
      21.1.25.12 Instruction pokel
      21.1.25.13 Instruction pokelu
      21.1.25.14 Instruction pokedi
      21.1.25.15 Instruction pokediu
      21.1.25.16 Instruction pokedl
      21.1.25.17 Instruction pokedlu
      21.1.25.18 Instruction peeks
      21.1.25.19 Instruction pokes
    21.1.26 Exceptions handling instructions
      21.1.26.1 Instruction pushe
      21.1.26.2 Instruction pope
      21.1.26.3 Instruction raise
      21.1.26.4 Instruction popexite
    21.1.27 Debugging Instructions
      21.1.27.1 Instruction strace
      21.1.27.2 Instruction disas
      21.1.27.3 Instruction note
    21.1.28 System Interaction Instructions
      21.1.28.1 Instruction getenv
    21.1.29 Miscellaneous Instructions
      21.1.29.1 Instruction nop
      21.1.29.2 Instruction rand
      21.1.29.3 Instruction time
      21.1.29.4 Instruction sleep
      21.1.29.5 Instruction siz
      21.1.29.6 Instruction sel
Appendix A Table of ASCII Codes
Appendix B GNU Free Documentation License
Concept Index
GNU poke Manual
***************

This manual describes GNU poke (version 2.0.91, 4 February 2022).

   Copyright (C) 2019-2022 The poke authors.

     You can redistribute it and/or modify this manual under the terms
     of the GNU General Public License as published by the Free Software
     Foundation, either version 3 of the License, or (at your option)
     any later version.

1 Introduction
**************

1.1 Motivation
==============

The main purpose of GNU poke is to manipulate structured binary data in
terms of abstractions provided by the user.  The Poke type definitions
can be seen as a sort of declarative specifications for decoding and
encoding procedures.  The user specifies the structure of the data to be
manipulated, and poke uses that information to automagically decode and
encode the data.  Under this perspective, struct types correspond to
sequences of instructions, array types to repetitions or loops, union
types to alternatives or conditionals, and so on.

1.1.1 Decode-Compute-Encode
---------------------------

Computing with data whose form is not the most convenient way to be
manipulated, like is often the case in unstructured binary data,
requires performing a preliminary step that transforms the data into a
more convenient representation, usually featuring a higher level of
abstraction.  This step is known in computer jargon as "unmarshalling",
when the data is fetch from some storage or transmission media or, more
generally, "decoding".

   Once the computation has been performed, the result should be
transformed back to the low-level representation to be stored or
transmitted.  This is performed in a closing step known as "marshalling"
or, more generally, "encoding".

   Consider the following C program whose purpose is to read a 32-bit
signed integer from a byte-oriented storage media at a given offset,
multiply it by two, and store the result at the same offset.

     void double_number (int fd, off_t offset, int endian)
     {
        int number, i;
        unsigned char b[4];

        /* Decode.  */
        lseek (fd, offset, SEEK_SET);
        for (i = 0; i < 4; ++i)
           read (fd, &b[i], 1);

        if (endian == BIG)
          number = b[0] << 24 | b[1] << 16 | b[2] << 8 | b[3];
        else
          number = b[3] << 24 | b[2] << 16 | b[1] << 8 | b[0];

        /* Compute.  */
        number = number * 2;

        /* Encode.  */
        if (endian == BIG)
        {
          b[0] = (number >> 24) & 0xff;
          b[1] = (number >> 16) & 0xff;
          b[2] = (number >> 8) & 0xff;
          b[3] = number & 0xff;
        }
        else
        {
          b[3] = (number >> 24) & 0xff;
          b[2] = (number >> 16) & 0xff;
          b[1] = (number >> 8) & 0xff;
          b[0] = number & 0xff;
        }

        lseek (fd, offset, SEEK_SET);
        for (i = 0; i < 4; ++i)
           write (fd, &b[i], 1);
     }

   As we can see, decoding takes care of fetching the data from the
storage in simple units, bytes.  Then it mounts the more abstract entity
on which the computation will be performed, in this case a signed 32-bit
integer.  Considerations like endianness, negative encoding (which is
assumed to be two's complement in this example and handled automatically
by C) and error conditions (omitted in this example for clarity) should
be handled properly.

   Conversely, encoding turns the signed 32-bit integer into a sequence
of bytes and then writes them out to the storage at the desired offset.
Again, this requires taking endianness into account and handling error
conditions.

   This example may look simplistic and artificial, and it is, but too
often the computation proper (like multiplying the integer by two) is
way more straightforward than the decoding and encoding of the data used
for the computation.

   Generally speaking, decoding and encoding binary data is laborious
and error prone.  Think about sequences of elements, variable-length and
clever compact encodings, elements not aligned to byte boundaries, the
always bug-prone endianness, and a long etc.  Dirty business, sometimes
risky, and always _boring_.

1.1.2 Describe-Compute
----------------------

This is where poke comes into play.  Basically, it allows you to
describe the characteristics of the data you want to compute on, and
then decodes and encodes it for you, taking care of the gory details.
That way you can concentrate your energy on the _fun_ part: computing on
the data at your pleasure.

   Of course, you are still required to provide a description of the
data.  In the Poke language, these descriptions take the form of "type
definitions", which are "declarative": you specify _what_ you want, and
poke extracts the _how_ from that.

   For example, consider the following Poke type definition:

     type Packet =
       struct
       {
         uint<16> magic = 0xef;
         uint<32> size;
         byte[size] data @ 8#B;
       };

   This tells poke that, in order to decode a 'Packet', it should
perform the following procedure (a similar procedure is implied for
encoding):

   - Read two bytes from the IO space, mount them into an unsigned
     16-bit integer using whatever current endianness, and put it in
     'magic'.  If this unsigned 16-bit integer doesn't equal to '0xef',
     then stop and emit a "data integrity" error.
   - Read four bytes, mount them into an unsigned 32-bit integer using
     the same endianness, and put it in 'size'.
   - Seek the IO space to advance 16 bits.
   - Do 'size' times:

        - Read one byte and mount it into an unsigned 8-bit integer.
        - Put the integer in the proper place in the 'data' array.

   If during this procedure an end of file is encountered, or some other
erroneous condition happens, an appropriate error is raised.

   In the procedure sketched above we find a sequence of operations,
implied by the struct type, and a loop, implied by the array type.  As
we shall see later in this book, it is also possible to decode
conditionally.  Union types are used for that purpose.

1.2 Nomenclature
================

GNU poke is a new program and it introduces many a new concept.  It is a
good idea to clarify how we call things in the poke community.  Unless
everyone uses the same nomenclature to refer to pokish thingies, it is
gonna get very confusing very soon!

   First of all we have 'poke', the program.  Since "poke" is a very
common English word, when the context is not clear we either use the
full denomination 'GNU poke', or quote the word using some other
notation.

   Then we have "Poke", with upper case P, which is the name of the
domain-specific programming language implemented by 'poke', the program.

   This distinction is important.  For example, when people talk about
"poke programmers" they refer to the group of people hacking GNU poke.
When they talk about "Poke programmers" they refer to the people who
write programs using the Poke programming language.

   Finally, a "pickle" is a Poke source file containing definitions of
types, variables, functions, etc, that conceptually apply to some
definite domain.  For example, 'elf.pk' is a pickle that provides
facilities to poke ELF object files.  Pickles are not necessarily
related to file formats: a set of functions to work with bit patterns,
for example, could be implemented in a pickle named 'bitpatterns.pk'.

   We hope this helps to clarify things.

1.3 Invoking poke
=================

Synopsis:

     poke [OPTION...] [FILE]

The following options are available.

'-l'
'--load=FILE'
     Load the given file as a Poke program.  Any number of '-l' options
     can be specified, and they are loaded in the given order.

'-L FILE'
     Load the given file as a Poke program and exit.  The rest of the
     command-line is not processed by poke, and is available to the Poke
     script in the 'argv' variable.  This is commonly used along with a
     shebang (*note Scripts::) to implement Poke scripts.

Commanding poke from the command line:

'-c'
'--command=CMD'
     Execute the given command.  Any number of '-c' options can be
     specified, and they are executed in the given order.

'-s'
'--source=FILE'
     Load FILE as a command file.  Any number of '-s' options may be
     specified, and they are loaded in the given order.  *Note Command
     Files::.

Styling text output:

'--color=HOW'
     Whether to use styled output, and how.  Valid options for HOW are
     'yes', 'no', 'auto', 'html' and 'test'.

'--style-dark'
     Use the default style that works better for dark backgrounds.  This
     is the default.

'--style-bright'
     Use the default style that works better for bright backgrounds.

'--style=FILE'
     Use FILE as the CSS to use for styling poke, instead of the default
     style.

Machine interface:

'--mi'
     Run poke in MI mode.  In this mode, poke communicates with a client
     using JSON messages in the standard input and output.

Other options:

'-q'
'--no-init-file'
     Do not load the '~/.pokerc' init file.

'--no-auto-map'
     Do not load map-files automatically when poke opens IO spaces.

'--no-hserver'
     Do not run the terminal hyperlinks server.

'--quiet'
     Be as terse as possible.

'--help'
     Print a help message and exit.

'--version'
     Show version and exit.

1.4 Commanding poke
===================

GNU poke is primarily an interactive editor that works in the command
line.  However, it is also possible to use it in a non-interactive way.
This chapter documents both possibilities.

1.4.1 The REPL
--------------

If poke is invoked with an interactive TTY connected to the standard
input, it greets you with a welcome message, licensing information and
such, and finally a prompt that looks like:

     (poke)

   At this point, the program is ready to be commanded.  You are
expected to introduce a line and press <enter>.  At that point poke will
examine the command, notify you if there is some error condition,
process the line and maybe displaying something in the terminal.

   Repeatedly typing complex commands can be tiresome.  To help you,
poke uses the readline library *Note (rluserman)Top::.  This provides
shortcuts and simple keystrokes to repeat previous commands with or
without modification, fast selection of file names and entries from
other multiple choice contexts, and navigation within a command and
among previous commands.  When the REPL starts, the history of your
previous sessions are loaded from the file '.poke_history' located in
your home directory (if it exists).

   There are several kinds of lines that can be provided in the REPL:

   * A "dot-command" invocation, that starts with a dot character ('.').
   * A command invocation.
   * A Poke statement.
   * A Poke expression.

   These are explained in the following sections.

1.4.2 Evaluation
----------------

You can evaluate a Poke statement by typing it at the REPL's prompt.
Only a single statement, including expression statements (*note
Expression Statements::) and compound statements (*note Compound
Statements::), can be evaluted this way.  It needs not be terminated by
a semicolon.

   When an expression is evaluated, the result of the evaluation is
printed back to you.  For example:

     (poke) 23
     23
     (poke) [1,2,3]
     [1,2,3]
     (poke) Packet @ 0#B
     Packet {i=1179403647,j=65794L}

   When a statement other than an expression statement is executed in
the REPL no result is printed, but of course the statement can print on
its own:

     (poke) fun do_foo = void: {}
     (poke) do_foo
     (poke) for (i in [1,2,3]) printf "elem %i32d\n", i;
     elem 1
     elem 2
     elem 3

   If there is an error compiling the line, you are notified with a nice
error message, showing the location of the error.  For example:

     (poke) [1,2,3 + "foo"]
     <stdin>:1:6: error: invalid operands in expression
     [1,2,3 + "foo"];
          ^~~~~~~~~

1.4.3 Commands and Dot-Commands
-------------------------------

There are two kinds of commands in poke: the "dot-commands", which are
written in C and have their own conventions for handling sub-commands
and passing arguments and flags, and normal commands, which are written
in Poke.

1.4.3.1 Dot-Commands
....................

Dot-commands are so called because their names start with the dot
character ('.').  They can feature subcommands.  Example:

     (poke) .vm disassemble mapper int[] @ 0#B
     (poke) .vm disassemble writer int[] @ 0#B

   When there is no ambiguity, the command name and the subcommands can
be shortened to prefixes.  The commands above can also be written as:

     (poke) .vm dis m int[] @ 0#B
     (poke) .vm dis w int[] @ 0#B

   Some commands also get flags, which are one-letter indicators that
can be appended to the command name (including subcommands) after a
slash character ('/').  For example, the '.vm' disassembler commands
accept a 'n' flag to indicate we want a native disassemble.  We can pass
it as follows:

     (poke) .vm disassemble mapper/n int[] @ 0#B
     (poke) .vm disassemble writer/n int[] @ 0#B

   If a dot-command accepts more than one argument, they are separated
using comma characters (',').  Spaces are generally ignored.

1.4.3.2 Commands
................

Regular poke commands are written in Poke and use different conventions.
The name of commands follow the same rules as normal Poke identifiers,
and do not start with a dot character.

   An example is the 'dump' command:

     (poke) dump
     76543210  0011 2233 4455 6677 8899 aabb ccdd eeff  0123456789ABCDEF
     00000000: 7f45 4c46 0201 0100 0000 0000 0000 0000  .ELF............
     00000010: 0100 f700 0100 0000 0000 0000 0000 0000  ................
     00000020: 0000 0000 0000 0000 8001 0000 0000 0000  ................
     00000030: 0000 0000 4000 0000 0000 4000 0800 0700  ....@.....@.....
     00000040: 1800 0000 0000 0000 0000 0000 0000 0000  ................
     00000050: 7900 0000 0000 0000 b701 0000 9a02 0000  y...............
     00000060: 7b10 0000 0000 0000 1800 0000 0000 0000  {...............
     00000070: 0000 0000 0000 0000 7900 0000 0000 0000  ........y.......

   After the name of the command, arguments can be specified by name,
like this:

     (poke) dump :from 0#B :size 8#B
     (poke) dump :from 0#B :size 8#B
     76543210  0011 2233 4455 6677 8899 aabb ccdd eeff  0123456789ABCDEF
     00000000: 7f45 4c46 0201 0100                      .ELF....

   The 'dump' command is discussed in greater detail below (*note
dump::).  The order of arguments is irrelevant in principle:

     (poke) dump :from 0#B :size 8#B :ascii 0 :ruler 0
     00000000: 7f45 4c46 0201 0100
     (poke) dump :ruler 0 :from 0#B :size 8#B :ascii 0
     00000000: 7f45 4c46 0201 0100

   However, beware side effects while computing the values you pass as
the arguments!  The expressions themselves are evaluated from left to
right.

   Which arguments are accepted, and their kind, depend on the specific
command.

   Note that the idea is to restrict the number of dot-commands to the
absolutely minimum.  Most of the command-like functionality provided in
poke shall be implemented as regular commands.

1.4.4 Command Files
-------------------

Command files contain poke commands.  A poke command may be a dot
command, a Poke statement or a Poke expression.  Lines starting with '#'
are comments will be ignored.  However a comment must start at the
beginning of a line.  Here is an example of a script:

     # The following two lines are dot commands
     .load my-pickle.pk
     .set obase 16

     # The following line is a Poke statement
     dump :size 0x100#B :from 0x10#B

     # The following line is a Poke expression statement without any side effect.
     # Consequently it is valid, but rather useless.
     4 == 4

   A command file contains commands, not Poke code.  This means it gets
read line by line and commands cannot occupy more than one line.  Hence
the following is a valid command file:

     type foo = struct {int this; int that;}

but this is not valid as a command file (although it is a valid Poke
statement) and will provoke an error:

     type foo = struct
     {
      int this;
      int that;
     }

   Command files can be loaded at startup using the '-s' command line
option (*note Invoking poke::).  The '~/.pokerc' startup file is also an
example of a poke command file (*note pokerc::).

1.4.5 Scripts
-------------

Following the example of Guile Scheme, the Poke syntax includes support
for multi-line comments using the '#!' and '!#' delimiters.  This, along
with the '-L' command line option, allows to write Poke scripts and
execute them in the command line like if they were normal programs.
Example of a script:

     #!/usr/bin/poke -L
     !#

     print "Hello world!\n";

   The resulting script can process command-line options by accessing
the 'argv' array.  The following Poke script prints its arguments:

     #!/usr/bin/poke -L
     !#

     for (arg in argv)
       printf ("Argument: %s\n", arg);

   If you want to pass additional flags to the poke command, you need to
use a slightly different kind of shebang:

     #!/usr/bin/env sh
     exec poke -L "$0" "$@"
     !#

     load elf;
     printf ("%v\n", Elf64_Ehdr @ 0#B);

2 Setting Up
************

2.1 Setting up Hyperlinks
=========================

GNU poke uses "terminal hyperlinks" in order to improve the interactive
usage of the tool: clicking on terminal hyperlinks requests poke to
execute certain actions.  This is used to implement buttons and other
interactive goodies.

   Many terminal emulators support terminal hyperlinks.  However, we are
using a very simple protocol called 'app://' that is not supported (yet)
on GNU/Linux distros.  Fortunately, it is very easy to set your system
to use this protocol, and this chapter shows you how.

2.1.1 Make sure your poke speaks hyperlinks
-------------------------------------------

The first step in having an hyperlinks-capable poke is to make sure to
have a recent enough version of libtextstyle when building poke.  If
your poke can emit hyperlinks you will see a message like this when
running it on the terminal:

     hserver listening in port 43713.

2.1.2 Use a terminal emulator that supports hyperlinks
------------------------------------------------------

Gnome Terminal has support for displaying hyperlinks as do many other
emulators that rely on VTE. Check the list at
<https://gist.github.com/egmontkob/eb114294efbcd5adb1944c9f3cb5feda#supporting-apps>
for a mostly up-to-date, non-exhaustive list of emulators that support
printing hyperlinks.

2.1.3 Get and install the app-client utility
--------------------------------------------

Since 'app://' is a new URI protocol that we designed, common terminal
emulators don't know what to do when they encounter such a URI. To work
around this problem we use the XDG Desktop Specification and a little C
utility called 'app-client', which can be found at
<https://gitlab.com/darnir/hyperlink-app-client>.

   By setting 'app-client' as the default handler for 'app://' URIs, the
terminal emulator does not need to understand the syntax or semantics of
the 'app://' protocol.  It offloads the handling of the URI entirely to
'app-client'.  In order to use this, first download and install
'app-client':

     $ git clone https://gitlab.com/darnir/hyperlink-app-client
     $ cd hyperlink-app-client
     $ make
     $ cp app-client /location/in/$PATH/variable

   Next step is to copy the 'app-client.desktop' file in the git
repository to '$HOME/.local/share/applications'.  This is a XDG Desktop
Entry for the app-client.  And let's most applications on your system
know that this should be used to handle 'app://' URIs.  (See the
'MimeType' field)

   This is enough for any utility (like terminals) that use 'xdg-open'
to do the right thing with hyperlinks.  However, certain terminals
require additional setup.  See below if that is your case.

2.1.4 GNOME Terminal
--------------------

Gnome Terminal doesn't use 'xdg-open' to start the applications.
Instead, it parses the 'mimeapps.list' file manually to find the right
application.

   Edit your 'mimeapps.list', it is usually located at
'$HOME/.local/share/applications/mimeapps.list', but it might also be at
'$XDG_CONFIG_DIR/mimeapps.list', and add the following line to it:

     x-scheme-handler/app=app-client.desktop

   This let's Gnome Terminal know how to open 'app://' links.

2.2 Simple Init File
====================

GNU poke is a spartan program that tries to be as simple as possible by
default, without fancy displays.  Therefore, before exploring poke you
may want to configure it minimally.  This section contains a few
recommendations in that respect.

   First we must say that poke reads a per-user configuration from the
'~/.pokerc' file.  *Note pokerc::.

   We recommend new users to set the following options:

     .set endian little
     .set omode tree
     .set oacutoff 5
     .set pretty-print yes

These options are explained later in this manual.  *Note set command::.

3 Basic Editing
***************

In this chapter you will learn how to shuffle binary data around with
poke, in terms of fundamental predefined entities: bits, bytes,
integers, and the like.

3.1 Binary Files
================

GNU poke is an editor for "binary files".  Right, so what is a binary
file?  Strictly speaking, every file in a computer's file system is
binary.  This is because, in a very fundamental level, files are just
sequences of bytes.

   Colloquially, however, it is very common to talk about "binary files"
as opposed to "text files".  In this informal meaning, a text file is
basically a file composed, mostly, of bytes (and byte sequences) that
can be translated into printable characters in some character set, such
as ASCII, EBCDIC or Unicode.  It follows that binary files would then be
files composed, mostly, of bytes not intended to be interpreted as
encoded characters.

   Some text files contain non-printable characters, such as form feed
characters, and many binary files contain printable strings, such as a
string table in an ELF object file.  That is why we used the word
"mostly" in the definitions above.  In practice, however, the
distinction is almost always clear and there is common consensus on
whether a given file format can be considered as a binary format, or
not.

   GNU poke can edit any file, and as we shall see, it provides some
nice features to manipulate sequences of bytes interpreted as character
strings.  However, it is called a "binary editor" because it is
especially designed to be particularly useful editing binary files, in
the sense of the term defined above.

   In this chapter, we will be using ELF object files as the experiment
subject in most of the examples.  ELF files are good for this purpose,
because they are eminently binary, highly structured, and still strings
play a role in them, encoding names of entities like sections and
symbols.  You don't need to have a perfect knowledge of the ELF format
in order to follow the examples, but being familiarized with the concept
of object file formats should surely help.

   Obtaining a simple ELF object file is easy, if you have a C compiler
installed:

     $ echo 'int foo () { return 0; }' | gcc -c -xc -o foo.o -

   The command above compiles a very simple ELF object file that
contains the compiled form of a little dummy function.  This object file
will be our companion for a while, and will be the subject of much
analysis and abuse, as we poke it.

3.2 Files as IO Spaces
======================

Now that we have a binary file ('foo.o') it is time to open it with
poke.  There are two ways to do that.

   One way is to pass the name of the file in the poke invocation.  The
program will start, open the file, and present you with the REPL, like
in:

     $ poke foo.o
     [...]
     (poke)

   The other way is to fire up poke without arguments, and then use the
'.file' dot-command to open the file:

     $ poke
     [...]
     (poke) .file foo.o
     The current IOS is now `./foo.o'.
     (poke)

Note how poke replies to the dot-command, stating that the "current IOS"
is now the file we opened.

   You may be wondering, what is this IOS thing?  It is an acronym for
Input/Output Space, often written IO Space.  This is the denomination
used to refer to the entities being edited with poke.  In this case the
IO space being edited is a file, but we will see that is not always the
case: poke can also edit other entities such as memory buffers and
remote block-oriented devices over the network.  For now, let's keep in
mind that IOS, or IO space, refers to the file being edited.

   And why "current"?  GNU poke is capable of editing several files
(more generally, several IO spaces) simultaneously.  At any time, one of
these files is the "current one".  In our case, the current IO space is
the file 'foo.o', since it is the only file poke knows about:

     (poke) .info ios
       Id	Type	Mode	Bias		Size		Name
     * #0	FILE	rw	0x00000000#B	0x00000398#B	./foo.o

   The command '.info ios' gives us information about all the IO spaces
that are currently open.  The first column tells us a "tag" that
identifies the IOS. In this example, the tag corresponding to 'foo.o' is
'#0'.  The second column tells us the type of IO space.  The third
column tells us that 'foo.o' allows both reading and writing.  The
fourth column tells us the size of the file, in hexadecimal.

   You may wonder what is that weird suffix '#B'.  It is a unit, and
tells us that the size '0x398' is measured in bytes, i.e.  the size of
'foo.o' is '0x398' bytes (or, in decimal, '920' bytes.)

   Finally, the asterisk character at the left of the entry for 'foo.o'
identifies it as the current IO space.  To see this more clearly, let's
open another file:

     (poke) .file bar.o
     The current IOS is now `./bar.o'.
     (poke) .info ios
       Id	Type	Mode	Bias		Size		Name
     * #1	FILE	rw	0x00000000#B	0x00000398#B	./bar.o
       #0	FILE	rw	0x00000000#B	0x00000398#B	./foo.o

   Ah, there we have both 'foo.o' and 'bar.o'.  Now the current IO space
(the entry featuring the asterisk at the left) is the file that we just
opened, 'bar.o'.  This is because poke always sets the most recently
open file as the current one.  We can switch back to 'foo.o' using yet
another dot-command, '.ios', which gets an IO space tag as an argument:

     (poke) .ios #0
     The current IOS is now `./foo.o'.
     (poke) .info ios
       Id	Type	Mode	Bias		Size		Name
       #1	FILE	rw	0x00000000#B	0x00000398#B	./bar.o
     * #0	FILE	rw	0x00000000#B	0x00000398#B	./foo.o

We are back to 'foo.o'.  Since we are not really interested in 'bar.o',
let's close it:

     (poke) .close #1
     (poke) .info ios
       Id	Type	Mode	Bias		Size		Name
     * #0	FILE	rw	0x00000000#B	0x00000398#B	./foo.o

Awesome.  Now we can focus on 'foo.o''s contents...

3.3 Dumping File Contents
=========================

Data stored in modern computers, in both volatile memory and persistent
files, is fundamentally a sequence of entities called "bytes".  The
bytes can be addressed by its position in the sequence, starting with
zero:

     +--------+--------+--------+ ... +--------+
     | byte 0 | byte 1 | byte 2 |     | byte N |
     +--------+--------+--------+ ... +--------+

Each byte has capacity to store a little unsigned integer in the range
'0..255'.  Therefore, the IO spaces that we edit with poke (like the
file 'foo.o') can be seen as a sequence of little numbers, like depicted
in the figure above.

   GNU poke provides a command whose purpose is to display the values of
these bytes: 'dump'(1) .  It is called like that because it dumps ranges
of bytes to the terminal, allowing the user to inspect them.

   So let's use our first poke command!  Fire up poke, open the file
'foo.o' as explained above, and execute the 'dump' command:

     (poke) dump
     76543210  0011 2233 4455 6677 8899 aabb ccdd eeff  0123456789ABCDEF
     00000000: 7f45 4c46 0201 0100 0000 0000 0000 0000  .ELF............
     00000010: 0100 f700 0102 0000 0000 0000 0000 0000  ................
     00000020: 0102 0000 0000 0000 9801 0000 0000 0000  ................
     00000030: 0000 0000 4000 0000 0000 4000 0800 0700  ..............
     00000040: 2564 0a00 0000 0000 0000 0000 0000 0000  %d..............
     00000050: b702 0000 0100 0000 1801 0000 0000 0000  ................
     00000060: 0000 0000 0000 0000 8510 0000 ffff ffff  ................
     00000070: b700 0000 0000 0000 9500 0000 0000 0000  ................
     (poke)

What are we looking at?

   The first line of the output, starting with '76543210', is a "ruler".
It is there to help us to visually determine the location (or offset) of
the data.

   The rest of the lines show the values of the bytes that are stored in
the file, 16 bytes per line.  The first column in these data lines shows
the offset, in hexadecimal and measured in number of bytes, from which
the row of data starts.  For example, the offset of the first byte shown
in the third data line has offset 0x20 in the file, the second byte has
offset 0x21, and so on.  Note how the data rows show the values of the
individual bytes, in hexadecimal.  Generally speaking, when dealing with
bytes (and binary data in general) it is useful to manipulate magnitudes
in hexadecimal, or octal.  This is because it is easy to group digits in
these bases to little groups of bits (four and three respectively) in
the equivalent binary representation.  In this case, each couple of
hexadecimal digits denote the value of a single byte(2).  For example,
the value of the first byte in the third data row is 0x01, the value of
the second byte 0x02, and so on.

   Using the ruler and the column of offsets, locating bytes in the data
is very easy.  Let's say for example we are interested in the byte at
offset 0x68: we use the first column to quickly find the row starting at
0x60, and the ruler to find the column marked with '88'.  Cross column
and row and... voila!  The byte in question has the value 0x85.  The
reverse process is just as easy.  What is the offset of the first 0x40
in the file?  Try it!

   The section at the right of the output is the ASCII output.  It shows
the row of bytes at the left interpreted as ASCII characters.
Non-printable characters are shown as '.' to avoid scrambling the
terminal, and yes, there is actually way to customize what character to
use, so they are not confused from real ASCII dot characters ('0x2e') :P
In this particular dump we can see that near the beginning of the file
there are three bytes whose value, if interpreted as ASCII characters,
conform the string "ELF". As we shall see, this is part of the ELF magic
number.  Again, the ruler is very useful to locate the byte
corresponding to some character in the ASCII section, or the other way
around.  What is the value of the byte corresponding to the 'F' in
'ELF'?  Try it!

   Something to notice in the 'dump' output above is that these are not,
by any mean, the complete contents of the file 'foo.o'.  The '.info ios'
dot-command informed us in the last section that 'foo.o' contains 920
bytes, of which the 'dump' command only showed us... '0x80' bytes, or
'128' bytes in decimal.

   'dump' is certainly capable of showing more (and less) than '128'
bytes.  We can ask 'dump' to display some given amount of data by
specifying its size using a "command argument".  For example:

     (poke) dump :size 64#B
     76543210  0011 2233 4455 6677 8899 aabb ccdd eeff  0123456789ABCDEF
     00000000: 7f45 4c46 0201 0100 0000 0000 0000 0000  .ELF............
     00000010: 0100 f700 0102 0000 0000 0000 0000 0000  ................
     00000020: 0102 0000 0000 0000 9801 0000 0000 0000  ................
     00000030: 0000 0000 4000 0000 0000 4000 0800 0700  ..............

   The command above asks poke to "dump 64 bytes".  In this example
':size' is the name of the argument, and '64#B' is the argument's value.
Again, the suffix '#B' tells poke we want to dump 64 bytes, not 64
kilobits nor 64 potatoes.

   Another interesting aspect of our first dump (ahem) is that the
dumped bytes start from the beginning of the file, i.e.  the offset of
the first byte is 0x0.  Certainly there should be other areas of the
file with interesting contents for us to inspect.  To that purpose, we
can use yet another option, ':from':

     (poke) dump :size 64#B :from 128#B
     76543210  0011 2233 4455 6677 8899 aabb ccdd eeff  0123456789ABCDEF
     00000080: 1400 0000 0000 0000 0000 0000 0000 0000  ................
     00000090: 0000 0000 0000 0000 0000 0000 0000 0000  ................
     000000a0: 0000 0000 0300 0100 0000 0000 0000 0000  ................
     000000b0: 0000 0000 0000 0000 0000 0000 0300 0300  ................

   The command above asks poke to "dump 64 bytes starting at 128 bytes
from the beginning of the file".  Note how the first row of bytes start
at offset '0x80', i.e.  '128' in decimal.

   Passing options to commands is easy and natural, but we may find
ourselves passing the same values again and again to certain command
options.  For example, if the default size of 'dump' of 128 bytes is not
what you prefer, because you have a particularly tall monitor, or you
are one of these people using sub-atomic sized fonts, it can be tiresome
and error-prone to pass ':size' to 'dump' every time you use it.
Fortunately, the default size can be customized by setting a "global
variable":

     (poke) pk_dump_size = 160#B

This tells poke to set '160' bytes as the new value for the
'pk_dump_size' variable.  This is a global variable that the 'dump'
command uses to determine how much data to show if the user doesn't
specify an explicit value with the ':size' option.  Many other commands
use the same strategy in order to alter their default behavior, not just
'dump'.

   And now that we are talking about that, it is also cumbersome to have
to set the default size used by 'dump' every time we run poke.  But no
problem, just set the variable in a file called '.pokerc' in your home
directory, like this:

     pk_dump_size = 160#B

Every time poke starts, it reads '~/.pokerc' and executes the commands
contained in it.  See *note pokerc::.

   The 'dump' command is very flexible, and accepts a lot of options and
customization variables that we won't be covering in this chapter.  For
a complete description of the command, see *note dump::.

   ---------- Footnotes ----------

   (1) Note that this is not a dot-command like '.file', '.ios' or
'.close': 'dump' does not start with a dot!  We will see later how
dot-commands differ from "normal commands" like 'dump', but for now,
let's ignore the distinction.

   (2) Do not be fooled by the fact 'dump' shows the hexadecimal digits
in groups of four: this is just a visual aid and, as we shall see, it is
possible to change the grouping by passing arguments to 'dump'.

3.4 Poking Bytes
================

Let's look again at the first bytes of the file 'foo.o':

     (poke) dump :size 64#B
     76543210  0011 2233 4455 6677 8899 aabb ccdd eeff  0123456789ABCDEF
     00000000: 7f45 4c46 0201 0100 0000 0000 0000 0000  .ELF............
     00000010: 0100 f700 0102 0000 0000 0000 0000 0000  ................
     00000020: 0102 0000 0000 0000 9801 0000 0000 0000  ................
     00000030: 0000 0000 4000 0000 0000 4000 0800 0700  ..............

   At this point we know how to use the ruler to localize specific bytes
just by looking at the displayed data.  If we wanted to operate on the
values of some given bytes, we could look at the dump and type the
values in the REPL. For example, if we wanted to add the values of the
bytes at offsets 0x2 and 0x4, we could look at the dump and then type:

     (poke) 0x4c + 0x02
     0x4e

   GNU poke supports many operators that take integers as arguments, to
perform arithmetic, relational, logical and bit-wise operations on them
(*note Integers::).  Since bytes are no more (and no less) than little
unsigned integers, we can use these operators to perform calculations on
bytes.

   For example, this is how we would calculate whether the highest bit
in the second byte in 'foo.o' is set:

     (poke) 0x45 & 0x80
     0

Note how booleans are encoded in Poke as integers, 0 meaning false, any
other value meaning true.

   Looking at the output of 'dump' and writing the desired byte value in
the prompt is cumbersome.  Fortunately, there is a much more convenient
way to access the value of a byte, given its offset in the file: it is
called "mapping" a byte value.  This operation is implemented by a
binary operator, called the map operator.

   This is how it works.  Assuming we were interested in the byte at
'64' bytes from the beginning of the file, this is how we would refer to
it (or "map" it):

     (poke) byte @ 64#B
     37UB

This application of the map operator tells poke to map a byte at the
offset 64 bytes.  It can be read as "byte at 64 bytes".  Note how poke
replies with the value 37UB. The suffix 'UB' means "unsigned byte", and
is an indication for the user about the nature of the preceding number:
it is unsigned, and it occupies a byte when stored.

   As we can see in this example, poke uses decimal by default when
showing values in the REPL. We already noted how it is usually better to
work in hexadecimal when dealing with byte values.  Fortunately, we can
change the numeration base used by poke when printing numbers, using the
'.set obase' ("set output base") dot-command as this:

     (poke) .set obase 16

After this, we can map the byte again, this time getting the result
expressed in hexadecimal:

     (poke) byte @ 64#B
     0x25UB

Again, you may find it useful to add the '.set obase 16' command to your
'.pokerc' file, if you want the customization to be persistent between
poke invocations.

   Going back to the example of calculating whether the highest bit in
the second byte in 'foo.o' is set, this is how we would do it with a
map:

     (poke) (byte @ 2#B) & 0x80
     0

Turns out the answer is no.

   The map operator can also be used at the left side of an assignment
operator:

     (poke) byte @ 0x28#B = 0xff

Which reads "assign 0xff to the byte at offset 0x4a bytes".  Dumping
again, we can verify that the byte actually changed:

     76543210  0011 2233 4455 6677 8899 aabb ccdd eeff  0123456789ABCDEF
     00000000: 7f45 4c46 0201 0100 0000 0000 0000 0000  .ELF............
     00000010: 0100 f700 0102 0000 0000 0000 0000 0000  ................
     00000020: 0102 0000 0000 0000 ff01 0000 0000 0000  ................
     00000030: 0000 0000 4000 0000 0000 4000 0800 0700  ..............

Does this mean that 'foo.o' changed accordingly, in disk?  The answer is
yes.  poke always commits changes immediately to the file being edited.
This, that is an useful feature, can also be a bit tricky if you forget
about it, leading to data corruption, so please be careful.

   Incidentally, altering the byte at offset 0x28 most probably have
caused 'foo.o' to stop being a valid ELF file, but since we are just
editing bytes (and not ELF structures) we actually don't care much.

3.5 Values and Variables
========================

Up to now we have worked with byte values, either writing them in the
REPL or mapping them at the current IO space.  Often it is useful to
save values under meaningful names, and access to them by name.  In poke
we do that by storing the values in "variables".

   Before being used, variables shall be defined using the 'var'
construction.  Let's get the byte at offset '64' bytes and save it in a
variable called 'foo':

     (poke) var foo = byte @ 64#B

This defines a new variable ('foo') and initializes it to the value of
the byte at offset 64 bytes.  This results on 'foo' to hold the value
37.

   Once defined, we can get the value of a variable by just giving its
name to poke:

     (poke) foo
     37UB

Several variables can be defined in a single 'var' declaration.  Each
definition is separated by a comma.  This is equivalent of issuing
several 'var's:

     (poke) var a = 10, b = 20

In general, a variable containing a byte value can be used in any
context where the contained value would be expected.  If we wanted to
check the highest bit in the byte value stored in 'foo' we would do:

     (poke) foo & 0x80
     0x0

   Assigning a value to a variable makes the value the new contents of
the variable.  For example, we can increase the value of 'foo' by one
like this:

     (poke) foo = foo + 1

   At this point, an important question is: when we change the value of
the variable 'foo', are we also changing the value of the byte stored in
'foo.o' at offset 64 bytes?  The answer is no.  This is because when we
do the mapping:

     (poke) var foo = byte @ 64#B

The value stored in 'foo' is a _copy_ of the value returned by the map
operator '@'.  You can imagine the variable as a storage cell located
somewhere in poke's memory.  After the assignment above is executed
there are two copies of the byte value 0x25: one in 'foo.o' at offset 64
bytes, and the other in the variable 'foo'.

   It follows that if we wanted to increase the byte in the file, we
would need to do something like:

     (poke) var foo = byte @ 64#B
     (poke) foo = foo + 1
     (poke) byte @ 64#B = foo

Or, more succinctly, omitting the usage of a variable:

     (poke) byte @ 64#B = (byte @ 64#B) + 1

Or, even _more_ succinctly:

     (poke) (byte @ 64#B) += 1

Note how we have to use parenthesis around the map at the right hand
side, because the map operator '@' has less precedence than the plus
operator '+'.

3.6 From Bytes to Integers
==========================

The bytes we have been working with are unsigned whole numbers (or
integers) in the range '0..255'.  We saw how poke sees the contents of
the files as a sequence of bytes, and how each byte can be addressed
using an offset.  Mapping bytes using the map operator '@' gives us
these values, which are denoted in poke with literals like '10UB' or
'0x0aUB'.

   This very limited range of values have consequences when it comes to
do arithmetic with bytes.  Suppose for example we wanted to calculate
the average of the first byte values stored in 'foo.o'.  We could do
something like:

     (poke) a0 = byte @ 0#B
     (poke) a1 = byte @ 1#B
     (poke) a2 = byte @ 2#B
     (poke) a0
     0x7fUB
     (poke) a1
     0x45UB
     (poke) a2
     0x4cUB
     (poke) (a0 + a1 + a2) / 3UB
     5UB

That is obviously the wrong answer.  What happened?  Let's do it step by
step.  First, we add the first two bytes:

     (poke) a0 + a1
     0xc4UB

Which is all right.  0xc4 is 0x7f plus 0x45.  But, let's add now the
third byte:

     (poke) a0 + a1 + a2
     0x10UB

That's no good.  Adding the value of the third byte (0x4c) we overflow
the range of valid values for a byte value.  The calculation went banana
at this point.

   Another obvious problem is that we surely will want to store integers
bigger than 255 in our files.  Clearly we need a way to encode them
somehow, and since all we have in a file are bytes, the integers will
have to be composed of them.

   Integers bigger than 255 can be encoded by interpreting consecutive
byte values in a certain way.  First, let's consider a single byte.  If
we print a byte value using binary rather than decimal or hexadecimal,
we will observe that eight bits are what it takes to encode the numbers
between 0 and 0xff (255) using a "natural binary encoding":

     (poke) .set obase 2
     (poke) 0UB
     0b00000000UB
     (poke) 0xFFUB
     0b11111111UB

   This is the reason why people say bytes are "composed" of eight bits,
or that the width of a byte is eight bits.  But this way of talking
doesn't really reflect the view that the operating system has of devices
like files or memory buffers: both disk and memory controllers provide
and consume bytes, i.e.  little unsigned numbers in the range '0..255'.
At that level, bytes are indivisible.  We will see later that poke
provides ways to work on the "sub-byte" level, but that is just really
an artifact to make our life easier: underneath, all that goes in and
out are bytes.

   Anyhow, if we were to "concatenate" the binary representation of two
consecutive bytes, we would end with a much bigger range of possible
numbers, in the range '0b00000000_00000000..0b11111111_11111111'(1), or
'0x0000..0xffff' in hexadecimal.  poke provides a bit-concatenation
operator ':::' that does exactly that:

     (poke) 0x1UB
     0b00000001UB
     (poke) 0x1UB ::: 0x1UB
     0b0000000100000001UH

Note how the suffix of the resulting number is now 'UH'.  This indicates
that the number is no longer a byte value: it is too big for that.  The
'H' in this new suffix means "half", and it is a traditional way to call
an integer that is encoded using two bytes, or 16 bits.

   So, using our method of encoding bigger numbers concatenating bytes,
what would be the "half" integer composed of two bytes at the beginning
of 'foo.o'?

     (poke) .set obase 16
     (poke) (byte @ 0#B):::(byte @ 1#B)
     0x7f45UH

   Now, let's go back to the syntax we used to map a byte value.  In the
invocation of the map operator 'byte @ 0#B' the operand at the left (in
this case 'byte') tells the operator what kind of value to map.  This is
called a "type specifier"; 'byte' is the type specifier for a single
byte value, and 'byte[3]' is the type specifier for a group of three
byte values arranged in an array.

   As it happens, 'byte' is a synonym for another slightly more
interesting type specifier: 'uint<8>'.  You can probably infer the
meaning already: a byte is an unsigned integer which is 8 bits big.  We
can of course use this alternate specifier in a mapping operation,
achieving exactly the same result than if we were using 'byte':

     (poke) uint<8> @ 0#B
     0x7fUB

   You may be wondering: is it possible to use a similar type specifier
for mapping bigger integers, like these "halves" that are composed of
two bytes?  Yeah, it is indeed possible:

     (poke) uint<16> @ 0#B
     0x7f45UH

Mapping an unsigned integer of 16-bits at the offset 0 gives us an
unsigned "half" value, as expected.

   You can easily build bigger and bigger numbers concatenating more and
more bytes.  Three bytes?  sure:

     (poke) uint<24> @ 0#B
     (uint<24>) 0x7f454c

Note that in this case poke uses a prefix instead of a suffix to
indicate that the given value is 24-bits long.  Four bytes?

     (poke) uint<32> @ 0#B
     0x7f454c46U

   Certain integer widths are so often used that easier-to-type synonyms
for their type specifiers are provided.  We already know 'byte' for
'uint<8>'.  Similarly, 'ushort' is a synonym for 'uint<16>', 'uint' is a
synonym for 'uint<32>' and 'ulong' is a synonym for 'uint<64>'.  Try
them!

   GNU poke supports integers up to eight bytes, i.e.  up to 64-bits.
This may change in the future, as we are planning to support arbitrarily
large integers.

   ---------- Footnotes ----------

   (1) poke allows to insert underscore characters '_' anywhere in
number literals.  The only purpose of these characters is to improve
readability, and they are totally ignored by poke, i.e.  they do not
alter the value of the number.

3.7 Big and Little Endians
==========================

When talking about whole numbers (integers) we should distinguish
between their value (such as 123) and their "written form" that we would
use when writing the number on a piece of paper, such as '123'.

   The written form of a number is composed of digits, arranged in
certain order.  We all know that the ordering of the digits in the
written form of a number is important: if we write '123' we are
referring to a different value than if we write '321'.  The mathematical
reason for this is that depending on the position they occupy in the
written form, each digit contributes with a different "weight" to the
total value of the number.  This is always the case, regardless of the
numerical base used to denote the number.

   For example, the value of the number 123 (whose written form is
'123') is calculated as '1*10^2+2*10^1+3*10^0'.  If we swap the last two
digits in the written form of the number, we have
'1*10^2+3*10^1+2*10^0', which results in a different value: '132'.  When
we consider other numerical bases, the bases in the polynomial change
accordingly, but the correspondence between written form and value
stands: for example, the value of 0x123 is calculated as
'1*16^2+2*16^1+3*16^0'.

   The "higher" a digit is in the polynomial, the "more significant" it
is, i.e.  the more weight it has on the value of the number where it
appears.  In the written number '123', for example, the digit 1 is the
"most significant" digit of the number, and the digit 3 is the "least
significant" digit.

   This distinction between the written form of a number and its value
is very important.  Just like in certain languages letters are read
right-to-left (Arabic) or even down-to-up (Japanese) we could certainly
conceive a language in which the digits of numbers were arranged from
right-to-left instead of left-to-right.  In such a language the written
representation of 123 would be '321', not '123'.  In other words: the
least significant digit would come first, not last, in the written form
of the number.

   Now when it comes to store numbers in computers, rather than writing
them on a paper, the role of the paper is played by the computer's
memory, be it ephemeral (like RAM) or persistent (like a spinning hard
disk or a Flash memory), which is organized as a sequence of bytes.
Since we are composing numbers with bytes, it makes sense to have each
byte to play the role of a digit in the written form of the bigger
number.  Since bytes can have values from 0 to 255, the base is 256.
But what is the "written form" for our byte-composed numbers?

   In the last section we tried to compose bigger integers by
concatenating bytes together and interpreting the result.  In doing so,
we assumed (quite naturally) that in the written form of the resulting
integer the bytes are ordered in the same order than they appear in the
file, i.e.  we assume that the written form of the number
'b1*256^2+b2*256^1+b3*256^0' would be 'b1b2b3', where 'b1', 'b2' and
'b3' are bytes.  In other words, given a written form 'b1b2b3', 'b1'
would be the most significant byte (digit) and 'b3' would be the least
significant byte (digit).  In our world of IO spaces, the "written form"
is the disposition of the bytes in the IO space (file, memory buffer,
etc) being edited.

   That interpretation of the written form is exactly what the
bit-concatenation operator implements:

     (poke) dump :from 0#B :size 3#B
     76543210  0011 2233 4455 6677 8899 aabb ccdd eeff  0123456789ABCDEF
     00000000: 7f45 4c                                  .EL
     (poke) var b1 = byte @ 0#B
     (poke) var b2 = byte @ 1#B
     (poke) var b3 = byte @ 2#B
     (poke) b1:::b2:::b3
     (uint<24>) 0x7f454c

   However, much like in certain human languages the written form is
read from right to left, some computers also read numbers from right to
left in their "written form".  Actually, turns out that _most_ modern
computers do it like that.  This means that, in these computers, given
the written form 'b1b2b3' (i.e.  given a file where 'b1' comes first,
followed by 'b2' and then 'b3') the most significant byte is 'b3' and
the least significant byte is 'b1'.  Therefore, the value of the number
would be 'b3*256^2+b2*256^1+b3*256^0'.

   So, given the written form of a bigger number 'b1b2b3' (i.e.  some
ordering of bytes implied by the file they are stored in) there are at
least two ways to interpret them to calculate the value of the number.
When the written form is read from left to right, we talk about a "big
endian" interpretation.  When the written form is read from right to
left, we talk about a "little endian" interpretation.

   Given the first three bytes in 'foo.o', we can determine the value of
the integer composed of these three bytes in both interpretations:

     (poke) b1:::b2:::b3
     (uint<24>) 0x7f454c
     (poke) b3:::b2:::b1
     (uint<24>) 0x4c457f

   Remember how the type specifier 'byte' is just a synonym of
'uint<8>', and how we can use type specifiers like 'uint<24>' and
'uint<32>' to map bigger integers?  When we do that, like in:

     (poke) uint<24> @ 0#B
     (uint<24>) 0x7f454c

Poke should somehow decide what kind of interpretation to use, i.e.  how
to read the "written form" of the number.  As you can see from the
example, poke uses the left-to-right interpretation, or big-endian, by
default.  But you can change it using a new dot-command: '.set endian':

     (poke) .set endian little
     (poke) uint<24> @ 0#B
     (uint<24>) 0x4c457f

   The currently used interpretation (also called "endianness") is shown
if you invoke the dot-command without an argument(1):

     (poke) .set endian
     little

   Different systems use different endianness.  Into a given system, it
is to be expected that most files will be encoded following the same
conventions.  Therefore poke provides you a way to set the endianness to
whatever endianness is in the system.  You do it this way:

     (poke) .set endian host

   ---------- Footnotes ----------

   (1) This also applies to the other '.set' commands

3.8 Negative Integers
=====================

3.8.1 Negative Encodings
------------------------

Up to this point we have worked with unsigned integers, i.e.  whole
numbers which are zero or bigger than zero.  Much like it happened with
endianness, the interpretation of the value of several bytes as a
negative number depends on the specific interpretation.

   In computing there are two main ways to interpret the values of a
group of bytes as a negative number: "one's complement" and "two's
complement".

   At the moment GNU poke supports the two complement interpretation,
which is really ubiquitous and is the negative encoding used by the vast
majority of modern computers and operating systems.

   We may consider adding support for one's complement in the future,
but only if there are real needs that would justify the effort (which
wouldn't be a small one ;)).

3.8.2 Signed Integers
---------------------

Unsigned values are never negative.  For example:

     (poke) 0UB - 1UB
     0xffUB

Instead of getting a -1, we get the result of an unsigned underflow,
which is the biggest possible value for an unsigned integer of size 8
bits: 0xff.

   When using type specifiers like 'uint<8>' or 'uint<16>' in a map, we
get unsigned values such as 0UB. It follows that we need other type
specifiers to map signed values.  These look like 'int<8>' and
'int<16>'.

   For example, let's map a signed 16-bit value from 'foo.o':

     (poke) .set obase 10
     (poke) int<16> @ 0#B
     28515H

Note how the suffix of the value is now 'H' and not 'UH'.  This means
that the value is signed!  But in this case it is still positive, so
let's try to get an actual negative value:

     (poke) var h = int<16> @ 0#B
     (poke) h - h - 1H
     -1H

3.8.3 Mixing Signed and Unsigned Integers
-----------------------------------------

Adding two signed integers gives you a signed integer:

     (poke) 1 + 2
     3

Likewise, adding two unsigned integers results in an unsigned integer:

     (poke) 1U + 2U
     3U

But, what happens if we mix signed and unsigned values in an expression?
Is the result signed, or unsigned?  Let's find out:

     (poke) 1U + 2
     3U

Looks like combining an unsigned value with a signed value gives us an
unsigned value.  This actually applies to all the operators that work on
integer values: multiplication, division, exponentiation, etc.

   What actually happens is that the signed operand is converted to an
unsigned value before executing the expression.  You can also convert
signed values into unsigned values (and vice-versa) using "cast
constructions":

     (poke) 2 as uint<32>
     2U

Therefore, the expression '1U + 2' is equivalent to '1U + 2 as
uint<32>':

     (poke) 1U + 2 as uint<32>
     3U

   You may be wondering: why not doing it the other way around?  Why not
converting the unsigned operand into a signed value and then operate?
The reason is that, given an integer of some particular size, the
positive range that you can store in it is bigger when interpreted as an
unsigned integer than when interpreted as a signed integer.  Therefore,
converting signed into unsigned before operating reduces the risk of
positive overflow.  This of course assumes that we, as users, will be
working with positive numbers more often than with negative numbers, but
that is a reasonable assumption to do, as it is often the case!

3.9 Weird Integers
==================

Up to this point we have been playing with integers that are built using
a whole number of bytes.  However, we have seen that the type specifier
for an integer has the form 'int<N>' or 'uint<N>' for signed and
unsigned variants, where 'N' is the width of the integer, in bits.  We
have used bit-sizes that are multiple of 8, which is the size of a byte.
So, why is this so?  Why is 'N' not measured in bytes instead?

   The reason is that poke is not limited to integers composed of a
whole number of bytes.  You can actually have integers having _any_
number of bits, between 1 and 64.  So yes, 'int<3>' is a type specifier
for signed 3-bit integers, and 'uint<17>' is a type specifier for
unsigned 17-bit integers.

   We call integers like this "weird integers".

   The vast majority of programming languages do not provide any support
for weird integers.  In the few cases they do, it is often in a very
limited and specific way, like bitmap fields in C structs.  Such
constructions are often vague, obscure, and often their semantics depend
on the specific implementation of the language, and/or the
characteristics of the system where you run your program.

   In poke, on the contrary, weird numbers are first class citizens, and
they don't differ in any way from "normal" integers composed of a whole
number of bytes.  Their interpretation is also well defined, and they
keep the same semantics regardless of the characteristics of the
computer on which poke is running.

3.9.1 Incomplete Bytes
----------------------

Let's consider first weird numbers that span for more than one byte.
For example, an unsigned integer of 12 bits.  Let's visualize the
written form of this number, i.e.  the sequence of its constituent bytes
as they appear in the underlying IO space:

       byte 0  |  byte 1
     +---------+----+----+
     |::::::::::::::|    |
     +---------+----+----+
     |   uint<12>   |

All right, the first byte is used in its entirely, but only half of the
second byte is used to conform the value of the number.  The other half
of the second byte has no influence of the value of the 12 bits number.

   Now, we talk about the "second half of the byte", but what do that
means exactly?  We know that bytes in memory and files (bytes in IO
spaces) are indivisible at the system level: bytes are read and written
one at a time, as little integers in the range '0..255'.  However, we
can create the useful fiction that each byte is composed by "bits",
which are the digits in the binary representation of the byte value.

   So, we can look at a byte as composed of a group of eight bits, like
this:

                byte
     +-------------------------+
     | b7 b6 b5 b4 b3 b2 b1 b0 |
     +-------------------------+

Note how we decided to number the bits in descending order from left to
right.  This is because these bits correspond to the base of the
polynomial equivalent to the binary value of the byte, i.e.  the value
of the byte is
'b7*2^7+b6*2^6+b5*2^5+b4*2^4+b3*2^3+b2*2^2+b1*2^1+b0*2^0'.  In other
words: at the bit level poke always uses a big endian interpretation,
and the bit that "comes first" in this imaginary stream of bits is the
most significant bit in the binary representation of the number.  Please
note that this is just a convention, set by the poke authors: the
opposite could have been chosen, but it would have been a bit confusing,
as we would have to picture binary numbers in reverse order!

   With this new way of looking at bytes, we can now visualize what we
mean exactly with the "first half" and "second half" of the trailing
byte, in our 12 bits unsigned number:

                byte 0         |           byte 1
     +-------------------------+-------------+-------------+
     | a7 a6 a5 a4 a3 a2 a1 a0   b7 b6 b5 b4 :             |
     +-------------------------+-------------+-------------+
     |                uint<12>               |

Thus the first half of 'byte 1' is the sequence of bits 'b7 b6 b5 b4'.
The second half, which is not pictured since it doesn't contribute to
the value of the number, would be 'b3 b2 b1 b0'.

   So what would be the value of the 12-bit integer?  Exactly like with
non-weird numbers, this depends on the current selected endianness,
which determines the ordering of bytes.

   If the current endianness is big, then 'byte 0' provides the most
significant bits of the result number, and the used portion of 'byte 1'
provides the least significant bits of the result number:

     0b a7 a6 a5 a4 a3 a2 a1 a0 b7 b6 b5 b4

However, if the current selected endianness is little, then the used
portion of 'byte 1' provides the most significant bits of the result
number, and 'byte 0' provides the least significant bits of the result
number:

     0b b7 b6 b5 b4 a7 a6 a5 a4 a3 a2 a1 a0

   Let's see this in action.  Let's take a look to the value of the
first two bytes in 'foo.o', in binary:

     (poke) .set obase 2
     (poke) byte @ 0#B
     0b01111111UB
     (poke) byte @ 1#B
     0b01000101UB

Looking at these bytes as sequences of bits, we have:

             byte @ 0#B       |        byte @ 1#B
     +-------------------------+-------------+-------------+
     |  0  1  1  1  1  1  1  1    0  1  0  0 :  0  1  0  1
     +-------------------------+-------------+-------------+
     |                uint<12>               |

Let's map our weird number at offset 0 bytes, using big endian:

     (poke) .set endian big
     (poke) uint<12> @ 0#B
     (uint<12>) 0b011111110100

That matches what we explained before: the most significant bits of the
unsigned 12 bits number come from the byte at offset 0, i.e.
'01111111', whereas the least significant bits come from the byte at
offset 1, i.e.  '0100'.

   Now let's map it using little endian:

     (poke) uint<12> @ 0#B
     (uint<12>) 0b010001111111

This time the most significant bits of the unsigned 12 bits number come
from the byte at offset 1, i.e.  '0100', whereas the least significant
bits come from the byte at offset 0, i.e.  '01111111'.

   An important thing to note is that non-weird numbers, i.e.  numbers
built with a whole number of bytes, are basically a particular case of
weird numbers where the last byte in the written form (in the IO space)
provides all its bits.  The rules are exactly the same in all cases,
which makes it easy to obtain predictable and natural results when
building integers using poke.

3.9.2 Quantum Bytenics
----------------------

The second kind of weird numbers are integers using less than 8 bits.
These "sub-byte" numbers do not use all the bits of their containing
byte.  Consider for example the written form of an unsigned integer of
size 5 bits:

         byte
     +-----+----+
     |:::::|    |
     +-----+----+
     uint<5>

Now let's view the byte as a sequence of bits:

                  byte
     +----------------+----------+
     | b7 b6 b5 b4 b3 |          |
     +----------------+----------+
     |    uint<5>     |

What is the value of this number?  Applying the general rules for
building integers from bytes, we can easily see that regardless of the
current endianness the value, in binary, is:

     0b b7 b6 b5 b4 b3

   Let's see this in poke:

     (poke) .set obase 2
     (poke) .set endian big
     (poke) byte @ 0#B
     0b01111111UB
     (poke) uint<5> @ 0#B
     (uint<5>) 0b01111
     (poke) .set endian little
     (poke) uint<5> @ 0#B
     (uint<5>) 0b01111

3.9.3 Signed Weird Numbers
--------------------------

In the section discussing negative integers, we saw how the difference
between a signed number and an unsigned number is basically a different
interpretation of the most significant byte.  Exactly the same applies
to weird numbers.

   Let's summon our unsigned 12-bit integer at the beginning of the file
'foo.o':

     (poke) .set endian big
     (poke) uint<12> @ 0#B
     (uint<12>) 0b011111110100

The most significant byte of the resulting value (not of its written
form) indicates that this number would be positive if we were mapping
the corresponding signed value.  Let's see:

     (poke) int<12> @ 0#B
     (int<12>) 0b010001111111
     (poke) .set obase 10
     (poke) int<12> @ 0#B
     (int<12>) 1151

   Let's make it a bit more interesting, and change the value of the
first byte in the file so we get a negative number:

     (poke) .set obase 2
     (poke) byte @ 0#B = 0b1111_1111
     (poke) int<12> @ 0#B
     (int<12>) 0b111111110100
     (poke) .set obase 10
     (poke) int<12> @ 0#B
     (int<12>) -12

Now, let's switch to little endian:

     (poke) .set endian little
     (poke) .set obase 2
     (poke) int<12> @ 0#B
     (int<12>) 0b010011111111
     (poke) .set obase 10
     (poke) int<12>  0#B
     (int<12>) 1279

3.10 Unaligned Integers
=======================

We have mentioned above that the data stored in computers, that we edit
with poke, is arranged as a sequence of bytes.  The entities we edit
with poke (that we call "IO devices") are presented to us as IO spaces.
Up to now, we have accessed this IO space in terms of bytes, in commands
like 'dump :from 32#B' and in expressions like '2UB + byte @ 0#B'.  We
said that mapped integers are built from bytes read from the IO space.

   However, the IO space that poke offers to us is actually a space of
bits, not a space of bytes, and the poke values are mapped on this space
of bits.  The following figure shows this:

     poke values      |        uint<16> @ 2#b        |
     -----------      |                               |
     IO space     |b|b|b|b|b|b|b|b|b|b|b|b|b|b|b|b|b|b|b|b|b|b|b|b|
     -----------  |               |               |               |
     IO device    |     byte0     |     byte1     |     byte2     |

The main consequence of this, that you can see in the figure above, is
that we can use offsets in mapping operations that are not "aligned to
bytes".  You can specify an offset in bits, instead of bytes, using the
'#b' suffix instead of '#B'.  Little 'b' means bits, and big 'B' means
bytes.

   Let's map an unaligned 16 bit unsigned integer in 'foo.o':

     (poke) dump :from 0#B :size 3#B
     76543210  0011 2233 4455 6677 8899 aabb ccdd eeff  0123456789ABCDEF
     00000000: 7f45 4c                                  .EL
     (poke) .set obase 2
     (poke) byte @ 0#B
     0b01111111UB
     (poke) byte @ 1#B
     0b01000101UB
     (poke) byte @ 2#B
     0b01001100UB
     (poke) .set endian big
     (poke) uint<16> @ 2#b
     0b1111110100010101UH

   Graphically:

     poke values      |        uint<16> @ 2#b        |
     -----------      |                               |
     IO space     |0|1|1|1|1|1|1|1|0|1|0|0|0|1|0|1|0|1|0|0|1|1|0|0|
     -----------  |               |               |               |
     IO device    |     0x7f      |      0x45     |      0x4c     |

   These three levels of abstractions make it very easy and natural to
work with unaligned data.  Imagine for example that you are poking
packages in a network protocol that is bit-oriented.  This means that
the packages will generally not be aligned to byte boundaries, but still
the payload stored in the packages contains integers of several sizes.
poke allows you to directly map these integers as if they were aligned
to byte boundaries, and work with them.

   However, when one tries to determine the correspondence between a
given poke value and the underlying bytes in the IO device, things can
get complicated.  This is particularly true when we map what we called
"weird numbers", i.e.  numbers with partial bytes.  As we saw, the rules
to build these numbers were expressed in terms of bytes.

   In order to ease the visualization of the process used to build
integer values (especially if they are weird numbers, i.e.  integers
with partial bytes) one can imagine an additional layer of "virtual
bytes" above the space of bits provided by the IO space.  Graphically:

     poke values       |        uint<16> @ 2#b        |
     -----------       |                               |
     Virtual bytes     | virt. byte1   |  virt. byte2  |
     -----------       |               |               |
     IO space      |0|1|1|1|1|1|1|1|0|1|0|0|0|1|0|1|0|1|0|0|1|1|0|0|
     -----------   |               |               |               |
     IO device     |     0x7f      |      0x45     |      0x4c     |

   It is very important to understand that the IO space is an
abstraction provided by poke.  The underlying file, or memory buffer, or
whatever, is actually a sequence of bytes; poke translates the
operations on integers, bits, bytes, etc into the corresponding byte
operations, which are far from trivial.  Fortunately, you can let poke
to do that dirty job for you, and abstract yourself from that
complexity.

3.11 Integers of Different Sizes
================================

When integer values of different sizes are passed to an arithmetic or
relational operator, the "smaller" operand gets converted into the size
of the "bigger" operand.  For example:

     (poke) 1H + 2
     3

The operands are of size 16-bit and 32-bit respectively, and the result
is a 32-bit integer.  This is equivalent to:

     (poke) 1H as int<32> + 2
     3

3.12 Offsets and Sizes
======================

Early in the design of what is becoming GNU poke I was struck by a
problem that, to my surprise, would prove not easy to fix in a
satisfactory way: would I make a byte-oriented program, or a
bit-oriented program?  Considering that the program in question was
nothing less than an editor for binary data, this was no petty dilemma.

   Since the very beginning I had a pretty clear idea of what I wanted
to achieve: a binary editor that would be capable of editing user
defined data structures, besides bytes and bits.  I also knew I needed
some sort of domain specific language to describe these structures and
operate on them.  How that language would look like, and what kind of
abstractions it would provide, however, was not clear to me.  Not at
all.

   So once I sketched an initial language design, barely something very
similar to C structs, I was determined to not continue with the poke
implementation until I had described as many as binary formats in my
language as possible.  That, I reckoned, was the only way to make sure
the implemented language would be expressive, complete and useful enough
to fulfil my requirements.

   The first formats I implemented using my immature little language
included ELF, FLV, MP3, BSON... of them describing structures based on
whole bytes.  Even when they try to be compact, it is always by packing
bit-fields in elements that are, invariably, sized as a multiple of
bytes.  Consequently, the language I was evolving became byte oriented
as well.  No doubt also influenced by my C inheritance, I would think of
bit-fields either as a sort of second class citizen, or as mere results
of shifting and masking.

   This worked well.  The language evolved to be able to express many
different aspects of these formats in a very nice way, like
variable-length data and holes in structures.  Consider the following
definition, which is *not* valid in today's Poke:

     type Data =
       struct
       {
         byte magic;
         byte count;
         byte dstart;

         byte[count] data @ dstart;
       };

   The data starts with a byte that is a magic number.  Then the size of
the data stored, in bytes, and then the data itself.  This data,
however, doesn't start right after 'dstart': it starts at 'dstart',
which is expressed as an offset, in bytes, since the beginning of the
Data.  I conceived struct field labels to be any expression evaluating
to an integer, which would be..., bytes obviously.

   Then, one day, it was the turn for IETF RFC1951, which is the
specification of the DEFLATE algorithm and associated file format.  Oh
dear.  Near the beginning of the spec document it can be read:

     This document does not address the issue of the order in which bits
     of a byte are transmitted on a bit-sequential medium, since the
     final data format described here is byte- rather than bit-oriented.
     However, we describe the compressed block format in below, as a
     sequence of data elements of various bit lengths, not a sequence of
     bytes.

   Then it goes on describing rules to pack the DEFLATE elements into
bytes.  I was appalled, and certainly sort of deflated as well.  The
purpose of my program was precisely to edit binary in terms of the data
elements described by a format.  And in this case, these data elements
came in all sort of bit lengths and alignments.  This can be seen in the
following RFC1951 excerpt, that describes the header of a compressed
block:

     Each block of compressed data begins with 3 header bits containing
     the following data:
          first bit       BFINAL
          next 2 bits     BTYPE

     Note that the header bits do not necessarily begin on a byte
     boundary, since a block does not necessarily occupy an integral
     number of bytes.

   At this point I understood that my little language on the works would
never be capable to describe the DEFLATE structures naturally: C-like
bit-fields, masking and shifting, all based on byte-oriented containers
and boundaries, would never provide the slickness I wanted for my
editor.  I mean, just use C and get done with it.

   This pissed me off.  Undoubtedly other formats and protocols would be
impacted in a similar way.  Even when most formats are byte oriented,
what am I supposed to tell to the people hacking bit-oriented stuff?
"Sorry pal, this is not supported, this program is not for you"?  No
way, I thought, not on my watch.

   The obvious solution for the problem, is to be general.  In this
case, to express every offset and every memory size in bits instead of
bytes.  While this obviously gives the language maximum expressiveness
power, and is ideal for expressing the few bit-oriented formats, it has
the disadvantage of being very inconvenient for most situations.

   To see how annoying this is, let's revisit the little Data element we
saw above.  In a bit-oriented description language, we would need to
write something like:

     type BitData =
       struct
       {
         byte magic;
         byte count;
         byte dstart;

         byte[count] data @ dstart * 8;
       };

   Yeah... exactly.  The <*> and <8> keys in the keyboards of the poke
users would wear out very fast, not to mention their patience as well.
Also, should I provide both 'sizeof' and 'bitsizeof' operators?  Nasty.

   I am very fond of the maxim "Never write a program you would never
use yourself"(1), so I resigned myself to make GNU poke byte oriented,
and to provide as many facilities for operating on bit-fields as
possible.

Fortunately, I have smart friends...

   During one of the Rabbit Herd's Hacking Weekends(2) I shared my
frustration and struggle with the other rabbits, and we came to realize
that offsets and data sizes in Poke should not be pure magnitudes or
mere integer values: they should be united.  They should have units.

   It makes full sense when you come to think about it.  For a program
like poke, it is only natural to talk about different memory units, like
bits, bytes, kilobytes, megabits, and so on.  Bits and bytes are just
two common units.  Apart from allowing me to express values in different
units, this approach also has other benefits as we will see shortly.

   I'm really grateful to Bruno Haible, Luca Saiu and Nacho Gonzalez for
putting me on the right track.

   ---------- Footnotes ----------

   (1) Actually it is Lord Vetinari's "Never build a dungeon you can't
get out of."  but the point is the same.

   (2) <http://www.jemarch.net/rhhw>

3.13 Buffers as IO Spaces
=========================

We have mentioned already that files are not the only entities that can
be edited using poke.  Remember the dot-command '.file' that opened a
file as an IO space?

     (poke) .file foo.o
     The current IOS is now `./foo.o'.
     (poke) .info ios
       Id	Type	Mode	Bias		Size		Name
     * #0	FILE	rw	0x00000000#B	0x000004c8#B	./foo.o

Memory buffers can be created using a similar dot-command, '.mem':

     (poke) .mem foo
     The current IOS is now `*foo*'.
     (poke) .info ios
       Id	Type	Mode	Bias		Size		Name
     * #1	MEMORY		0x00000000#B	0x00001000#B    *foo*
       #0	FILE	rw	0x00000000#B	0x000004c8#B    ./foo.o

   Note how the name of the buffer is built by prepending and appending
asterisks.  Therefore, the name of the buffer created by the command
'.mem foo' is '*foo*'.  Note also that the new buffer is created with a
default size of 0x1000 bytes, or 4096 bytes.  The contents of the buffer
are zeroed:

     (poke) dump
     76543210  0011 2233 4455 6677 8899 aabb ccdd eeff  0123456789ABCDEF
     00000000: 0000 0000 0000 0000 0000 0000 0000 0000  ................
     00000010: 0000 0000 0000 0000 0000 0000 0000 0000  ................
     00000020: 0000 0000 0000 0000 0000 0000 0000 0000  ................
     00000030: 0000 0000 0000 0000 0000 0000 0000 0000  ................
     00000040: 0000 0000 0000 0000 0000 0000 0000 0000  ................
     00000050: 0000 0000 0000 0000 0000 0000 0000 0000  ................
     00000060: 0000 0000 0000 0000 0000 0000 0000 0000  ................
     00000070: 0000 0000 0000 0000 0000 0000 0000 0000  ................

   Memory buffer IO spaces grow automatically when a value is mapped
beyond their current size.  This is very useful when populating newly
created buffers.  However, for security reasons, there is a limit: the
IO spaces are only allow to grow 4096 bytes at a time.

   When it comes to map values, there is absolutely no difference
between an IO space backed by a file and an IO space backed by a memory
buffer.  Exactly the same rules apply in both cases.

3.14 Copying Bytes
==================

Memory buffer IO spaces are cheap, and they are often used as "scratch"
areas.

   Suppose for example we want to experiment with the ELF header of
'foo.o'.  We could open it in poke:

     (poke) .file foo.o
     The current IOS is now `./foo.o'.

The header of an ELF file comprises the first 64 bytes in the file:

     (poke) dump :size 64#B
     76543210  0011 2233 4455 6677 8899 aabb ccdd eeff  0123456789ABCDEF
     00000000: 7f45 4c46 0201 0100 0000 0000 0000 0000  .ELF............
     00000010: 0100 3e00 0100 0000 0000 0000 0000 0000  ..>.............
     00000020: 0000 0000 0000 0000 0802 0000 0000 0000  ................
     00000030: 0000 0000 4000 0000 0000 4000 0b00 0a00  ..............

   We know that as soon as we poke something on an IO space, the
underlying file is immediately modified.  So if we start playing with
'foo.o''s header, we may corrupt the file.  We could of course make a
copy of 'foo.o' and work on the copy, but it is much better to create a
memory IO space and copy the ELF header there:

     (poke) .mem scratch
     The current IOS is now `*scratch*'.
     (poke) copy :from_ios 0 :from 0#B :size 64#B
     (poke) dump :size 64#B
     76543210  0011 2233 4455 6677 8899 aabb ccdd eeff  0123456789ABCDEF
     00000000: 7f45 4c46 0201 0100 0000 0000 0000 0000  .ELF............
     00000010: 0100 3e00 0100 0000 0000 0000 0000 0000  ..>.............
     00000020: 0000 0000 0000 0000 0802 0000 0000 0000  ................
     00000030: 0000 0000 4000 0000 0000 4000 0b00 0a00  ..............

The command 'copy' above copies 64 bytes starting at byte 0 from the IO
with id 0 (the file 'foo.o') to the current IO space (the buffer
'*scratch*').

   Once we are done working with the copy of the ELF header, and
satisfied, we can copy it back to the file and close the memory IO
space:

     (poke) copy :from 0#B :size 64#B :to_ios 0
     (poke) .close
     The current IOS is now `./foo.o'.

Note how the command arguments ':from_ios' and ':to_ios' are assumed to
be the current IO space if they are not explicitly specified in the
command invocation.  For detailed information on the 'copy' command, see
*note copy::.

3.15 Saving Buffers in Files
============================

Another useful command when working with buffer IO spaces (and in
general, with any IO space) is 'save'.  Let's say we want to save a copy
of the header of an ELF file in another file.  We could do it the
pokeish way:

     $ poke foo.o
     [...]
     (poke) save :from 0#B :size 64#B :file "header.dat"

The command above saves the first 64 bytes in the current IO space
(which is backed by the file 'foo.o') into a new file 'header.dat' in
the current working directory.

   Let's now consider again the scenario where we are using a memory IO
space as a scratch area.  It is late in the night and we are tired, so
we would like to save the contents of the scratch buffer to a file, so
we can continue our work tomorrow.  This is how we would do that:

     (poke) .info ios
       Id	Type	Mode	Bias		Size		Name
     * #1	MEMORY		0x00000000#B	0x00001000#B    *scratch*
       #0	FILE	rw	0x00000000#B	0x000f4241#B    ./foo.o
     (poke) save :from 0#B :size iosize (1) :file "scratch.dat"

Here we used the built-in function 'iosize', that given an IO space
identifier returns its size.

3.16 Character Sets
===================

Computers understand text as a sequence of "codes", which are numbers
identifying some particular "character".  A character can represent
things like letters, digits, ideograms, word separators, religious
symbols, etc.  Collections of character codes are called "character
sets".

   Some character sets try to cover one or a few similar written
languages.  This is the case of ASCII and ISO Latin-1, for example.
These character sets are small, i.e.  just a few hundred codes.

   Other character sets are much more ambitious.  This is the case of
Unicode, that tries to cover the entire totality of human languages in
the globe, including the fictitious ones, like klingon.  Unicode is a
really big character set.

   In order to store character codes in a computer's memory, or a file,
we need to "encode" each character code in one or more bytes.  The
number of bytes needed to encode a given character code depends on the
range of codes in the containing set.

   ASCII, for example, defines 128 character codes: a single byte is
enough to encode every possible ASCII character.  It is very easy to
encode ASCII.

   Unicode, on the contrary, defines many thousand of character codes,
and has room for many more: we would need 31 bits in order to encode any
conceivable Unicode character code.  However, it would be wasteful to
use that many bits per character: most used character codes tend to be
in lower regions of the code space.  For example, the code corresponding
to the Latin letter ''a'' is a fairly small number, whereas the codes
corresponding to the Klingon alphabet are really big numbers.
Consequently, some systems opt to just encode a subset of Unicode, like
the first 16 bits of the Unicode space, which is called the Basic
Multilingual Plane, and contains all the characters that most people
will ever need.  There are also variable-length encodings of Unicode,
that try to use as less bytes as possible to encode any given code.  A
particularly clever encoding of Unicode, designed by Rob Pike, is
backwards compatible with the ASCII encoding, i.e.  it encodes all the
ASCII codes in one byte each, and the values of these byte are the same
than in ASCII. This clever encoding is called UTF-8.

3.17 From Bytes to Characters
=============================

3.17.1 Character Literals
-------------------------

poke has built-in support for ASCII, and its simple encoding: each ASCII
code is encoded using one byte.  Let's see:

     (poke) 'a'
     0x61UB

We presented poke with the character 'a', and it answered with its
corresponding code in the ASCII character set, which is 0x61.  In fact,
'a' and 0x61UB are just two ways to write exactly the same byte value in
poke:

     (poke) 'a' == 0x61UB
     1
     (poke) 'a' + 1
     0x62U

   In order to make this more explicit, poke provides yet another
synonym for the type specifier 'uint<8>': 'char'.

3.17.2 Classifying Characters
-----------------------------

When working with characters it is very useful to have some acquaintance
of the ASCII character set, which is designed in a very clever way with
the goal of easing certain code calculations.  See *note Table of ASCII
Codes:: for a table of ASCII codes in different numeration bases.

   Consider for example the problem of determining whether a byte we map
from an IO space is a digit.  Looking at the ASCI table, we observe that
digits all have consecutive codes, so we can do:

     (poke) var b = byte @ 23#B
     (poke) b >= '0' && b <= '9'
     1

   Now that we know that 'b' is a digit, how could we calculate its
digit value?  If we look at the ASCII table again, we will find that the
character codes for digits are not only consecutive: they are also
ordered in ascending order '0', '1', ... Therefore, we can do:

     (poke) b
     0x37UB
     (poke) b - '0'
     7UB

'b' contains the ASCII code 0x37UB, which corresponds to the character
'7', which is a digit.

   How would we check whether 'b' is a letter?  Looking at the ASCII
table, we find that lower-case letters are encoded consecutively, and
the same applies to upper-case letters.  This leads to repeat the trick
again:

     (poke) (b >= 'a' && b <= 'z') || (b >= 'A' && b <= 'Z')
     0

3.17.3 Non-printable Characters
-------------------------------

Not all ASCII code are printable using the glyph that are usually
supported in terminals.  If you look at the table in *note Table of
ASCII Codes::, you will find codes for characters described as "start of
text", "vertical tab", and so on.

   These character codes, which are commonly known as "non-printable
characters", can be represented in poke using its octal code:

     (poke) '\002'
     0x2UB

This is of course no different than using '2UB' directly, but in some
contexts the "character like" notation may be desirable, to stress the
fact that the byte value is used as an ASCII character.

   Some of the non-printable characters also have alternative notations.
This includes new-line and horizontal tab:

     (poke) '\n'
     0xaUB
     (poke) '\t'
     0x9UB

These '\' constructions in character literals are called "escape
sequences".  See *note Characters:: for a complete list of allowed
escapes in character literals.

3.18 ASCII Strings
==================

3.18.1 String Values
--------------------

Now that we know how to manipulate ASCII codes in poke, we may wonder
how can we combine them to conform words or, more generally, "strings"
of ASCII characters.

   GNU poke has support for native ASCII string values.  The characters
conforming the string are written between '"' and '"' characters, like
in:

     (poke) "word"
     "word"

Note, and this is important, that string values are as atomic as integer
values: they are not really composite values.  The fact that '"word"'
contains an 'r' at position 3 is like the fact that the value '123'
contains a digit '2' at position 2.

   Like in character literals, poke strings support several escape
sequences that help to denote non-printed characters, such as new lines
and horizontal tabs.  *Note String Literals::.

3.18.2 Poking Strings
---------------------

Let's start with a fresh memory buffer IOS '*scratch*':

     (poke) .mem scratch
     The current IOS is now `*scratch*'.
     (poke) dump :size 48#B
     76543210  0011 2233 4455 6677 8899 aabb ccdd eeff  0123456789ABCDEF
     00000000: 0000 0000 0000 0000 0000 0000 0000 0000  ................
     00000010: 0000 0000 0000 0000 0000 0000 0000 0000  ................
     00000020: 0000 0000 0000 0000 0000 0000 0000 0000  ................

If we wanted to, somehow, store the word 'word' in this IO space,
encoded in ASCII, we could proceed as:

     (poke) char @ 0x12#B = 'w'
     (poke) char @ 0x13#B = 'o'
     (poke) char @ 0x14#B = 'r'
     (poke) char @ 0x15#B = 'd'
     (poke) dump :size 48#B
     76543210  0011 2233 4455 6677 8899 aabb ccdd eeff  0123456789ABCDEF
     00000000: 0000 0000 0000 0000 0000 0000 0000 0000  ................
     00000010: 0000 776f 7264 0000 0000 0000 0000 0000  ..word..........
     00000020: 0000 0000 0000 0000 0000 0000 0000 0000  ................

This worked.  The ASCII part of the 'dump' output, which interprets the
bytes as ASCII, clearly shows the word 'word' at the offset where we
poked the character values.  However, we can do better: string values
can be mapped themselves.

   String values use the type specifier 'string'.  As any other kind of
value in poke, they can be mapped from an IO space:

     (poke) string @ 0x12#B
     "word"

Clearly that is the string resulting from the concatenation of the
character values that we poked before.

   The question now is: how did poke know that the last character of the
string was the 'd' at offset 0x15#B? The fact the character code 0 (also
known as the "NULL character") at offset 0x16#B is non-printable,
doesn't imply it is not part of the ASCII character set.  Clearly, we
have to pick an ASCII code and reserve it to mark the end of strings.
Like the C programming language, and countless formats and systems do,
poke uses the NULL character to delimit strings.

   Now consider this:

     (poke) "word"'length
     4UL
     (poke) "word"'size
     40UL#b

Using the 'length' and 'size' attributes, poke tells us that the length
of the string '"word"' is 4 characters, but the size of the string value
is 40 bits, or 5 bytes.  Why this discrepancy?  The 'size' value
attribute tells how much storage space a given value required once
mapped to an IO space, and in the case of strings it should count the
space occupied by the terminating NULL character.

   Poking string values on the IO space is as straightforward as poking
integers:

     (poke) string @ 0x22#B = "WORD"
     (poke) dump :size 48#B
     76543210  0011 2233 4455 6677 8899 aabb ccdd eeff  0123456789ABCDEF
     00000000: 0000 0000 0000 0000 0000 0000 0000 0000  ................
     00000010: 0000 776f 7264 0000 0000 0000 0000 0000  ..word..........
     00000020: 0000 574f 5244 0000 0000 0000 0000 0000  ..WORD..........

3.18.3 From Characters to Strings
---------------------------------

Strings can be concatenated using the string-concatenation '+'
operators:

     (poke) "foo" + "bar"
     "foobar"

The string resulting from the operation above has length 6 characters,
and size 7 bytes.  The terminating NULL character of '"foo"' is lost in
the operation.  This is easily seen:

     (poke) "foo"'size + "bar"'size
     0x40UL#b
     (poke) ("foo" + "bar")'size
     0x38UL#b

   The string concatenation operator requires two strings.  Therefore,
if we wanted to append a character to a string, we would get an error:

     (poke) "Putin" + 'a'
     <stdin>:1:1: error: invalid operands in expression
     "Putin" + 'a';
     ^~~~~~~~~~~~~

It is possible to transform a character value (i.e.  a byte value) into
a string composed of that character using a cast:

     (poke) 'a' as string
     "a"

Using that cast, we can now append:

     (poke) "Putin" + 'a' as string
     "Putina"

3.19 From Strings to Characters
===============================

Despite being atomic values, poke strings can be indexed in order to
retrieve individual characters:

     (poke) "word"[2]
     0x72UB

Note how the indexing is zero-based, i.e.  the first position in the
string is referred as '[0]', the second position with '[1]', and so on.

   If you specify a negative index, or an index that is too big, you
will get an error:

     (poke) "word"[-1]
     <stdin>:1:8: error: index is out of bounds of string
     "word"[-1];
            ^
     (poke) "word"[5]
     <stdin>:1:8: error: index is out of bounds of string
     "word"[5];
            ^

3.20 Strings are not Arrays
===========================

Arrays are collections of homogeneous data organized in a sequence.  We
have already seen (briefly) arrays of bytes, which use the type
specifier 'byte[]'.

   Similarly, it is possible to arrange characters (which are basically
little numbers) in an array, like in:

     (poke) ['a','b','c']
     [0x61UB,0x62UB,0x63UB]

However, the array above is _not_ equivalent to the string '"abc"'.  The
later is a simple value, whereas an array is a composite value, and also
is implicitly terminated with a NULL character, i.e.  a 0 byte.

   The Poke standard library provides a couple of utility functions to
convert between string values and character arrays: 'catos' and 'stoca'.

   'catos' gets an array of characters and returns an equivalent string.
For example:

     (poke) catos (['a','b','c'])
     "abc"

'stoca' gets a string and an array and sets the element of the array to
the characters composing the string.  For example:

     (poke) var a = char[3]()
     (poke) stoca ("abc", a)
     (poke) a
     [0x61UB,0x62UB,0x63UB]

4 Structuring Data
******************

In the previous chapter we learned how to manipulate pre-defined
entities like bytes, integers and strings.  One of the big points of
poke, however, is that it allows you to define your own data
abstractions, and to operate in term of them.  This is achieved by
defining data structures.

4.1 The SBM Format
==================

4.1.1 Images as Maps of Pixels
------------------------------

There are two main ways to store two-dimensional images in computers.

   One is to explicitly store the different "pixels" that compose the
image.  In these "bitmaps", the pixels are arranged sequentially and
implicitly organized into "lines".  A header typically provides
information to determine how many pixels fit in each line:

      <--- line_width  --->
     | pixel | pixel | ... | pixel | pixel | ... | ...
     |        line 1       |         line 2      |

   Several properties have to be encoded for each pixel, depending on
the sophistication of the image: for monochrome images each pixel can be
just either switched on or off, so a single bit could be used to encode
each pixel (this is the origin of the term "bitmap").  When color is
added, a bit is no longer sufficient: the color of the pixel shall be
encoded in some way, typically using a color schema such as RGB, that
requires triplet of little integers to be encoded.  If transparency is
to be supported, the degree of transparency of the pixel shall also be
encoded somehow.

   The other way to store an image is to store a functional description
of the "painted" parts of the image.  This functional description
usually contains instructions like "paint a line of thickness 1 and
color red from the coordinate 10,20 to coordinate 10,40".  Once executed
with certain parameters (like the desired resolution) the functional
description generates a bitmap.  Image formats using this approach are
commonly known as "vectorial formats".

   When it comes to bitmaps, there are a plethora of different formats
out there (bmp, jpg, png) competing in terms of capabilities such as
higher color depths, better resolution, support for transparency (alpha
channels), higher compression level, and the like.  These capabilities
greatly complicate these formats, but ultimately any bitmap can be
reduced to a sequence of pixels, which can be further structured in
terms of "lines".

   We don't know enough poke yet to handle the complications of these
real-life bitmap formats, so in subsequent sections we introduce a very
simple format for bitmaps, the Stupid BitMap format, or simply SBM, that
we will use for learning purposes.

   But please do not feel disappointed: later in this book, when we
become more proficient pokers, we will be messing with these shiny
complex formats as well :)

4.1.2 SBM header
----------------

A SBM file starts with a header that contains a "magic number" composed
of the three bytes 'S' (0x53UB), 'B' (0x42UB) and 'M' (0x4dUB). The next
two bytes indicate the number of pixels per line, and the number of
lines, respectively.  In summary:

                     SBM header
     +-------+-------+-------+-------+-------+
     |  'S'  |  'B'  |  'M'  |  ppl  | lines |
     +-------+-------+-------+-------+-------+
       byte0   byte1   byte2   byte3   byte4

Here 'ppl' stands for pixels-per-line.  The encoding of these fields
implies that the bigger image that can be encoded in SBM has dimensions
255x255.

4.1.3 SBM data
--------------

Albeit stupid, SBM is a colorful format.  It supports more than a
million colors, encoding each color in what is known as "RGB24".  In
RGB24, each color is encoded using three little integers, specifying the
amount of red, green and blue that, added together, compose the color.

   The name RGB24 comes from the fact that each color is encoded using
three bytes, or 24 bits.  Therefore, each pixel in a SBM image is
encoded using three consecutive bytes:

             SBM pixel
     +-------+-------+-------+
     |  Red  | Green | Blue  |
     +-------+-------+-------+
       byte0   byte1   byte2

Each byte in this encoding determines the amount of the base color (red,
green or blue) that compose the color.  We will talk more about these
components later.

4.2 Poking a SBM Image
======================

4.2.1 P is for poke
-------------------

Let's compose our first SBM image, using poke.  The image we want to
encode is the very simple rendering of the letter 'P' shown in the
figure below.

       | 0 | 1 | 2 | 3 | 4 |
       +---+---+---+---+---+
     0 |   | * | * |   |   |
     1 |   | * |   | * |   |
     2 |   | * |   | * |   |
     3 |   | * | * |   |   |
     4 |   | * |   |   |   |
     5 |   | * |   |   |   |
     6 |   | * |   |   |   |

The image has seven lines, and there are five pixels per line, i.e.  the
dimension of the image in pixels is 5x7.  Also, the pixels denoted by
asterisks are red, whereas the pixels denoted with empty spaces are
white.  In other words, our image uses a red foreground over a white
background.  The "painted" pixels are called foreground pixels, the non
painted pixels are called background pixels.

4.2.2 Preparing the Canvas
--------------------------

The first thing we need is some suitable IO space where to encode the
image.  Let's fire up poke and create a memory buffer:

     $ poke
     [...]
     (poke) .mem image
     The current IOS is now `*image*'.
     (poke) dump
     76543210  0011 2233 4455 6677 8899 aabb ccdd eeff  0123456789ABCDEF
     00000000: 0000 0000 0000 0000 0000 0000 0000 0000  ................
     00000010: 0000 0000 0000 0000 0000 0000 0000 0000  ................
     00000020: 0000 0000 0000 0000 0000 0000 0000 0000  ................
     00000030: 0000 0000 0000 0000 0000 0000 0000 0000  ................
     00000040: 0000 0000 0000 0000 0000 0000 0000 0000  ................
     00000050: 0000 0000 0000 0000 0000 0000 0000 0000  ................
     00000060: 0000 0000 0000 0000 0000 0000 0000 0000  ................
     00000070: 0000 0000 0000 0000 0000 0000 0000 0000  ................

Freshly created memory IO spaces are 4096 bytes long, and that's big
enough for our little image.  If we wanted to work with more data,
remember that memory IO spaces will grow automagically when poked past
their size.

4.2.3 Poking the Header
-----------------------

The first three bytes of the header of a SBM file contains the magic
number that identifies the file as a SBM bitmap.  We can poke these
bytes very easily:

     (poke) byte @ 0#B = 'S'
     (poke) byte @ 1#B = 'B'
     (poke) byte @ 2#B = 'M'

The next couple of bytes encode the dimensions of the bitmap, in this
case 5x7:

     (poke) byte @ 3#B = 5
     (poke) byte @ 4#B = 7

There is something worth noting in this last mapping.  Even tough we
were poking bytes (passing the 'byte' type specifier to the map
operators) we specified the 32-bit signed integers '5' and '7' instead
of '5UB' and '7UB'.  When poke finds a situation like this, where
certain kind of integers are expected but other kind are provided, it
converts the value from the provided type to the expected type.  This
conversion may result in truncation (think about converting, say 0xfff
to an unsigned byte, whose maximum possible value is 0xff) but certainly
not in the case at hands.

   The final header looks like:

     (poke) dump :size 16#B
     76543210  0011 2233 4455 6677 8899 aabb ccdd eeff  0123456789ABCDEF
     00000000: 5342 4d05 0800 0000 0000 0000 0000 0000  SBM.............

4.2.4 Poking the Pixels
-----------------------

Now that we have written a SBM header, we have to encode the sequence of
pixels composing the image.

   Recall that every pixel is encoded using three bytes, that conform a
RGB24 color.  We have two kinds of pixels in our image: white pixels,
and red pixels.  In RGB24 white is encoded as '(255,255,255)'.  Pure red
is encoded as '(255,0,0)', but to make things more interesting we will
be using a nicer tomato-like red '(255,99,71)'.

   Therefore, poking a white pixel at some offset OFFSET would involve
the following operations:

     (poke) byte @ OFFSET = 255
     (poke) byte @ OFFSET+1#B = 255
     (poke) byte @ OFFSET+2#B = 255

Likewise, the operations to poke a tomato pixel would look like:

     (poke) byte @ OFFSET = 255
     (poke) byte @ OFFSET+1#B = 99
     (poke) byte @ OFFSET+2#B = 71

To ease things a bit, we can define variables with the color components
for both foreground and background pixels:

     (poke) var bg1 = 255
     (poke) var bg2 = 255
     (poke) var bg3 = 255
     (poke) var fg1 = 255
     (poke) var fg2 = 99
     (poke) var fg3 = 71

Then to poke a foreground pixel would involve doing:

     (poke) byte @ OFFSET = fg1
     (poke) byte @ OFFSET+1#B = fg2
     (poke) byte @ OFFSET+2#B = fg3

   At this point, you may feel that the perspective of mapping the
pixels of our image is not very appealing, considering we have 5x7 = 35
pixels in our image.  We will need to poke 35 * 3 = 105 bytes.  We may
feel tempted to, somehow, use a bigger integer to "encapsulate" the
bytes.  Using the bit-concatenation operator, we could do something
like:

     (poke) var bg = 255UB::255UB::255UB
     (poke) var fg = 255UB::99UB::71UB
     (poke) bg
     (uint<24>) 0xffffff
     (poke) fg
     (uint<24>) 0xff6347

This encodes each color with a 24-bit unsigned integer.  When looking at
the hexadecimal values of 'bg' and 'fg' above, note that 0xff = 255,
0x63 = 99 and 0x47 = 71.  Each byte seems to be in the right position in
the 24-bit containing number.  Now, poking a pixel at some given offset
should be as easy as issuing just one map operation, right?  Let's see,
using some arbitrary offset 10#B:

     (poke) uint<24> @ 10#B = fg
     (poke) dump :from 10#B :size 4#B
     76543210  0011 2233 4455 6677 8899 aabb ccdd eeff  0123456789ABCDEF
     0000000a: 4763 ff00                                Gc..

If your current endianness is little (i.e.  you are running on a x86
system or similar) you will get the dump above.  The bytes are reversed,
and consequently the resulting pixel has the wrong color.  Our little
trick didn't work :(

   So are we doomed to poke three bytes for each pixel we want to poke
in our image?  No, not really.  The Poke language provides a
construction oriented to alleviate cases like this, where several
similar elements are to be "encapsulated" in a container.  These
constructions are called "arrays".

   Using array values, we can define the foreground and background
colors like this:

     (poke) var bga = [255UB, 255UB, 255UB]
     (poke) var fga = [255UB, 99UB, 71UB]

All the elements on an array should be of the same kind, i.e.  of the
same type.  Therefore, this is not allowed:

     (poke) [1,"foo"]
     <stdin>:1:1: error: array initializers should be of the same type
     [1,"foo"];
     ^~~~~~~~~

   Given an array value, it is possible to query for the number of
values contained in it (called "elements") by using the ''length' value
attribute.  For example:

     (poke) bga'length
     3UL

Tells us that the array value stored in the variable 'bga' has three
elements.

   How can we poke an array value?  We know that the map operator
accepts two operands: a type specifier and the value to map.  The type
specifier of an array of three bytes is denoted as 'byte[3]'.
Therefore, we can again try to poke a foreground pixel at offset 10#B,
this time using 'fga':

     (poke) byte[3] @ 10#B = fga
     (poke) dump :from 10#B :size 4#B
     76543210  0011 2233 4455 6677 8899 aabb ccdd eeff  0123456789ABCDEF
     0000000a: ff63 4700                                 .cG.

   This time, the bytes were written in the right order.  This is
because array elements are always written using their "written"
ordering, with no mind to endianness.  We can also map a pixel from a
given offset:

     (poke) byte[3] @ 10#B
     [255UB,99UB,71UB]

4.2.5 Poking Lines
------------------

At this point, we could encode the 40 pixels composing the image, by
issuing the same number of pokes of 'byte[3]' arrays.  However, we can
simplify the task even further.

   Our pixels are arrays of bytes, denoted by the type specifier
'byte[3]'.  Similarly, we could conceive arrays of 32-bit signed
integers, denoted by 'int[3]', or arrays of bits, denoted by
'uint<1>[3]'.  But, is it possible to have arrays of other arrays?  Yes,
it is:

     (poke) [[1,2],[3,4]]

The value above is an array of two arrays of two integers each.  If we
wanted to map such an array, what would be the type specifier we would
need to use?  It would be 'int[2][2]', which should be read from
right-to-left as "array of two arrays of two integers".  Let's map one
from an arbitrary offset in our IO space:

     (poke) int[2][2] @ 100#B
     [[0,0],[0,0]]

   Consider again the sequence of pixels composing the image.  Using the
information we have in the SBM header, we can group the pixels in the
sequence into "lines".  In our example image, each line contains 5
pixels.  It would be natural to express each line as a sequence of
pixels.  The first line in our image would be:

     (poke) var l0 = [bga,fga,fga,bga,bga]
     (poke) l0
     [[255UB,255UB,255UB],[255UB,99UB,71UB],...]

Let's complete the image lines:

     (poke) var l0 = [bga,fga,bga,fga,bga]
     (poke) var l1 = [bga,fga,bga,fga,bga]
     (poke) var l2 = [bga,fga,fga,bga,bga]
     (poke) var l3 = [bga,fga,bga,bga,bga]
     (poke) var l4 = l3
     (poke) var l5 = l4

Note how we exploited the fact that the three last lines of our image
are identical, to avoid to write the same array thrice.  Array values
can be assigned, and in general manipulated, like any other kind of
value, such as integers or strings.

   At this point, we could poke the pixels line-by-line.  What would be
the type specifier for a line?  A line is an array of five arrays of 3
bytes each, so the type specifier would be 'byte[3][5]'.  Let's do that:

     (poke) byte[3][5] @ 5#B = l0
     (poke) byte[3][5] @ 10#B = l1
     (poke) byte[3][5] @ 15#B = l2
     (poke) byte[3][5] @ 20#B = l3
     (poke) byte[3][5] @ 25#B = l4
     (poke) byte[3][5] @ 30#B = l5
     (poke) byte[3][5] @ 35#B = l6

Not bad, we went from poking 105 bytes in the IO space to poking six
lines.  But we can still do better...

4.2.6 Poking Images
-------------------

When we poked the lines at the end of the previous section, we had to
increase the offset in every map operation.  This is inconvenient.

   In the same way that a sequence of bytes can be abstracted in a line,
a sequence of lines can be abstracted in an image.  It follows that we
can look at the image data as an array of lines.  But lines are
themselves arrays of arrays... no matter, there is no limit on the
number of arrays-of levels that you can nest.

   So, let's define our image as an array of the lines defined above:

     (poke) var image_data = [l0,l1,l2,l3,l4,l5]
     (poke) image_data
     [[[255UB,255UB,255UB],[255UB,99UB,71UB],[255UB,99UB,71UB]...]...]

What would be the type specifier for an image?  It would be an array of
seven arrays of five arrays of three bytes each, in other words
'byte[3][5][7]'.  Let's poke the pixels:

     (poke) byte[3][5][6] @ 5#B = image_data

   This is an example of how abstraction can simplify the handling of
binary data: we switched from manipulating bytes to manipulate higher
abstractions such as colors, lines and images.  We achieved that by
structuring the data in a way that reflects these abstractions.  That's
the way of the Poker.

4.2.7 Saving the Image
----------------------

Now that we have completed the SBM image in our buffer '*image*', it is
time to save it to disk.  For that, we can use the 'save' command we are
already familiar with.

   We know that the SBM image starts at offset 0#B, but what is the size
of its entire binary representation?  The header is easy: it spans for 5
bytes.  The size of the sequence of pixels can be derived from the
pixels per line byte, and the number of lines byte.  We know that each
pixel occupies 3 bytes, so calculating...

     (poke) var ppl = byte @ 3#B
     (poke) var lines = byte @ 4#B
     (poke) save :from 0#B :size 5#B + ppl#B * lines#B :file "p.sbm"

Note how we expressed "ppl bytes" as 'ppl#B', and "lines bytes" as
'lines#B'.  This is the same than expressing "10 bytes" as '10#B'.  We
will talk more about these united values later.

   There is another way of getting the size of the stream of pixels.
Recall that we have the entire set of pixels, structured as lines,
stored in the variable 'image_data'.  Given an array, it is possible to
query for its size using the ''size' "attribute":

     (poke) .set obase 10
     (poke) [1,2,3]'size
     96UL#b

The above indicates that the size of the array of the three integers 1,
2 and 3 is 96 bits.  Using that attribute, we can also obtain the size
of the pixels in the image:

     (poke) image_data'size
     720UL#b

And we can use it in the save command:

     (poke) save :from 0#B :size 5#B + image_data'size :file "p.sbm"

   Using either strategy, at this point a file named 'p.sbm' should have
been written in the current working directory, containing our "P is for
poke" image.  Keep that file around, because we will be poking it
further!

4.3 Modifying SBM Images
========================

4.3.1 Reading a SBM File
------------------------

Let's open with poke the cute image we created in the last section,
'p.sbm':

     $ poke p.sbm
     [...]
     (poke) dump
     76543210  0011 2233 4455 6677 8899 aabb ccdd eeff  0123456789ABCDEF
     00000000: 5342 4d05 07ff ffff ff63 47ff 6347 ffff  SBM......cG.cG..
     00000010: ffff ffff ffff ffff 6347 ffff ffff 6347  ........cG....cG
     00000020: ffff ffff ffff ff63 47ff ffff ff63 47ff  .......cG....cG.
     00000030: ffff ffff ffff 6347 ff63 47ff ffff ffff  ......cG.cG.....
     00000040: ffff ffff ff63 47ff ffff ffff ffff ffff  .....cG.........
     00000050: ffff ffff 6347 ffff ffff ffff ffff ffff  ....cG..........
     00000060: ffff ff63 47ff ffff ffff ffff ffff       ...cG.........

You can see the 'P' in the ASCII column, right?  If it wasn't for the
header, it would be pictured almost straight.  This is because dump
shows 16 bytes per row, and our image has lines that are 15 bytes long.
This is a happy coincidence: you definitely shouldn't expect to see
ASCII art in the dump output of SBM files in general!  :)

   Now let's read the image's metadata from the header: pixels per line
and how many lines are contained in the image:

     (poke) var ppl = byte @ 3#B
     (poke) ppl
     5UB
     (poke) var lines = byte @ 4#B
     (poke) lines
     7UB

All right, our image is 7x5.  Knowing that each pixel occupies three
bytes, and that each line contains 'ppl' pixels, and that we have
'lines' lines, we can map the entire image data using an array type
specifier:

     (poke) var image_data = byte[3][ppl][lines] @ 5#B
     (poke) image_data
     [[[255UB,255UB,255UB],[255UB,99UB,71UB], ...]...]

4.3.2 Painting Pixels
---------------------

The "P is for poke" slogan was so successful and widely appraised that
the recutils(1) chaps wanted to do a similar campaign "R is for
recutils".  For that purpose, they asked us for a tomato-colored SBM
image with an 'R' in it.

   Our creative department got at it, and after a lot of work they came
with the following design:

       | 0 | 1 | 2 | 3 | 4 |
       +---+---+---+---+---+
     0 |   | * | * |   |   |
     1 |   | * |   | * |   |
     2 |   | * |   | * |   |
     3 |   | * | * |   |   |
     4 |   | * | * |   |   |
     5 |   | * |   | * |   |
     6 |   | * |   | * |   |

Observe that this design really looks like our 'P' (so much for a
creative department).  The bitmap has exactly the same dimensions, and
difference are just three pixels, that pass from being background pixels
to foreground pixels.

   Therefore, it makes sense to read our 'p.sbm' and use it as a base,
completing the missing pixels.  We saw in the last section how to read a
SBM image.  This time, however, we will copy the image first to a memory
IO space to avoid overwriting 'p.sbm':

     $ poke p.sbm
     [...]
     (poke) .mem scratch
     The current IOS is now `*scratch*'.
     (poke) .info ios
       Id	Type	Mode	Bias		Size		Name
     * #1	MEMORY		0x00000000#B	0x00001000#B	*scratch*
       #0	FILE	rw	0x00000000#B	0x0000006e#B	./p.sbm
     (poke) copy :from_ios 0 :from 0#B :to 0#B :size iosize (0)
     (poke) dump
     76543210  0011 2233 4455 6677 8899 aabb ccdd eeff  0123456789ABCDEF
     00000000: 5342 4d05 07ff ffff ff63 47ff 6347 ffff  SBM......cG.cG..
     00000010: ffff ffff ffff ffff 6347 ffff ffff 6347  ........cG....cG
     00000020: ffff ffff ffff ff63 47ff ffff ff63 47ff  .......cG....cG.
     00000030: ffff ffff ffff 6347 ff63 47ff ffff ffff  ......cG.cG.....
     00000040: ffff ffff ff63 47ff ffff ffff ffff ffff  .....cG.........
     00000050: ffff ffff 6347 ffff ffff ffff ffff ffff  ....cG..........
     00000060: ffff ff63 47ff ffff ffff ffff ffff 0000  ...cG...........
     00000070: 0000 0000 0000 0000 0000 0000 0000 0000  ................

Good.  Now let's map the contents of the image, both header information
and the sequence of pixels:

     (poke) var ppl = byte @ 3#B
     (poke) var lines = byte @ 4#B
     (poke) var image_data = byte[3][ppl][lines] @ 5#B

Let's modify the image.  Since the dimensions of the new image are
exactly the same, the header remains the same.  It is the pixel sequence
that is different.  We basically need to turn the pixels at coordinates
'(4,2)', '(5,3)' and '(6,3)' from background pixels to foreground
pixels.

   Remember how we would change the value of some integer in the IO
space?  First, we would map it into a variable, change the value, and
then poke it back to the IO space.  Something like this:

     (poke) var n = int @ OFFSET
     (poke) n = n + 1
     (poke) int @ OFFSET = n

This three-steps process is necessary because in the 'n = n + 1' above
we are modifying the value of the variable 'n', not the integer actually
stored at offset OFFSET in the current IO space.  Therefore we have to
explicitly poke it back if we want the IO space to be updated as well.

   Array values (and, as we shall see, other "composited" values) are
different: when they are the result of the application of the map
operator, the resulting values are "mapped" themselves.

   When a Poke value is mapped, updating their elements have a side
effect: the area corresponding to the updated element, in whatever IO
space it is mapped on, is updated as well!

   Why is this?  The map operator, regardless of the kind of value it is
mapping, always returns a _copy_ of the value found stored in the IO
space.  We already saw how this worked with integers.  However, in Poke
values are copied around using a mechanism called "shared value".  This
means that when a composite value like an array is copied, its elements
are shared by both the original value and the new value.

   We can depict this graphically for better understanding.  A Poke
array like:

     (poke) var a = [1,2,3]

is stored in memory like this:

        +---+---+---+
      a | 1 | 2 | 3 |
        +---+---+---+

   If we make a copy of the array in another variable 'b':

     (poke) var b = a

we get

        +---+---+---+      +---+---+---+
      a | 1 | 2 | 3 |    b | 1 | 2 | 3 |
        +---+---+---+      +---+---+---+

Note how each of the integer elements has been copied to the new value.
The resulting two arrays can then be modified independently:

     (poke) a[1] = 5

resulting in:

        +---+---+---+      +---+---+---+
      a | 1 | 5 | 3 |    b | 1 | 2 | 3 |
        +---+---+---+      +---+---+---+

   However, consider the following array whose elements are also arrays:

     (poke) var a = [[1,2],[3,4],[5,6]]

This array is stored in memory like this:

        +---+---+---+
      a |   |   |   |
        +---+---+---+
          |   |   |   +---+---+
          |   |   +---| 5 | 6 |
          |   |       +---+---+
          |   |
          |   |       +---+---+
          |   +-------| 3 | 4 |
          |           +---+---+
          |
          |           +---+---+
          +-----------| 1 | 2 |
                      +---+---+

   If now we make a copy of the same array into another variable 'b':

     (poke) var b = a

The elements of the array are copied "by shared value", i.e.

        +---+---+---+           +---+---+---+
      a |   |   |   |         b |   |   |   |
        +---+---+---+           +---+---+---+
          |   |   |   +---+---+   |   |   |
          |   |   +---| 5 | 6 |---+   |   |
          |   |       +---+---+       |   |
          |   |                       |   |
          |   |       +---+---+       |   |
          |   +-------| 3 | 4 |-------+   |
          |           +---+---+           |
          |                               |
          |           +---+---+           |
          +-----------| 1 | 2 |-----------+
                      +---+---+

The elements are indeed shared between the original array and the copy!
If now we modify any of the shared values, this will be reflected in
both containing values:

     (poke) a[1][1] = 9
     (poke) a
     [[1,2],[3,9],[5,6]]
     (poke) b
     [[1,2],[3,9],[5,6]]

or graphically:

        +---+---+---+           +---+---+---+
      a |   |   |   |         b |   |   |   |
        +---+---+---+           +---+---+---+
          |   |   |   +---+---+   |   |   |
          |   |   +---| 5 | 6 |---+   |   |
          |   |       +---+---+       |   |
          |   |                       |   |
          |   |       +---+---+       |   |
          |   +-------| 3 | 9 |-------+   |
          |           +---+---+           |
          |                               |
          |           +---+---+           |
          +-----------| 1 | 2 |-----------+
                      +---+---+

   Back to our image, it follows that if we wanted to change the color
of some SBM pixel stored at offset OFFSET, we would do this:

     (poke) var a = byte[3] @ OFFSET
     (poke) a[1] = 10

There is no need to poke the array back explicitly: the side effect of
assigning 10 to a[1] is that the byte at offset 'OFFSET+1' is poked.

   Generally speaking, mapped values can be handled in exactly the same
way than non-mapped values.  This is actually a very central concept in
poke.  However, it is possible to check whether a given value is mapped
or not using the ''mapped' attribute.

   As we said, "simple values" such as integers and strings are never
mapped, regardless of where they come from.  Both 'ppl' and 'lines' are
integers, therefore:

     (poke) ppl'mapped
     0
     (poke) lines'mapped
     0

However, 'image_data' is an array that was the result of the application
of a map operator, so:

     (poke) image_data'mapped
     1

When a value is mapped, you can ask for the offset where it is mapped,
and the IO space where it is mapped, using the attributes ''offset' and
''ios' respectively.  Therefore:

     (poke) image_data'ios
     1
     (poke) image_data'offset
     40UL#b

In other words, 'image_data' is mapped in the IO space with id 1 (the
'*scratch*' buffer) at offset 40 bits, or 5 bytes.  We already knew
that, because we mapped the image data ourselves, but in other
situations these attributes are most useful.  We shall see how later.

   Well, at this point it should be clear how to paint pixels.  First,
let's define our background and foreground pixels:

     (poke) var bga = [255UB, 255UB, 255UB]
     (poke) var fga = [255UB, 99UB, 71UB]

Then, we just update the pixels in the image data using the right
coordinates:

     (poke) image_data[4][2] = fga
     (poke) image_data[5][3] = fga
     (poke) image_data[6][3] = fga
     (poke) dump
     76543210  0011 2233 4455 6677 8899 aabb ccdd eeff  0123456789ABCDEF
     00000000: 5342 4d05 07ff ffff ff63 47ff 6347 ffff  SBM......cG.cG..
     00000010: ffff ffff ffff ffff 6347 ffff ffff 6347  ........cG....cG
     00000020: ffff ffff ffff ff63 47ff ffff ff63 47ff  .......cG....cG.
     00000030: ffff ffff ffff 6347 ff63 47ff ffff ffff  ......cG.cG.....
     00000040: ffff ffff ff63 47ff 6347 ffff ffff ffff  .....cG.cG......
     00000050: ffff ffff 6347 ffff ffff 6347 ffff ffff  ....cG....cG....
     00000060: ffff ff63 47ff ffff ff63 47ff ffff 0000  ...cG....cG.....
     00000070: 0000 0000 0000 0000 0000 0000 0000 0000  ................

4.3.3 Cropping the R
--------------------

Looking at our new image, we realize that the first and the last column
are all background pixels.  We are aware that the recutils project is
always short of resources, so we would like to modify the image to
remove these columns, "cropping" it so it looks like this:

       | 0 | 1 | 2 |
       +---+---+---+
     0 | * | * |   |
     1 | * |   | * |
     2 | * |   | * |
     3 | * | * |   |
     4 | * | * |   |
     5 | * |   | * |
     6 | * |   | * |

In order to perform this operation we need to rework the stream of
pixels to reflect the desired result, and then to update the header
metadata accordingly.

4.3.4 Shortening and Shifting Lines
-----------------------------------

Let's think in term of lines.  In the original image, each line has 5
pixels, that we can enumerate as:

          +----+----+----+----+----+
     line | p0 | p1 | p2 | p3 | p4 |
          +----+----+----+----+----+

What we want is to crop out the first and the last column, so the
resulting line would look like:

          +----+----+----+
     line | p1 | p2 | p3 |
          +----+----+----+

   Let's get the first line from the original 'image_data':

     (poke) var l0 = image_data[0]
     (poke) l0
     [[255UB,255UB,255UB],[255UB,99UB,71UB],...]

We could create the corresponding cropped line, by doing something like
this:

     (poke) var cl0 = [l0[1],l0[2],l0[3]]

And the same for the other lines.  However, Poke provides a better way
to easily obtain sub portions of arrays.  It is called "trimming".
Given an array like the line 'l0', we can obtain the desired portion of
it by issuing:

     (poke) l0[1:4]
     [[255UB,99UB,71UB],[255UB,99UB,71UB]]

Note how the limits of the semi-open interval specified in the trim
reflect array indexes, and hence they are 0 based.  Also, the left limit
is closed and the right limit is open.  The result of an array trimming
is always another array, which may be empty:

     (poke) l0[1:3]
     [[255UB,99UB,71UB]]

   Armed with this new operation, we can very easily mount the sequence
of pixels for our cropped image:

     (poke) var l0 = image_data[0]
     (poke) var l1 = image_data[1]
     (poke) var l2 = image_data[2]
     (poke) var l3 = image_data[3]
     (poke) var l4 = image_data[4]
     (poke) var l5 = image_data[5]

And then update the lines in the mapped image data:

     (poke) image_data[0] = l0[1:4]
     (poke) image_data[0] = l1[1:4]
     (poke) image_data[1] = l2[1:4]
     (poke) image_data[2] = l3[1:4]
     (poke) image_data[3] = l4[1:4]
     (poke) image_data[4] = l5[1:4]

4.3.5 Updating the Header
-------------------------

The last step is to update the header to reflect the new dimensions of
the image:

     (poke) byte[] @ 0#B = ['S','B','M']
     (poke) byte @ 3#B = 3
     (poke) byte @ 4#B = 7

And we are done.  Note how this time we wrote the magic bytes as an
array, to save some typing and silly manual offset arithmetic.  You may
have noticed that the type specifier we used this time in the map is
'byte[]' instead of 'byte[3]'.  This type specifier denotes an array of
any number of bytes, which certainly includes arrays of three bytes,
like in the example.

4.3.6 Saving the Result
-----------------------

And finally, let's write out the new file as 'r.sbm':

     (poke) save :from 0#B :size 5#B + image_data'size :file "r.sbm"

   ---------- Footnotes ----------

   (1) http://www.gnu.org/s/recutils

4.4 Defining Types
==================

4.4.1 Naming your Own Abstractions
----------------------------------

During the process of creating and manipulating SBM files we soon
started talking about things like lines, pixel sequences, pixels,
colors, and so on.  What is more, very naturally we started _thinking_
in terms of these entities: let's drop this or that line, or let's
change the green component of this pixel.

   Consider for example RGB colors.  We know that each color is defined
by three levels of light: red, green and blue.  These components are
also called color beams.  Since each color beam has a range of 0 to 255,
many formats, like SBM, use bytes to encode them.

   Therefore, in the previous sections we used the type specifier 'byte'
when we needed to map RGB color beams, like in:

     (poke) byte[3] @ 5#B

Recall that the mapping operation above means "map three bytes at the
offset 5 bytes in the current IO space".  But what we really want is to
map color beams, not bytes!

   Poke provides a way to assign names to type specifiers:

     (poke) type RGB_Color_Beam = byte

The definition above tells poke that a RGB color beam is composed of a
byte, i.e.  an unsigned 8-bit integer.  Any type specifier can be used
at the right side of the assignment sign, and also names of already
defined types.  From this point on, we can map in terms of color beams:

     (poke) RGB_Color_Beam[3] @ 5#B

Meaning "map three RGB color beams at the offset 5 in the current IO
space".

   Once a type is defined, the name can be used anywhere where a type
specifier is expected.

   By the way, we mentioned many times how 'byte' is a synonym for
'uint<8>', 'int' is a synonym for 'int<32>' and so on.  These synonyms
are actually result of type definitions that are in the poke "standard
library".  This library is loaded by poke at startup time.

4.4.2 Abstracting the Structure of Entities
-------------------------------------------

Since we didn't know better, during our work with SBM images we had to
remember how these entities were constructed from more simple entities
such as bits and bytes, every time we needed to map them, or to poke
them.  For example, if we wanted to map a pixel at some particular
offset, we had to issue the following command:

     (poke) var pixel = byte[3] @ 5#B

Now that we made poke aware of what a RGB color beam is, we can rewrite
the above as:

     (poke) var pixel = RGB_Color_Beam[3] @ 5#B

This is better, but still adoleces from a big problem: what if at some
point the SBM pixels get expanded to also have a transparency index,
stored in a fourth byte?  If that ever happens (and _will_ happen later
in this book) then we would need to remember it before issuing commands
like:

     (poke) var image_data = RGB_Color_Beam[4][ppl][lines] @ 5#B

   To avoid this problem, we define yet another type, this time
describing the structure of a SBM pixel:

     (poke) type SBM_Pixel = RGB_Color_Beam[3]

And then we can define 'image_data' as a table of SBM pixels, instead of
as a table of triplets of RGB color beams:

     (poke) var image_data = SBM_Pixel[ppl][lines] @ 5#B

   This later definition doesn't need to be changed if we change the
definition of what a SBM pixel is.  This is called "encapsulation" and
is a very useful abstraction in computing.

4.5 Pickles
===========

4.5.1 poke Commands versus Poke Constructions
---------------------------------------------

In *note Nomenclature:: we mentioned that poke, the program, implements
a domain specific programming language called Poke, with a big p.  In
the examples so far we have already used the Poke language, somewhat
extensively, while interacting with the program using the REPL.

   For example, in:

     (poke) 10 + 2
     12

We are giving poke a "Poke expression" '10 + 2' to be evaluated.  Once
the expression is evaluated, the REPL prints the resulting value for us.

   Similarly, when we define a variable or a type with 'var' and 'type'
respectively, we are providing poke definitions to be evaluated.  When
we assign a value to a variable we are actually providing a Poke
statement to be executed.

   So the REPL accepts "poke commands", some of which happen to be Poke
expressions, definitions or statements.  But we also have used
dot-commands like '.file' or '.info'.  These dot-commands are not part
of the Poke programming language.

   Every time we insert a line in the REPL and hit <enter>, poke
recognizes the nature of the line, and then does the right thing.  If
the line is recognized as a Poke expression, for example, the Poke
compiler is used to compile the statement into a routine, that is
executed by the Poke Virtual Machine.  The resulting value is then
printed for the benefit of the user.

4.5.2 Poke Files
----------------

Poke programs are basically a collection of definitions and statements,
which most often than not are stored in files, which avoids the need to
type them again and again.  By convention, we use the '.pk' file
extension when naming files containing Poke programs.

   Remember how we defined the foreground and background pixels for
'p.sbm' in the REPL?

     (poke) var bga = [255UB, 255UB, 255UB]
     (poke) var fga = [255UB, 99UB, 71UB]

Where 'bga' is a white pixel and 'fga' is a tomato colored pixel.  We
could write these definitions in a file 'colors.pk' like this:

     var white = [255UB, 255UB, 255UB];
     var tomato = [255UB, 99UB, 71UB];

Note that variable definitions in Poke are terminated by a semicolon
(';') character, but we didn't need to specify them when we issued the
definitions in the REPL. This is because poke adds the trailing
semicolon for us when it detects a Poke construction requiring it is
introduced in the REPL.

Another difference is that Poke constructions can span for multiple
lines, like in most programming languages.  For example, we could have
the following variable definition in a file:

     var matrix = [[10, 20, 30],
                   [40, 50, 60],
                   [70, 80, 90]];

   Once we have written our 'colors.pk' file, how can we make poke aware
of it?  A possibility is to use the 'load' construction:

     (poke) load colors

Assuming a file named 'colors.pk' exists in the current working
directory, poke will load it and evaluate its contents.  After this, we
can use the colors:

     (poke) tomato
     [0xffUB,0x63UB,0x47UB]

Before, we would need to define these colors every time we would like to
poke SBM files or, in fact, any RGB24 data.  Now we just have to load
the file 'colors.pk' and use the variables defined there.

   If you try to load a file whose name contains a dash character ('-')
you will get an error message:

     (poke) load my-colors
     <stdin>:1:8: error: syntax error, unexpected '-', expecting ';'
     load my-colors;
            ^

This is because the argument to 'load' is interpreted as a Poke
identifier, and dash characters are not allowed in identifiers.  To
alleviate this problem, you can also specify the name of the file to
load encoded in a string, line in:

     (poke) load "my-colors.pk"

If you use this form of 'load', however, you are required to specify the
complete name of the file, including the '.pk' file extension.

   Since loading files is such a common operation, poke provides a
dot-command '.load' that does auto-completion:

     (poke) .load my-colors.pk

Which is equivalent to 'load "my-colors.pk"'.

   Since 'load' is part of the Poke language, it can also be used in
Poke programs stored in files.  We will explore this later.

4.5.3 Pickling Abstractions
---------------------------

In the last section we defined a couple of RGB colors 'white' and
'tomato' in a file called 'colors.pk'.  If we keep adding colors to the
file, we may end with a nice collection of colors that are available to
us at any time, by just loading the file.

   Since there are many ways to understand the notion of "color", and
also many ways to implement these many notions, it would be better to be
more precise and call our file 'rgb24.pk', since the notion of color we
are using is of that RGB24.  While are at it, let's also rename the
variables to reflect the fact they denote not just any kind of colors,
but RGB24 colors:

     var rgb24_white = [255UB, 255UB, 255UB];
     var rgb24_tomato = [255UB, 99UB, 71UB];

At this point, we can also add the definitions of a couple of types to
our 'rgb24.pk':

     type RGB_Color_Beam = byte;
     type RGB24_Color = RGB_Color_Beam[3];

     var rgb24_white = [255UB, 255UB, 255UB];
     var rgb24_tomato = [255UB, 99UB, 71UB];

Any time we want to manipulate RGB24 colors we can just load the file
'rgb24.pk' and use these types and variables.

   In poke parlance we call files like the above, that contain
definitions of conceptually related entities, "pickles".  Pickles can be
very simple, like the 'rgb24.pk' sketched above, or fairly complicated
like 'dwarf.pk'.

   It is common for pickles to 'load' other pickles.  For example, if we
were to write a SBM pickle, we would load the RGB24 pickle from it:

     load rgb24;

     [... SBM definitions ...]

This way, when we 'load sbm' from the repl, the dependencies get loaded
as well.

   GNU poke includes several already written pickles for commonly used
file formats, and other domains.  The 'load' construction knows where
these pickles are installed, so in order to load the pickle to
manipulate ELF files, for example, all you have to do is to:

     (poke) load elf

4.5.4 Exploring Pickles
-----------------------

As we have seen, a pickle provides Poke variables, types and functions
related to some definite domain.  Let's say we are interested in editing
an ELF file.  We know that GNU poke comes with a pre-installed pickle
named 'elf.pk'.  We can load it like this:

     (poke) load elf

   By convention, the entities provided by a pickle 'foo.pk' are usually
prefixed like:

   - Variables, units and functions are prefixed with 'foo_'.
   - Types are prefixed with 'Foo_'.

Therefore, once the pickle is loaded we can use the '.info' dot-command
in order to get a glimpse of the functionality provided by the pickle.
*Note info command::.  Example:

     (poke) .info variables elf
     Name            Declared at
     elf_stb_names   elf-common.pk:53
     elf_stt_names   elf-common.pk:72
     elf_stv_names   elf-common.pk:84

The above tells us that the 'elf.pk' pickle provides these three
variables.  Types are way more interesting:

     (poke) .info types Elf64
     Name                 Declared at
     Elf64_Ehdr           elf-64.pk:184
     Elf64_Off            elf-64.pk:26
     Elf64_SectionFlags   elf-64.pk:130
     Elf64_Shdr           elf-64.pk:152
     Elf64_RelInfo        elf-64.pk:36
     Elf64_Chdr           elf-64.pk:85
     Elf64_Rela           elf-64.pk:61
     Elf64_Phdr           elf-64.pk:169
     Elf64_Sxword         elf-64.pk:24
     Elf64_Sym            elf-64.pk:71
     Elf64_Rel            elf-64.pk:45
     Elf64_File           elf-64.pk:207
     Elf64_Addr           elf-64.pk:25
     Elf64_Group          elf-64.pk:121
     Elf64_Dyn            elf-64.pk:98
     Elf64_Xword          elf-64.pk:23

We immediately notice the 'Elf64_File'.  Looks like what we would need
to map in order to access the entire ELF data in some given IO space
(like a file.)  We can ask for more details about it:

     (poke) .info type Elf64_File
     Class:      struct
     Name:       Elf64_File
     Complete:   no
     Fields:
       Name     Type
       ehdr     Elf64_Ehdr
       shdr     Elf64_Shdr[]
       phdr     Elf64_Phdr[]
     Methods:
       Name                     Type
       get_section_name         (offset<Elf_Word,8>)string
       get_symbol_name          (Elf64_Shdr,,offset<Elf_Word,8>)string
       get_sections_by_name     (string)Elf64_Shdr[]
       get_sections_by_type     (Elf_Word)Elf64_Shdr[]
       section_name_p           (string)int
       get_string               (offset<Elf_Word,8>)string
       get_group_signature      (Elf64_Shdr)string
       get_group_signatures     ()string[]
       get_section_group        (string)Elf64_Shdr[]

We see that an 'Elf64_File' has three fields, and the nature of these
files: a header 'ehdr', an array of section headers 'shdr' and an array
of program headers 'phdr'.

   The type description above also lists the methods defined in the
type.  Looking at the method name and its type is very usually revealing
enough to use it.  For example, we see that 'get_sections_by_name'
returns an array of section headers, characterized by some given section
name:

     (poke) var elf = Elf64_File @ 0#B
     (poke) elf.get_sections_by_name (".text")
     [Elf64_Shdr {
       sh_name=0x1bU#B,
       sh_type=0x1U,
       sh_flags=#<ALLOC,EXECINSTR>,
       sh_addr=0x0UL#B,
       sh_offset=0x40UL#B,
       sh_size=0xbUL#B,
       sh_link=0x0U,
       sh_info=0x0U,
       sh_addralign=0x1UL,
       sh_entsize=0x0UL#b
     }]

   Ultimately, of course the best way to see what a pickle provides is
to go read its source code.

4.5.5 Startup
-------------

When poke starts it loads the file '.pokerc' located in our home
directory, if it exists.  This initialization file contains poke
commands, one per line.

   If we wanted to get some Poke file loaded at startup, we could do it
by adding a load command to our '.pokerc'.  For example:

     # My poke configuration - jemarch
     [...]
     .load ~/.poke.d/mydefs.pk

4.6 Poking Structs
==================

4.6.1 Heterogeneous Related Data
--------------------------------

Let's recap the structure of the header of a Stupid BitMap:

                     SBM header
     +-------+-------+-------+-------+-------+
     |  'S'  |  'B'  |  'M'  |  ppl  | lines |
     +-------+-------+-------+-------+-------+
       byte0   byte1   byte2   byte3   byte4

The header is composed of five fields, which actually compose three
different logical "fields": a magic number, the number of pixels per
line, and the number of lines.

   We could of course abstract the header using an array of five bytes,
like this:

     type SBM_Header = byte[5];

   However, this would not capture the properties of the fields
themselves, which would need to be remembered by the user: which of
these five bytes correspond to the magic number?  Is the pixels per line
number signed or unsigned?  etc.

   Poke provides a much better way to abstract collections of
heterogeneous data: "struct types".  Using a struct type we can abstract
the SBM header like this:

     type SBM_Header =
      struct
      {
        byte[3] magic;
        uint<8> ppl;
        uint<8> lines;
      }

   Note how the struct has three named fields: 'magic', 'ppl' and
'lines'.  'magic' is an array of three bytes, while 'ppl' and 'lines'
are both unsigned integers.

4.6.2 Mapping Structs
---------------------

Once defined, struct types can be referred by name.  For example, we can
map the SBM header at the beginning of our file 'p.sbm' like this:

     (poke) SBM_Header @ 0#B
     SBM_Header {
       magic=[0x53UB,0x42UB,0x4dUB],
       ppl=0x5UB,
       lines=0x7UB
     }

The value resulting from the mapping is a struct value.  The fields of
struct values are accessed using the familiar dot-notation:

     (poke) var header = SBM_Header @ 0#B
     (poke) header.ppl * header.lines
     35UB

The total number of pixels in the image is 35.  Note how both
'header.ppl' and 'header.lines' are indeed unsigned byte values, and
thus the result of the multiplication is also an unsigned byte.  This
could be problematic if the image contained more than 255 pixels, but
this can be prevented by using a cast:

     (poke) header.ppl as uint * header.lines
     35U

Now the second operand 'header.lines' is promoted to a 32-bit unsigned
value before the multiplication is performed.  Consequently, the result
of the operation is also 32-bit wide (note the suffix of the result.)

4.6.3 Modifying Mapped Structs
------------------------------

Remember when we wanted to crop a SBM image by removing the first and
last row?  We updated the header in a byte by byte manner, like this:

     (poke) byte @ 3#B = 3
     (poke) byte @ 4#B = 7

Now that we have the header mapped in a variable, updating it is much
more easy and convenient.  The dot-notation is used to update the
contents of a struct field, by placing it at the left hand side of an
assignment:

     (poke) header.ppl = 3
     (poke) header.lines = 7

This updates the pixel per line and the number of lines, in the IO
space:

     (poke) dump :size 5#B
     76543210  0011 2233 4455 6677 8899 aabb ccdd eeff  0123456789ABCDEF
     00000000: 5342 4d03 07                             SBM..

4.7 How Structs are Built
=========================

First we need to define some structure to use as an example.  Let's say
we are interesting in poking Packets, as defined by the Packet
Specification 1.2 published by the Packet Foundation (none less).

   In a nutshell, each Packet starts with a byte whose value is always
'0xab', followed by a byte that defines the size of the payload.  A
stream of bytes conforming the payload follows, themselves followed by
another stream of the same number of bytes with "control" values.

   We could translate this description into the following Poke struct
type definition:

     type Packet =
       struct
       {
         byte magic = 0xab;
         byte size;
         byte[size] payload;
         byte[size] control;
       };

   There are some details described by the fictitious Packet
Specification 1.2 that are not covered in this simple definition, but we
will be attending to that later in this manual.

   So, given the definition of a struct type like Packet, there are only
two ways to build a struct value in Poke.

   One is to map it from some IO space.  This is achieved using the map
operator:

     (poke) Packet @ 12#B
     Packet {
       magic = 0xab,
       size = 2,
       payload = [0x12UB,0x30UB],
       control = [0x1UB,0x1UB]
     }

   The expression above maps a Packet starting at offset 12 bytes, in
the current IO space.  See the Poke manual for more details on using the
map operator.

   The second way to build a struct value is to _construct_ one,
specifying the value to some, all or none of its fields.  It looks like
this:

     (poke) Packet {size = 2, payload = [1UB,2UB]}
     Packet {
       magic = 0xab,
       size = 2,
       payload = [0x1UB,0x2UB],
       control = [0x0UB,0x0UB]
     }

   In either case, building a struct involves to determine the value of
all the fields of the struct, one by one.  The order in which the struct
fields are built is determined by the order of appearance of the fields
in the type description.

   In our example, the value of 'magic' is determined first, then
'size', 'payload' and finally 'control'.  This is the reason why we can
refer to the values of previous fields when defining fields, such as in
the size of the 'payload' array above, but not the other way around: by
the time 'payload' is mapped or constructed, the value of 'size', has
already been mapped or constructed.

   What happens behind the curtains is that when poke finds the
definition of a struct type, like Packet, it compiles two functions from
it: a mapper function, and a constructor function.  The mapper function
gets as arguments the IO space and the offset from which to map the
struct value, whereas the constructor function gets the template
specifying the initial values for some, or all of the fields; reasonable
default values (like zeroes) are used for fields for which no initial
values have been specified.

   These functions, mapper and constructor, are invoked to create fresh
values when a map operator @ or a struct constructor is used in a Poke
program, or at the poke prompt.

4.8 Variables in Structs
========================

Fields are not the only entity that can appear in the definition of a
struct type.

   Suppose that after reading more carefully the Packet Specification
1.2 (that spans for several thousand of pages) we realize that the field
'size' doesn't really store the number of bytes of the payload and
control arrays, like we thought initially.  Or not exactly: the Packet
Foundation says that if 'size' has the special value 0xff, then the size
is zero.

   We could of course do something like this:

     type Packet =
       struct
       {
         byte magic = 0xab;
         byte size;

         byte[size == 0xff ? 0 : size] payload;
         byte[size == 0xff ? 0 : size] control;
       };

   However, we can avoid replicating code by using a variable instead:

     type Packet =
       struct
       {
         byte magic = 0xab;
         byte size;

         var real_size = (size == 0xff ? 0 : size);

         byte[real_size] payload;
         byte[real_size] control;
       };

   Note how the variable can be used after it gets defined.  In the
underlying process of mapping or constructing the struct, the variable
is incorporated into the lexical environment.  Once defined, it can be
used in constraint expressions, array sizes, and the like.  We will see
more about this later.

   Incidentally, it is of course possible to use global variables as
well.  For example:

     var packet_special = 0xff;
     type Packet =
       struct
       {
         byte magic = 0xab;
         byte size;

         var real_size = (size == packet_special ? 0 : size);

         byte[real_size] payload;
         byte[real_size] control;
       };

   In this case, the global 'packet_special' gets captured in the
lexical environment of the struct type (in reality in the lexical
environment of the implicitly created mapper and constructor functions)
in a way that if you later modify 'packet_special' the new value will be
used when mapping/constructing _new_ values of type Packet.  Which is
really cool, but let's not get distracted from the main topic...  :)

4.9 Functions in Structs
========================

Further reading of the Packet Specification 1.2 reveals that each Packet
has an additional 'crc' field.  The content of this field is derived
from both the payload bytes and the control bytes.

   But this is no vulgar CRC we are talking about.  On the contrary, it
is a special function developed by the CRC Foundation in partnership
with the Packet Foundation, called superCRC (patented, TM).

   Fortunately, the CRC Foundation distributes a pickle 'supercrc.pk',
that provides a 'calculate_crc' function with the following spec:

     fun calculate_crc = (byte[] data, byte[] control) int:

   So let's use the function like this in our type, after loading the
supercrc pickle:

     load supercrc;

     type Packet =
       struct
       {
         byte magic = 0xab;
         byte size;

         var real_size = (size == 0xff ? 0 : size);

         byte[real_size] payload;
         byte[real_size] control;

         int crc = calculate_crc (payload, control);
       };

   However, there is a caveat: it happens that the calculation of the
CRC may involve arithmetic and division, so the CRC Foundation warns us
that the 'calculate_crc' function may raise E_div_by_zero.  However, the
Packet 1.2 Specification tells us that in these situations, the 'crc'
field of the packet should contain zero.  If we used the type above, any
exception raised by 'calculate_crc' would be propagated by the
mapper/constructor:

     (poke) Packet @ 12#B
     unhandled division by zero exception

   A solution is to use a function that takes care of the extra needed
logic, wrapping calculate_crc:

     load supercrc;

     type Packet =
       struct
       {
         byte magic = 0xab;
         byte size;

         var real_size = (size == 0xff ? 0 : size);

         byte[real_size] payload;
         byte[real_size] control;

         fun corrected_crc = int:
         {
           try return calculate_crc (payload, control);
           catch if E_div_by_zero { return 0; }
         }

         int crc = corrected_crc;
       };

   Again, note how the function is accessible after its definition.
Note as well how both fields and variables and other functions can be
used in the function body.  There is no difference to define variables
and functions in struct types than to define them inside other functions
or in the top-level environment: the same lexical rules apply.

4.10 Struct Methods
===================

At this point you may be thinking something on the line of "hey, since
variables and functions are also members of the struct, I should be able
to access them the same way than fields, right?".

   So you will want to do:

     (poke) var p = Packet  12#B
     (poke) p.real_size
     (poke) p.corrected_crc

But sorry, this won't work.

   To understand why, think about the struct building process we
sketched above.  The mapper and constructor functions are
derived/compiled from the struct type.  You can imagine them to have
prototypes like:

     Packet_mapper (IOspace, offset) -> Packet value
     Packet_constructor (template)   -> Packet value

   You can also picture the fields, variables and functions in the
struct type specification as being defined inside the bodies of
Packet_mapper and Packet_constructor, as their contents get
mapped/constructed.  For example, let's see what the mapper does:

     Packet_mapper:

       . Map a byte, put it in a local `magic'.
       . Map a byte, put it in a local `size'.
       . Calculate the real size, put it in a local `real_size'.
       . Map an array of real_size bytes, put it in a local `payload'.
       . Map an array of real_size bytes, put it in a local `control'.
       . Compile a function, put it in a local `corrected_crc'.
       . map a byte, call the function in the local `corrected_crc',
         complain if the values are not the same, otherwise put the
         mapped byte in a local `crc'.
       . Build a struct value with the values from the locals `magic',
         `size', `payload', `control' and `crc', and return it.

   The pseudo-code for the constructor would be almost identical.  Just
replace "map a byte" with "construct a byte".

   So you see, both the values for the mapped fields and the values for
the variables and functions defined inside the struct type end as locals
of the mapping process, but only the values of the fields are actually
put in the struct value that is returned in the last step.

   This is where methods come in the picture.  A method looks very
similar to a function, but it is not quite the same thing.  Let me show
you an example:

     load supercrc;

     type Packet =
       struct
       {
         byte magic = 0xab;
         byte size;

         var real_size = (size == 0xff ? 0 : size);

         byte[real_size] payload;
         byte[real_size] control;

         fun corrected_crc = int:
         {
           try return calculate_crc (payload, control);
           catch if E_div_by_zero { return 0; }
         }

         int crc = corrected_crc;

         method c_crc = int:
         {
           return corrected_crc;
         }
       };

   We have added a method 'c_crc' to our Packet struct type, that just
returns the corrected superCRC (patented, TM) of a packet.  This can be
invoked using dot-notation, once a Packet value is mapped/constructed:

     (poke) var p = Packet  12#B
     (poke) p.c_crc
     0xdeadbeef

   Now, the important bit here is that the method returns the corrected
crc _of a 'Packet'_.  That's it, it actually operates on a Packet value.
This Packet value gets implicitly passed as an argument whenever a
method is invoked.

   We can visualize this with the following "pseudo Poke":

     method c_crc = (Packet SELF) int:
     {
        return SELF.corrected_crc;
     }

   Fortunately, poke takes care to recognize when you are referring to
fields of this implicit struct value, and does The Right Thing(TM) for
you.  This includes calling other methods:

     method foo = void: { ... }
     method bar = void:
     {
      [...]
      foo;
     }

   The corresponding "pseudo-poke" being:

     method bar = (Packet SELF) void:
     {
      [...]
      SELF.foo ();
     }

   It is also possible to define methods that modify the contents of
struct fields, no problem:

     var packet_special = 0xff;

     type Packet =
       struct
       {
         byte magic = 0xab;
         byte size;
         [...]

         method set_size = (byte s) void:
         {
           if (s == 0)
             size = packet_special;
           else
             size = s;
         }
       };

   This is what is commonly known as a "setter".  Note, incidentally,
how a method can also use regular variables.  The Poke compiler knows
when to generate a store in a normal variable such as 'packet_special',
and when to generate a set to a 'SELF' field.

   Given the different nature of the variables, functions and methods,
there are a couple of restrictions:

   * Functions can't set fields defined in the struct type.

     This will be rejected by the compiler:

          type Foo =
            struct
            {
               int field;
               fun wrong = void: { field = 10; }
            };

     Remember the construction/mapping process.  When a function
     accesses a field of the struct type like in the example above, it
     is not doing one of these pseudo 'SELF.field = 10'.  Instead, it is
     simply updating the value of the local created in this step in
     Foo_mapper:

          Foo_mapper:

           . Map an int, put it in a local `field'.
           . [...]

     Setting that local would impact the mapping of the subsequent
     fields if they refer to 'field' (for example, in their constraint
     expression) but it wouldn't actually alter the value of the field
     'field' in the struct value that is created and returned from the
     mapper!

     This is very confusing, so we just disallow this with a compiler
     error "invalid assignment to struct field", for your own sanity 8-)

   * Methods can't be used in field constraint expressions, nor in
     variables or functions defined in a struct type.

     How could they be?  The field constraint expressions, the
     initialization expressions of variables, and the functions defined
     in struct types are all executed as part of the mapper/constructor
     and, at that time, there is no struct value yet to pass to the
     method.

     If you try to do this, the compiler will greet you with an "invalid
     reference to struct method" message.

   Something to keep in mind about methods is that they can destroy the
integrity of the data stored in a struct.  Consider for example the
following struct type:

     type Packet =
       struct
       {
         byte magic = 0xab;
         byte size : size <= 4096;
         [...]

         method set_size = (byte s) void:
         {
           if (s == 0)
             size = packet_special;
           else
             size = s;
         }
       };

Observe how this new version of 'Packet' has an additional constraint
that specifies 'size' should not exceed 4096.  However, when the method
'set_size' is executed the constraints are not checked again.  This is
useful at times, but can also lead to unintended data corruption.

   A solution for this problem is to make methods aware of the
restrictions.  Like in this case:

     type Packet =
       struct
       {
         var MAXSIZE = 4096;

         byte magic = 0xab;
         byte size : size <= MAXSIZE;
         [...]

         method set_size = (byte s) void:
         {
           if (s == 0)
             size = packet_special;
           else if (s > MAXSIZE)
             raise E_inval;
           else
             size = s;
         }
       };

Note how we use a variable 'MAXSIZE' in order to avoid hard-coding 4096
twice in the struct definition.

4.11 Padding and Alignment
==========================

It is often the case in binary formats that certain elements are
separated by some data that is not really used for any meaningful
purpose other than occupy that space.  The reason for keeping that space
varies from case to case; sometimes to reserve it for future use,
sometimes to make sure that the following data is aligned to some
particular alignment.  This is known as "padding".  There are several
ways to implement padding in GNU poke.  This article shows these
techniques and discusses their advantages and disadvantages.

4.11.1 Esoteric and exoteric padding
------------------------------------

Padding is the technique of keeping some amount of space between two
different elements in some data stream.  GNU poke provides two different
ways to express sequences of data elements: the fields of a struct type,
which are defined one after the other, and elements in an array.

   We call adding space between two struct fields "esoteric" (or
internal) padding.

   We call adding space between two array elements "exoteric" (or
external) padding.

   The following sections contain examples of the two kinds of padding
and how to better handle them in Poke.

4.11.2 Reserved fields
----------------------

People designing binary encoded formats tend to be cautious and try to
avoid future backward incompatibilities by keeping some unused fields
that are reserved for future use.  This is the first kind of padding we
will be looking at, and is particularly common in structures like
headers.

   See for example the header used to characterize compressed section
contents in ELF files:

     type Elf64_Chdr =
       struct
       {
         Elf_Word ch_type;
         Elf_Word ch_reserved;
         offset<Elf64_Xword,B> ch_size;
         offset<Elf64_Xword,B> ch_addralign;
       };

   where the 'ch_reserved' field is reserved for future use.  When the
time comes the space occupied by that field (32 bits in this case) will
be used to hold additional data in the form of one or more fields.  The
idea is that implementations of the older formats will still work.

   The most obvious way to handle this in Poke is using a named field
like 'ch_reserved' above.  This field will be decoded/encoded by poke
when constructing/mapping/writing struct values of this type, and will
be available to the user as 'chdr.ch_reserved'.

   Sometimes reserved space is required to be filled with certain data
values, such as zeroes.  This may be to simplify things, or to force
data producers to initialize the memory in order to avoid potential
leaking of sensible information.  In these cases we can use Poke initial
values:

     type Elf64_Chdr =
       struct
       {
         Elf_Word ch_type;
         Elf_Word ch_reserved = 0;
         offset<Elf64_Xword,B> ch_size;
         offset<Elf64_Xword,B> ch_addralign;
       };

   This will make poke to check that 'ch_reserved' is zero when
constructing or mapping headers for compressed sections raising a
constraint violation exception otherwise.  It will also make poke to
make sure 'ch_reserved' to 0 when constructing 'Elf64_Chdr' struct
values:

     (poke) Elf64_Chdr { ch_reserved = 23 }
     unhandled constraint violation exception

   An alternative way to characterize reserved space in Poke is to use
anonymous fields.  For example:

     type Elf64_Chdr =
       struct
       {
         Elf_Word ch_type;
         Elf_Word;
         offset<Elf64_Xword,B> ch_size;
         offset<Elf64_Xword,B> ch_addralign;
       };

   Using Poke anonymous fields to implement reserved fields has at least
two advantages.  First, the user cannot anymore temper with the data in
the reserved space in an easy way, i.e.  'chdr.ch_reserved = 666' is no
longer valid.  Second, the printed representation of anonymous struct
fields is more compact and denotes better than the involved space is not
to be messed with:

     (poke) Elf64_Chdr {}
     Elf64_Chdr {
       ch_type=0x0U,
       0x0U,
       ch_size=0x0UL#B,
       ch_addralign=0x0UL#B
     }

   A disadvantage of using anonymous fields is that you cannot specify
constraint expressions for them, nor initial values.  At some point we
will probably add syntax to declare certain struct fields as read-only.

   At this point, it is important to note that anonymous fields are
still encoded/decoded by poke every time the struct value is mapped or
written, exactly like regular fields.  Therefore using them doesn't pose
any advantage in terms of performance.

4.11.3 Payloads
---------------

The reserved fields discussed in the previous section are most often
discrete units like words, double-words, and the like, they are usually
of some fixed size, and they are used to delimit some space that is not
to be used.

   Another kind of padding happens when an entity contains space to be
used to store some kind of payload whose contents are not determined.
This would be such an example:

     type Packet =
       struct
       {
         offset<uint<32>,B> payload_size;
         byte[payload_size] payload;
         int flags;
       };

   In this example we are using a 'payload' field which is an array of
bytes.  The size of the payload is determined by the packet header, and
the contents are not determined.  Of course this assumes that the
payload sizes are divisible in whole bytes; a bit-oriented format may
need to use an array of bits instead.

   This approach of using a byte (or bit) array like in the example
above has the advantage of providing a field with the bytes (or bits) to
the user, for inspection and modification:

     (poke) packet.payload
     [23UB, ...]
     (poke) packet.payload[0] = 0

   The user can still map whatever payload structure in that space using
the attributes of a mapped 'Packet'.  For example, if the packet
contains an array of ULEB128 numbers, we could do:

     (poke) var numbers = ULEB128[packet.payload'size] @ packet.payload'offset

   But this approach has a disadvantage: every time the packet structure
is mapped or written the entire payload array gets decoded and encoded.
If the payloads are big enough (think about the data blocks of a file
described by a file system i-node for example) this can be a big problem
in terms of performance.

   Another problem of using byte (or bit) arrays for payloads is that
the printed representation of the struct values include the contents of
the arrays, and most often the user won't be interested in seeing that:

     (poke) Packet { payload_size = 23#B }
     Packet {
       payload_size=0x17U#B,
       payload=[0x0UB,0x0UB,0x0UB,0x0UB,0x0UB,...],
       flags=0x0
     }

   Another alternative is to implement the padding implied by a payload
using field labels:

     type Packet =
       struct
       {
         offset<uint<32>,B> payload_size;:
         int flags @ OFFSET + payload_size;
       };

   Note how a 'payload' field no longer exists in the struct type, and
the field 'flags' is defined to start at offset 'OFFSET + payload_size'.
This way no explicit array is encoded/decoded when manipulating 'Packet'
values:

     (poke) .set omaps yes
     (poke) Packet { payload_size = 500#Mb }
     Packet {
       payload_size=62500000U#B @ 0UL#b,
       flags=0 @ 4000000032UL#b
     } @ 0UL#b

   In this example we used the 'omaps' option, which asks poke to print
the offsets of the fields.  The offset of 'flags' is 4000000032 bits, or
500 megabytes:

     (poke) 4000000032UL #b/#MB
     500UL

   Mapping this new 'Packet' involves reading and decoding five bytes,
for the 'payload_size' and 'flags' only.  This is clearly much faster
and avoids unneeded IO.

   However you may be wondering, if there is no explicit 'payload'
field, how to access the payload space?  A way is to define a method to
the struct to provide the payload attributes:

     type Packet =
       struct
       {
         offset<uint<32>,B> payload_size;:
         var payload_offset = OFFSET;
         int flags @ OFFSET + payload_size;

         method get_payload_offset = off64:
         {
           return payload_offset;
         }
       };

   Note how we captured the offset of the payload using a variable in
the strict type definition.  Returning 'OFFSET' in 'get_payload_offset'
wouldn't work for obvious reasons: in the body of the method 'OFFSET'
evaluates to the end of 'flags' in this case.

   Using this method you can easily access the payload (again as an
array of ULEB128 numbers) like this:

     var numbers = ULEB128[packet.payload_size @ packet.get_payload_offset

   Finally, using labels for this purpose makes the printed
representation of the struct values more readable by not including the
payload bytes in it:

     (poke) Packet {}
     Packet {
       payload_size=0x0U#B,
       flags=0x0
     }

4.11.4 Aligning struct fields
-----------------------------

Another kind of esoteric padding happens when certain fields in entities
are required to be aligned to some particular alignment.  For example,
suppose that the 'flags' field in the packets used in the previous
sections is required to always be aligned to 4 bytes regardless of the
size of the payload.  This would be a common requirement if the format
is intended to be implemented in systems where data is to be accessed
using its "natural" alignment.

   Using explicit fields for both the payload and the additional
padding, we could come with:

     type Packet =
       struct
       {
         offset<uint<32>,B> payload_size;
         byte[payload_size] payload;
         byte[alignto (OFFSET, 4#B)] padding;
         int flags;
       };

   Where 'alignto' is a little function defined in the Poke standard
library, like this:

     fun alignto = (uoff64 offset, uoff64 to) uoff64:
     {
       return (to - (offset % to)) % to;
     }

   Alternatively, using the labels approach (which is generally better
as we discussed in the last section) the definition would become:

     type Packet =
       struct
       {
         offset<uint<32>,B> payload_size;:
         var payload_offset = OFFSET;
         int flags @ OFFSET + payload_size + alignto (payload_size, 4#B);

         method get_payload_offset = off64:
         {
           return payload_offset;
         }
       };

   In this case, the payload space is still completely characterized by
the 'payload_size' field and the 'get_payload_offset' method.

4.11.5 Padding array elements
-----------------------------

Up to now all the examples of padding we have shown are in the category
of esoteric or internal padding, i.e.  it was intended to add space
between fields of some particular entity.

   However, sometimes we want to specify some padding between the
elements of a sequence of entities.  In Poke this basically means an
array.

   Suppose we have a simple file system that is conformed by a sequence
of inodes.  The contents of the file system have the following form:

     +-----------------+
     |      inode      |
     +-----------------+
     :                 :
     :      data       :
     :                 :
     +-----------------+
     |      inode      |
     +-----------------+
     :                 :
     :      data       :
     :                 :
     +-----------------+
     |      ...        |

   That's it, each inode describes a block of data of variable size that
immediately follows.  Then more pairs of inode-data follow until the end
of the device.  However, a requirement is that each inode has to be
aligned to 128 bytes.

   Let's start by writing a simple type definition for the inodes:

     type Inode =
       struct
       {
         string filename;
         int perms;
         offset<uint<32>,B> data_size;
       };

   This definition is simple enough, but it doesn't allow us to just map
an array of inodes like this:

     (poke) Inode[]  0#B

   We could of course add the data and padding explicitly to the inode
structure:

     type Inode =
       struct
       {
         string filename;
         int perms;
         offset<uint<32>,B> data_size;
         byte[data_size] data;
         byte[alignto (data_size, 128#B) padding;
       };

   Then we could just map 'Inode[] @ 0#B' and we would the get expected
result.

   But this is not a good idea.  On one hand because, as we know, this
would imply mapping the full file system data byte by byte, and that
would be very very slow.  On the other hand, because the data is not
part of the inode, conceptually speaking.

   A better solution is to use this idiom:

     type Inode =
       struct
       {
         string filename;
         int perms;
         offset<uint<32>,B> data_size;

         byte[0] @ OFFSET + data_size + alignto (data_size, 128#B);
       };

   This uses an anonymous field at the end of the struct type, of size
zero, located at exactly the offset where the data plus padding would
end in the version with explicit fields.

   This later solution is fast and still allows us to get an array of
inodes reflecting the whole file system with:

     (poke) var inodes = Inode[] @ 0#B

   Like in the previous sections, a method =get_data_offset= can be
added to the struct type in order to allow accessing the data blocks
corresponding to a given inode.

4.12 Dealing with Alternatives
==============================

4.12.1 BSON
-----------

BSON (1) is a binary encoding for JSON documents.  The top-level entity
described by the spec is the 'document', which contains the total size
of the document, a sequence of elements, and a trailing null byte.

   We can describe the above in Poke with the following type definition:

     type BSON_Doc =
       struct
       {
         offset<int<32>,B> size;
         BSON_Elem[size - size'size - 1#B] elements;
         byte endmark : endmark == 0x0;
       };

   BSON elements come in different kinds, which correspond to the
different types of JSON entities: 32-bit integers, 64-bit integers,
strings, arrays, timestamps, and so on.  Every element starts with a
tag, which is a 8-bit unsigned integer that identifies it's kind, and a
name encoded as a NULL-terminated string.  What comes next depends on
the kind of element.

   The following Poke type definition describes a subset of BSON
elements, namely integers, big integers and strings:

     type BSON_Elem =
       struct
       {
         byte tag;
         string name;

         union
         {
           int32 integer32 : tag == 0x10;
           int64 integer64 : tag == 0x12;
           BSON_String str : tag == 0x02;
         } value;
       };

   The union in 'BSON_Elem' corresponds to the variable part.  When poke
decodes an union, it tries to decode each alternative (union field) in
turn.  The first alternative that is successfully decoded without
raising a constraint violation exception is the chosen one.  If no
alternative can be decoded, a constraint violation exception is raised.

   To see this process in action, let's use the BSON corresponding to
the following little JSON document:

     {
         "name" : "Jose E. Marchesi",
         "age" : 40,
         "big" : 1076543210012345
     }

   Let's take a look to the different elements:

     (poke) .load bson.pk
     (poke) var d = BSON_Doc  0#B
     (poke) d.elements'length
     0x3UL
     (poke) d.elements[0]
     BSON_Elem {
       tag=0x2UB,
       name="name",
       value=struct {
         str=BSON_String {
           size=0x11,
           value="Jose E. Marchesi",
           chars=[0x4aUB,0x6fUB,0x73UB,0x65UB...]
         }
       }
     }
     (poke) d.elements[1]
     BSON_Elem {
       tag=0x10UB,
       name="age",
       value=struct {
         integer32=0x27
       }
     }
     (poke) d.elements[2]
     BSON_Elem {
       tag=0x12UB,
       name="big",
       value=struct {
         integer64=0x3d31c3f9e3eb9L
       }
     }

   Note how unions decode into structs featuring different fields.  What
field is available depends on the alternative chosen while decoding.

   In the session above, 'd.elements[1].value' contains an 'integer32'
field, whereas 'd.elements[2].value' contains an 'integer64' field.
Let's see what happens if we try to access the wrong field:

     (poke) d.elements[1].value.integer64
     unhandled invalid element exception

   We get a run-time exception.  This kind of errors cannot be catched
at compile time, since both 'integer32' and 'integer64' are potentially
valid fields in the union value.

4.12.2 Unions are Tagged
------------------------

Unlike in C, Poke unions are tagged.  Unions have their own lexical
closures, and it is the capured values that determine what field is
chosen at every time.  Wherever the union goes, its tag accompanies it.

   To see this more clearly, consider the following alternative Poke
description of the BSON elements:

     type BSON_Elem =
       union
       {
         struct
         {
           byte tag : tag == 0x10;
           string name;
           int32 value;
         } integer32;

         struct
         {
           byte tag : tag == 0x12;
           string name;
           int64 value;
         } integer64;

         struct
         {
           byte tag : tag == 0x12;
           string name;
           BSON_String value;
         } str;
       };

   This description is way more verbose than the one used in previous
sections, but it shows a few important properties of Poke unions.

   First, the constraints guiding the decoding are not required to
appear in the union itself: it is a recursive process.  In this example,
'BSON_String' could have constraints on it's own, and these constraints
will also impact the decoding of the union.

   Second, there are generally many different ways to express the same
plain binary using different type structures.  This is no different than
getting different parse trees from the same sequence of tokens using
different grammars denoting the same language.

   See how different a BSON element looks (and feels) using this
alternative description:

     (poke) d.elements[0]
     BSON_Elem {
       str=struct {
         tag=0x2UB,
         name="name",
         value=BSON_String {
           size=0x11,
           value="Jose E. Marchesi",
           chars=[0x4aUB,0x6fUB,0x73UB,0x65UB...]
         }
       }
     }
     (poke) d.elements[1]
     BSON_Elem {
       integer32=struct {
         tag=0x10UB,
         name="age",
         value=0x27
       }
     }
     (poke) d.elements[2]
     BSON_Elem {
       integer64=struct {
         tag=0x12UB,
         name="big",
         value=0x3d31c3f9e3eb9L
       }
     }

   What is the best way?  It certainly depends on the kind of data you
want to manipulate, and the level of abstraction you want to achieve.
Ultimately, it is up to you.

   Generally speaking, the best structuring is the one that allows you
to manipulate the data in terms of the structured abstractions as
naturally as possible.  That's the art and craft of writing good
pickles.

   ---------- Footnotes ----------

   (1) http://bsonspec.org/spec.html

4.13 Structured Integers
========================

When we structure data using Poke structs, arrays and the like, we often
use the same structure than a C programmer would use.  For example, to
model ELF RELA structures, which are defined in C like:

     typedef struct
     {
       Elf64_Addr   r_offset;  /* Address */
       Elf64_Xword  r_info;    /* Relocation type and symbol index */
       Elf64_Sxword r_addend;  /* Addend */
     } Elf64_Rela;

we could use something like this in Poke:

     type Elf64_Rela =
       struct
       {
         Elf64_Addr r_offset;
         Elf64_Xword r_info;
         Elf64_Sxword r_addend;
       };

   Here the Poke struct type is pretty equivalent to the C incarnation.
In both cases the fields are always stored in the given order,
regardless of endianness or any other consideration.

   However, there are situations where stored integral values are to be
interpreted as composite data.  This is the case of the 'r_info' field
above, which is a 64-bit unsigned integer ('Elf64_Xword') which is
itself composed by several fields, depicted here:

      63                                          0
     +----------------------+----------------------+
     |       r_sym          |      r_type          |
     +----------------------+----------------------+
     MSB                                         LSB

   In order to support this kind of composition of integers, C
programmers usually resort to either bit masking (most often) or to the
often obscure and undefined behaviour-prone C bit fields.  In the case
of ELF, the GNU implementations define a few macros to access these
"sub-fields":

     #define ELF64_R_SYM(i)         ((i) >> 32)
     #define ELF64_R_TYPE(i)        ((i) & 0xffffffff)
     #define ELF64_R_INFO(sym,type) ((((Elf64_Xword) (sym)) << 32) + (type))

   Where 'ELF64_R_SYM' and 'ELF64_R_TYPE' are used to extract the fields
from an 'r_info', and 'ELF64_R_INFO' is used to compose it.  This is
typical of C data structures.

   We could of course mimic the C implementation in Poke:

     fun Elf64_R_Sym = (Elf64_Xword i) uint<32>:
        { return i .>> 32; }
     fun Elf64_R_Type = (Elf64_Xword i) uint<32>:
        { return i & 0xffff_ffff; }
     fun Elf64_R_Info = (uint<32> sym, uint<32> type) Elf64_Xword:
        { return sym as Elf64_Xword <<. 32 + type; }

   However, this approach has a huge disadvantage: since we are not able
to encode the logic of these "sub-fields" in proper Poke fields, they
become second class citizens, with all that implies: no constraints on
their own, can't be auto-completed, can't be assigned individually, etc.

   But we can use the so-called "integral structs"!  These are structs
that are defined exactly like your garden variety Poke structs, with a
small addition:

     type Elf64_RelInfo =
       struct uint<64>
       {
         uint<32> r_sym;
         uint<32> r_type;
       };

   Note the 'uint<64>' addition after 'struct'.  This can be any integer
type (signed or unsigned).  The fields of an integral struct should be
integral themselves (this includes both integers and offsets) and the
total size occupied by the fields should be the same size than the one
declared in the struct's integer type.  This is checked and enforced by
the compiler.

   The Elf64 RELA in Poke can then be encoded like:

     type Elf64_Rela =
       struct
       {
         Elf64_Addr r_offset;
         struct Elf64_Xword
         {
           uint<32> r_sym;
           uint<32> r_type;
         } r_info;
         Elf64_Sxword r_addend;
       };

   When an integral struct is mapped from some IO space, the total
number of bytes occupied by the struct is read as a single integer
value, and then the values of the fields are extracted from it.  A
similar process is using when writing.  That is what makes it different
with respect a normal Poke struct.

   It is possible to obtain the integral value corresponding to an
integral struct using a cast to an integral type:

     (poke) type Foo = struct int<32> { int<16> hi; uint<16> lo; };
     (poke) Foo { hi = 1 } as int<32>
     0x10000

   An useful idiom, that doesn't require to specify an explicit integral
type, is this:

     (poke) type Foo = struct int<32> { int<16> hi; uint<16> lo; };
     (poke) var x = Foo @ 0#B;
     (poke) +x
     0xfe00aa

   These casts allow to "integrate" struct values explicitly, but the
compiler also implicitly promotes integral struct values to integers in
all contexts where an integer is expected:

   * Operator to an addition, subtraction, multiplication, division,
     ceil division, bit left shift, bit right shift, bit-wise not,
     bit-wise and, bit-wise or, bit-wise xor, logical and, logical or,
     logical not, logical implication, positive, negative,
     bit-concatenation, condition in conditional expression.
   * Argument to a function whose formal expects an integer.
   * Value returned from a function that returns an integer.
   * Condition in loop statement.
   * Constraint expression in a struct field.
   * Expression in a conditional struct field.

   Note that the above list doesn't include relational operators, since
these operators work with struct values.

4.14 Working with Incorrect Data
================================

We have seen how Poke type definitions very often include constraint
expressions reflecting the integrity of the data stored in these types.

   Consider for example the following abbreviated description of the
header of an ELF file:

     type Elf64_Ehdr =
       struct
       {
         struct
         {
           byte[4] ei_mag = [0x7fUB, 'E', 'L', 'F'];
           byte ei_class;
           [...]
         } e_ident;

         Elf_Half e_type;
         Elf_Half e_machine;
         Elf_Word e_version = EV_CURRENT;
         [...]
         Elf_Half e_shstrndx : e_shnum == 0 || e_shstrndx < e_shnum;
       };

There are three constraint expressions in the definition above:

   * The constraint expression in the field 'ei_mag' makes sure that a
     right magic number begins the header.
   * The constraint expression in 'e_version' checks that the ELF
     version is the current version.
   * The constraint expression in 'e_shstrndx' checks that the index
     stored in the field doesn't overflow the section header table of
     the file.

   Every time we construct or map an 'Elf64_Ehdr' value the constraint
expressions are checked.  In case any of the constraints are not
satisfied we get a "constraint violation exception".  This is useful to
realize that we are facing invalid data (when mapping) or that we are
trying to build an invalid ELF header (when constructing.)

   However, the exception avoids the completion of the mapping or
construction operation.  Sometimes this is inconvenient:

   * When we are given invalid data that we want to explore or fix.
     Imagine for example a programmer investigating a possible assembler
     or linker bug that results in an invalid ELF file.
   * When we are discovering or inferring the structure of some data: we
     suspect at some point there may be something that is similar to an
     ELF header.
   * When we want to create invalid data.  Suppose for example a
     programmer who is building a test case for the linker, that
     requires a corrupted ELF file.

   The way poke provides support for these situation is using the
concept of "map strictness".  Suppose we map an ELF header in some IO
space:

     (poke) load elf
     (poke) var ehdr = Elf64_Ehdr @ 0#B

The mapping operator '@' performs a "strict mapping".  This means that
constraint expressions are evaluated and the operation is interrupted if
any constraint fails, as described above.  Also, this means the
integrity will be checked when the data is modified:

     (poke) ehdr.e_version = 666
     unhandled constraint violation exception

So the mapping operator generates "strict" values.  We can check the
strictness of a mapped value using the ''strict' attribute:

     (poke) ehdr'strict
     1

   If we know that we are facing corrupted data, or if we want to
corrupt the ELF header, we perform a "non-strict mapping" using a
variant of the mapping operator:

     (poke) var ehdr_ns = Elf64_Ehdr @! 0#B
     (poke) ehdr_ns'strict
     0

We can corrupt the header using this non-strict value:

     (poke) ehdr_ns.e_version = 666

5 Maps and Map-files
********************

Editing data with GNU poke mainly involves creating mapped values and
storing them in Poke variables.  However, this may not be that
convenient when poking several files simultaneously, and when the
complexity of the data increases.  poke provides a convenient mechanism
for this: maps and map files.

5.1 Editing using Variables
===========================

Editing data with GNU poke mainly involves creating mapped values and
storing them in Poke variables.  For example, if we were interested in
altering the fields of the header in an ELF file, we would map an
'Elf64_Ehdr' struct at the beginning of the underlying IO space (the
file), like in:

     (poke) .file foo.o
     (poke) load elf
     (poke) var ehdr = Elf64_Ehdr @ 0#B

   At this point the variable 'ehdr' holds an 'Elf64_Ehdr' structure,
which is mapped.  As such, altering any of the fields of the struct will
update the corresponding bytes in 'foo.o'.  For example:

     (poke) ehdr.e_entry = 0#B

   A Poke value has three mapping related attributes: whether it is
mapped, the offset at which it is mapped in an IO space, and in which IO
space.  This information is accessible for both the user and Poke
programs using the following attributes:

     (poke) ehdr'mapped
     1
     (poke) ehdr'offset
     0UL#b
     (poke) ehdr'ios
     0

   Thats it, 'ehdr' is mapped at offset zero byte in the IO space '#0',
which corresponds to 'foo.o':

     (poke) .info ios
       Id	Type	Mode	Bias		Size		Name
     * #0	FILE	rw	0x00000000#B	0x000004c8#B   ./foo.o

   Now that we have the ELF header, we may use it to get access to the
ELF section header table in the file, that we will reference using
another variable 'shdr':

     (poke) var shdr = Elf64_Shdr[ehdr.e_shnum] @ ehdr.e_shoff
     (poke) shdr[1]
     Elf64_Shdr {
       sh_name=0x1bU#B,
       sh_type=0x1U,
       sh_flags=#<ALLOC,EXECINSTR>,
       sh_addr=0x0UL#B,
       sh_offset=0x40UL#B,
       sh_size=0xbUL#B,
       sh_link=0x0U,
       sh_info=0x0U,
       sh_addralign=0x1UL,
       sh_entsize=0x0UL#b
     }

   Variables are convenient entities to manipulate in Poke.  Let's
suppose that the file has a lot of sections and we want to do some
transformation in every section.  It is a time consuming operation, and
we may forget which sections we have already processed and which not.
We could create an empty array to hold the sections already processed:

     (poke) var processed = Elf64_Shdr[] ()

   And then, once we have processed some given section, add it to the
array:

     ... edit shdr[23] ...
     (poke) processed += [shdr[23]]

   Note how the array =processed= is not mapped, but the sections
contained in it are mapped: Poke uses copy by shared value.  So, after
we spend the day carefully poking our ELF file, we can ask poke, are we
done with all the sections in the file?

     (poke) shdr'length == processed'length
     1

   Yes, we are.  This can be made as sophisticated as desired.  We could
easily write a function that saves the contents of 'processed' in files,
so we can continue hacking tomorrow, for example.

   We can then concluding that using mapped variables to edit data
structures stored in IO spaces works well in common and simple cases
like the above: we make our ways mapping here and there, defining
variables to hold data that interests us, and it is easy to remember
that the variables 'ehdr' and 'shdr' are mapped, where are they mapped,
and that they are mapped in the file 'foo.o'.

   However, GNU poke allows to edit more than one IO space
simultaneously.  Let's say we now want to poke the sections of another
ELF file: 'bar.o'.  We would start by opening the file:

     (poke) .file bar.o
     (poke) .info ios
       Id	Type	Mode	Bias		Size		Name
     * #1	FILE	rw	0x00000000#B	0x000004c8#B	./bar.o
       #0	FILE	rw	0x00000000#B	0x000004c8#B	./foo.o

   Now that 'bar.o' is the current IO space, we can map its header.  But
now, what variable to use?  We would rather not redefine 'ehdr', because
that is already holding the header of 'foo.o'.  We could adapt our
naming schema on the fly:

     (poke) var foo_ehdr = ehdr
     (poke) var bar_ehdr = Elf64_Ehdr @ 0#B

   But then we would need to do the same for the other variables too:

     (poke) var foo_shdr = shdr
     (poke) var bar_shdr = Elf64_Shdr[bar_ehdr.e_shnum] @ bar_ehdr.e_shoff

   However, we can easily see how this can degenerate quickly: what
about 'processed', for example?  In general, as the number of IO spaces
being edited increases it becomes more and more difficult to manage our
mapped variables, which are associated to each IO space.

5.2 poke Maps
=============

As we have seen mapping variables is a very powerful, general and
flexible mean to edit stored binary data in one or more IO spaces.
However it is easy to lose track of where the variables are mapped and,
ideally speaking, we would want to have a mean to refer to, say, the
"ELF header", and get the header as a mapped value regardless of what
specific file we are editing.  Sort of a "meta variable".  GNU poke
provides a way to do this: "maps".

   A "map" can be conceived as a sort of "view" that can be applied to a
given IO space.  Maps have entries, which are values mapped at some
given offset, under certain conditions.  For example, we have seen an
ELF file contains, among other things, a header at the beginning of the
file and a table of section headers of certain size and located at
certain location determined by the header.  These would be two entries
of a so-called ELF map.

   poke maps are defined in "map files".  These files use the '.map'
extension.  A map file 'self.map' (for sectioned/simple elf) defining
the view of an ELF file as a header and a table of section header would
look like this:

     /* self.map - map file for a simplified view of an ELF file.  */

     load elf;

     %%

     %entry
     %name ehdr
     %type Elf64_Ehdr
     %offset 0#B

     %entry
     %name shdr
     %type Elf64_Shdr[(Elf64_Ehdr @ 0#B).e_shnum]
     %condition (Elf64_Ehdr @ 0#B).e_shnum > 0
     %offset (Elf64_Ehdr @ 0#B).e_shoff

   This map file defines a view of an ELF file as a header entry 'ehdr'
and an entry with a table of section headers 'shdr'.

   The first section of the file, which spans until the separator line
containing '%%', is arbitrary Poke code which as we shall see, gets
evaluated before the map entries are processed.  This is called the map
"prologue".  In this case, the prologue contains a comment explaining
the purpose of the file, and a single statement 'load' that loads the
'elf.pk' pickle, since the entries below use definitions like
'Elf64_Ehdr' that are defined by that pickle.  The prologue is useful to
define Poke functions and other entities that are then used in the
definitions of the entries.

   A separator line containing only '%%' separates the prologue from the
next section, which is a list of entries definitions.  Each entry
definition starts with a line '%entry', and has the following
attributes:

'%name'
     Like 'ehdr' and 'shdr'.  These names should follow the same rules
     than Poke variables, but as we shall see later, map entries are not
     Poke variables.  This attribute is mandatory.

'%type'
     This can be any Poke expression denoting a type, like 'int',
     'Elf64_Ehdr' or 'Elf64_Shdr[(Elf64_Ehdr @ 0#B).e_shnum]'.  This
     attribute is mandatory.

'%condition'
     If specified, will determine whether to include the entry in the
     map.  In the example above, the map will have an entry 'shdr' only
     if the ELF file has one or more sections.  Any Poke expression
     evaluating to a boolean can be used as conditions.  This attribute
     is optional: entries not having a condition will always be included
     in the map.

'%offset'

     Offset in the IO space where the entry will be mapped.  Any Poke
     expression evaluating to an offset can be used as entry offset.
     This attribute is mandatory.

5.3 Loading Maps
================

So we have written our =self.map=, which denotes a view or structure of
ELF files we are interested on, and that resides in the current working
directory.  How to use it?

   The first step is to fire up poke and open some object file.  Let's
start with 'foo.o':

     (poke) .file foo.o

Now, we can load the map using the '.map load' dot-command:

     (poke) .map load self
     [self](poke)

   The '.map load self' command makes poke to look in certain
directories for a file called 'self.map', and to load it.  The list of
directories where poke looks for map files is encoded in the variable
'map_load_path' as a string containing a maybe empty list of directories
separated by ':' characters.  Each directory is tried in turn.  This
variable is initialized with suitable defaults:

     (poke) map_load_path
     "/home/jemarch/.poke.d:.:/home/jemarch/.local/share/poke:[...]"

   Once a map is loaded, observe how the prompt changed to contain a
prefix '[self]'.  This means that the map 'self' is loaded for the
current IO space.  You can choose to not see this information in the
prompt by setting the 'prompt-maps' option either at the prompt or in
your '.pokerc':

     poke) .set prompt-maps no

   By default 'prompt-maps' is 'yes'.  This prompt aid is intended to
provide a cursory look of the "views" or maps loaded for the current IO
space.  If we load another IO space and switch to it, the prompt changes
accordingly:

     (poke) [self](poke) .mem foo
     The current IOS is now `*foo*'.
     (poke) .ios #0
     The current IOS is now `./foo.o'.
     [self](poke)

   At any time the '.info maps' dot-command can be used to obtain a full
list of loaded maps, with more information about them:

     (poke) .info maps
     IOS   Name   Source
     #0    self   ./self.map

   In this case, there is a map 'self' loaded in the IO space '#0',
which corresponds to 'foo.o'.

   Once we make 'foo.o' our current IO space, we can ask poke to show us
the entries corresponding to this map using another dot-command:

         : (poke) .map show self
         : Offset     Entry
         : 0x0UL#B    $self::ehdr
         : 0x208UL#B  $self::shdr

   This tells us there are two entries for 'self' in 'foo.o':
'$self::ehdr' and '$self::shdr=' Note how map entries use names that
start with the '$' character, then contain the name of the map an the
name of the entry we defined in the map file, separated by '::'.

   We can now use these entries at the prompt like if they were regular
mapped variables:

     [self](poke) $self::ehdr
     Elf64_Ehdr {
       e_ident=struct {
         ei_mag=[0x7fUB,0x45UB,0x4cUB,0x46UB],
         [...]
       },
      e_type=0x1UH,
      e_machine=0x3eUH,
      [...]
     }
     (poke) $self::shdr'length
     11UL

   It is important to note, however, that map entries like '$foo::bar'
are _not_ part of the Poke language, and are only available when using
poke interactively.  Poke programs and scripts can't use them.

   Let's now open another ELF file, and the =self= map in it:

     (poke) .file /usr/local/lib/libpoke.so.0.0.0
     (poke) .map load self
     [self](poke)

   So now we have two ELF files loaded in poke: 'foo.o' and
'libpoke.so.0.0.0', and in both IO spaces we have the 'self' map loaded.
We can easily see that the map entries are different depending on the
current IO space:

     [self](poke) .map show self
     Offset       Entry
     0UL#B        $self::ehdr
     3158952UL#B  $self::shdr
     [self](poke) .ios #0
     The current IOS is now `./foo.o'.
     [self](poke) .map show self
     Offset   Entry
     0UL#B    $self::ehdr
     520UL#B  $self::shdr

'foo.o' is an object file, whereas 'libpoke.so.0.0.0' is a DSO:

     (poke) .ios #0
     The current IOS is now `./foo.o'.
     [self](poke) $self::ehdr.e_type
     1UH
     [self](poke) .ios #2
     The current IOS is now `/usr/local/lib/libpoke.so.0.0.0'.
     [self](poke) $self::ehdr.e_type
     3UH

   The interpretation of the map entry '$self::ehdr' is different
depending on the current IO space.  This makes it possible to refer to
the "ELF header" of the current file.

   Underneath, poke implements this by defining mapped variables and
"redirecting" the entry names '$foo::bar' to the right variable
depending on the IO space that is currently selected.  It hides all that
complexity from us.

5.4 Multiple Maps
=================

It is perfectly possible (and useful!)  to load more than one map in the
same IO space.  It is very natural for a single file, for example, to
contain data that can be interpreted in several ways, or of different
nature.

   Let's for example open again an ELF file, this time compiled with
'-g':

     (poke) .file foo.o

   We now load our 'self' map, to get a view of the file as a collection
of sections:

     (poke) .map load self
     [self](poke)

   And now we load the 'dwarf' map that comes with poke, to get a view
of the file as having debugging information encoded in DWARF:

     [self(poke) .map load dwarf
     [dwarf,self](poke)

   See how the prompt now reflects the fact that the current IO space
contains DWARF info!  Let's take a look:

     [dwarf,self](poke) .info maps
     IOS   Name    Source
     #0    dwarf   /home/jemarch/gnu/hacks/poke/maps/dwarf.map
     #0    self    ./self.map
     [dwarf,self](poke) .map show dwarf
     Offset    Entry
     0x5bUL#B  $dwarf::info

   Now we can access entries from any of the loaded maps, i.e.  access
the file in terms of different perspectives.  As an ELF file:

     [dwarf,self](poke) $self::shdr[1]
     Elf64_Shdr {
       sh_name=0xb5U#B,
       sh_type=0x11U,
       sh_flags=#<>,
       sh_addr=0x0UL#B,
       sh_offset=0x40UL#B,
       sh_size=0x8UL#B,
       sh_link=0x18U,
       sh_info=0xfU,
       sh_addralign=0x4UL,
       sh_entsize=0x4UL#b
     }

And as a file containing DWARF info:

     [dwarf,self](poke) $dwarf::info
     Dwarf_CU_Header {
       unit_length=#<0x0000004eU#B>,
       version=0x4UH,
       debug_abbrev_offset=#<0x00000000U#B>,
       address_size=0x8UB#B
     }

   If you are curious about how the DWARF entries are defined, look at
'maps/dwarf.map' in the poke source distribution, or in your installed
poke ('.info maps' will tell you the file the map got loaded from.)

   It is possible to unload or remove a map from a given IO space using
the '.map remove' dot-command.  Say we are done looking at the DWARF in
'foo.o', and we are no longer interested in it as a file containing
debugging info.  We can do:

     [dwarf,self](poke) .map remove dwarf
     [self](poke)

   Note how the prompt was updated accordingly: only 'self' remains as a
loaded map on this file.

5.5 Auto-map
============

Certain maps make sense when editing certain types of data.  For
example, 'dwarf.map' is intended to be used in ELF files.  In order to
ease using maps, poke provides a feature called "auto mapping", which is
disabled by default.

You can set auto mapping like this:

     (poke) .set auto-map yes

   When auto mapping is enabled, poke will look to the value of the
pre-defined variable 'auto_map', which must contain an array of pairs of
strings, associating a regular expression with a map name.

   For example, you may want to initialize 'auto_map' like this in your
'.pokerc' file:

     auto_map = [[".*\\.mp3$", "mp3"],
                 [".*\\.o$", "elf"],
                 ["a\\.out$", "elf"]];

   This will make poke to load =mp3.map= for every file whose name ends
with '.mp3', and 'elf.map' for files having names like 'foo.o' and
'a.out'.

   Following the usual pokeish philosophy of being as less as intrusive
by default as possible, the default value of 'auto_map' is the empty
string.

5.6 Constructing Maps
=====================

As we have seen, we can define our own maps using map files like
'self.map', which contain a prologue and a set of map entries.  However,
sometimes it is useful to create maps "on the fly" while we explore some
data with poke.

   To make this possible, poke provides a suitable set of dot-commands.
Let's say we are poking some data, and we want to create a map for it.
We can do that like this:

     (poke) .map create mymap

This creates an empty map named 'mymap', with no entries:

     [mymap](poke) .map show mymap
     Offset   Entry

   Adding entries is easy.  First, we have to map some variable, and
then use it as the base for the new entry:

     [mymap](poke) var foo = int[3]  0#B
     [mymap](poke) .map entry add mymap, foo
     [mymap](poke) .map show mymap
     Offset   Entry
     0x0UL#B  $mymap::foo

   Note how the entry '$mymap::foo' gets created, associated to the
current IO space and mapped at the same offset than the variable 'foo'.

   We can remove entries from existing maps using the '.map entry
remove' dot-command:

     [mymap](poke) .map entry remove mymap, foo
     [mymap](poke) .map show mymap
     Offset   Entry
     [mymap](poke)

   We plan to add an additional command to save maps to map files.  The
idea is that you can create your maps on the fly, save them, and then
load them back some other day when you are ready to continue poking.
This is not implemented yet though.

5.7 Predefined Maps
===================

GNU poke comes with a set of useful pre-written maps, which get
installed in a system location.  We want to expand this collection, so
please send us your map files!

6 Writing Pickles
*****************

GNU poke encourages the user to write little pieces of code in order to
face spontaneous needs and fix situations.  Is that name in the file
encoded in a fixed array of characters padded with white spaces?  No
problem, just write a three lines function so you can update the file
using a comfortable NULL-terminated string.  Better than waiting for
some poke maintainer to add that function for you, isn't it?

   Just save your functions in some personal '.pk' file that you load at
startup, and your magic tricks bag will increase in time, making your
poking more and more efficient.

   However, when it comes to share the code with other people, it is
important to follow certain conventions in order to achieve certain
uniformity.  This makes it easier for other people to discover what your
hack provides, and how it works.  It is this consistency, and these
conventions, that makes some random '.pk' file a "pickle".

   This guide contains guidelines and recommendations for the pickle's
writer.

6.1 Pretty-printers
===================

6.1.1 Convention for pretty-printed Output
------------------------------------------

Very often the structure of the data encoded in binary is not very
intelligible.  This is because it is usual for binary formats to be
designed with goals in mind other than being readable by humans:
compactness, detail etc.

   In our pickle we of course want to provide access to the very finer
detail of the data structures.  However, we also want for the user to be
able to peruse the data visually, and only look at the fine detail on
demand.

   Consider for example an ID3V1 tag data from some MP3 file.  This is
the result of mapping a 'ID3V1_Tag':

     ID3V1_Tag {
       id=[0x54UB,0x41UB,0x47UB],
       title=[0x30UB,0x31UB,0x20UB,0x2dUB,0x20UB,...],
       artist=[0x4aUB,0x6fUB,0x61UB,0x71UB,0x75UB,...],
       album=[0x4dUB,0x65UB,0x6eUB,0x74UB,0x69UB,...],
       year=[0x20UB,0x20UB,0x20UB,0x20UB],
       data=struct {
         extended=struct {
           comment=[0x20UB,0x20UB,0x20UB,0x20UB,0x20UB,...],
           zero=0x0UB,
           track=0x1UB
         }
       },
       genre=0xffUB
     }

   Not very revealing.  Fortunately, poke supports pretty printers.  If
a struct type has a method called '_print', it will be used by poke as a
pretty printer if the 'pretty-print' option is set:

     (poke) .set pretty-print yes

   By convention, the output of pretty-printers should always start with
'#<' and end with '>'.  The convention makes it explicit for the user
that everything she sees between '#<' and '>' is pretty-printed, and do
_not_ necessarily reflect the physical structure of the data.  Also some
information may be missing.  In order to get an exact and complete
description of the data, the user should '.set pretty-print no' and
evaluate the value again at the prompt.

   For example, in the following BPF instructions it is obvious at first
sight that the shown register values are pretty-printed:

     BPF_Insn = {
       ...
       regs=BPF_Regs {
          src=#<%r3>,
          dst=#<%r0>
       }
       ...
     }

   If the pretty-printed representation spans for more than one line,
please place the opening '#<' in its own line, then the lines with the
data, and finally '>' in its own line, starting at column 0.

   Example of the MP3 tag above, this time pretty-printed:

     #<
       genre: 255
       title: 01 - Eclipse De Mar
       artist: Joaquin Sabina
       album: Mentiras Piadosas
       year:
       comment:
       track: 1
     >

6.1.2 Pretty Printing Optional Fields
-------------------------------------

Let's say we are writing a pretty-printer method for a struct type that
has an optional field.  Like for example:

     type Packet =
       struct
       {
         byte magic : magic in [MAGIC1,MAGIC2];
         byte n;
         byte[n] payload;
         PacketTrailer trailer if magic == MAGIC2;
       }

In this case, the struct value will have a 'trailer' conditionally,
which has to be tackled on the pretty-printer somehow.

   An approach that often works good is to replicate the logic in the
optional field condition expression, like this:

       struct
       {
         byte magic : magic in [MAGIC1,MAGIC2];
         [...]
         PacketTrailer trailer if packet_magic2_p (magic);

         method _print = void:
         {
           [...]
           if (magic == MAGIC2)
             pretty_print_trailer;
         }
       }

This works well in this simple example.  In case the expression is big
and complicated, we can avoid rewriting the same expression by
encapsulating the logic in a function:

     fun packet_magic2_p = int: { return magic == MAGIC2; }
     type Packet =
       struct
       {
         byte magic : magic in [MAGIC1,MAGIC2];
         [...]
         PacketTrailer trailer if packet_magic2_p (magic);

         method _print = void:
         {
           [...]
           if (packet_magic2_p (magic))
             pretty_print_trailer;
         }
       }

However, this may feel weird, as the internal logic of the type somehow
leaks its natural boundary into the external function 'packet_magic2_p'.

   An alternative is to use the following idiom, that checks whether the
field actually exists in the struct:

     type Packet =
       struct
       {
         byte magic : magic in [MAGIC1,MAGIC2];
         [...]
         PacketTrailer trailer if packet_magic2_p (magic);

         method _print = void:
         {
           [...]
           try pretty_print_trailer;
           catch if E_elem {}
         }
       }

This approach also works with unions:

     type ID3V1_Tag =
       struct
       {
         [...]
         union
         {
           /* ID3v1.1  */
           struct
           {
             char[28] comment;
             byte zero = 0;
             byte track : track != 0;
           } extended;
           /* ID3v1  */
           char[30] comment;
         } data;
         [...]

         method _print = void:
         {
           [...]
           try print "  comment: " + catos (data.comment) + "\n";
           catch if E_elem
           {
             print "  comment: " + catos (data.extended.comment) + "\n";
             printf "  track: %u8d", data.extended.track;
           }
         }
       }

6.2 Setters and Getters
=======================

Given a struct value, the obvious way to access the value of a field is
to just refer to it using dot-notation.

   For example, for the following struct type:

     type ID3V1_Tag =
       struct
       {
         [...]
         char[30] title;
         char[30] artist;
         char[30] album;
         char[4] year;
         [...]
       }

   Suppose the find out the year in the tag is wrong, off by two years:
the song was release in 1980, not in 1978!.  Unfortunately, due to the
bizarre way the year is stored in the file (as a sequence of digits
encoded in ASCII, non-NULL terminated) we cannot just write:

     (poke) tag.year = tag.year + 2
     error

   Instead, we can use facilities from the standard library and a bit of
programming:

     (poke) stoca (format ("%d", atoi (catos (tag.year)) + 2), tag.year)

   The above line basically transforms the data we want to operate on
(the tag year) from the stored representation into a more useful
representation (from an array of character digits to an integer) then
operates with it (adds two) then converts back to the stored
representation.  Let's call this "more useful" representation the
"preferred representation".

   A well written pickle should provide "getter" and "setter" methods
for fields in struct types for which the stored representation is not
the preferred representation.  By convention, getter and setter methods
have the following form:

     method get_FIELD PREFERRED_TYPE: { ... }
     method set_FIELD (PREFERRED_TYPE val) void: { ... }

Using the 'get_' and 'set_' prefixes consistently is very important,
because the pokist using your pickle can easily find out the available
methods for some given value using tab-completion in the REPL.

   For example, let's add setter and getter methods for the field 'year'
in the ID3V1 tag struct above:

     type ID3V1_Tag =
       struct
       {
         [...]
         char[4] year;

         method get_year = int: { return atoi (catos (year)); }
         method set_year = (int val) void:
         {
           var str = format "%d", val;
           stoca (str, year);
         }
         [...]
       }

   What constitutes the preferred representation of a field is up to the
criteria of the pickle writer.  For the tag above, I would say the
preferred representations for the title, artist, album and year are
string for title, artist and album, and an integer for the year.

7 Writing Binary Utilities
**************************

GNU poke is, first and foremost, intended to be used as an interactive
editor, either directly on the command line or using a graphical user
interface built on it.  However, since its conception poke was intended
to also provide a suitable and useful foundation on which other
programs, the so-called "binary utilities", could be written.  This
chapter shows how to write Poke scripts and programs.

7.1 Poke Scripts
================

In interactive usage, there are two main ways to execute Poke code: at
the interactive prompt (or REPL), and loading "pickles".

   Executing Poke code at the REPL is as easy as introducing a statement
or expression:

     (poke) print "Hello\n"
     Hello

   Executing Poke code in a pickle is performed by loading the file
containing the code:

     (poke) .load say-hello.pk
     Hello

Where 'say-hello.pk' contains simply:

     print "Hello\n";

   However, we would like to have Poke scripts, i.e.  to be able to
execute Poke programs as standalone programs, from the shell.  In other
words, we want to use GNU poke as an interpreter.  This is achieved by
using a shebang, which should appear at the top of the script file.  The
poke shebang looks like this:

     #!/usr/bin/poke -L
     !#

   The '-L' command line option tells poke that it is being used as an
interpreter.  Additional arguments for poke can be specified before '-L'
(but not after).  The '#! ... !#' is an alternative syntax for
multi-line comments, which allows to have the shebang at the top of a
Poke program without causing a syntax error.  This nice trick has been
borrowed from guile.

   Therefore, we could write say-hello as a Poke script like this:

     #!/usr/bin/poke -L
     !#

     print "Hello\n";

And then execute it like any other program or script:

     $ ./say-hello

7.2 Command-Line Arguments
==========================

When a Poke script is executed, the command line arguments passed to the
script become available in the array argv.  Example:

     #!/usr/bin/poke -L
     !#

     for (arg in argv)
       print "Argument: " + arg + "\n";

Executing this script results in:

     $ ./printargs foo bar 'baz quux'
     Argument: foo
     Argument: bar
     Argument: baz quux

   Note how it is not needed to have an argc variable, since the number
of elements stored in a Poke array can be queried using an attribute:
'argv'length'.

   Note also that 'argv' is only defined when poke runs as an
interpreter:

     $ poke
     [...]
     (poke) argv
     <stdin>:1:1: error: undefined variable 'argv'
     argv;
     ^~~~

   Accessing the 'argv' is more than enough for many simple programs.
However, we may need a more sophisticated handling of command-line
options: support for both short and long style options, adherence to the
GNU coding standards, and the like.  For these cases poke provides a
pickle 'argp'.  *Note argp::.

7.3 Exiting from Scripts
========================

By default a Poke script will communicate a successful status to the
environment, upon exiting:

     $ cat hello
     #!/usr/bin/poke -L
     !#

     print "hello\n";
     $ ./hello && echo $?
     0

   In order to exit with some other status code, most typically to
signal an erroneous situation, the Pokeish way is to raise an 'E_exit'
exception with the desired exit status code:

     raise Exception { code = EC_exit, exit_status = 1 };

   This can be a bit cumbersome to write, so poke provides a more
conventional syntax in the form of an 'exit' function:

     fun exit = (int<32> exit_code = 0) void:
     {
       raise Exception { code = EC_exit, exit_status = exit_code };
     }

Using 'exit', the above raise statement becomes the much simpler:

     exit (1);

7.4 Loading pickles as Modules
==============================

Suppose we want to write a Poke program to extract the contents of
sections in a given ELF object file.  Extracting sections requires
dealing with several data structures encoded in the ELF file, such as
the header, the section header table, the string table (that contains
the names of the sections) and so on.  It would be of course possible to
define Poke types for these structures in the script itself but, as it
happens, GNU poke ships with an already written pickle that describes
the ELF structures.  It is called 'elf.pk'.

   So a script needing to mess with ELF data structures can just make
use of 'elf.pk' using the load construction:

     load elf;

   This looks for a file called 'elf.pk' in a set of directories, which
are predefined by poke, and loads it.  The list of directories where
poke looks for pickles is stored in the load_path variable as a colon
separated list of directory names, and can be customized:

     $ poke
     [...]
     (poke) load_path
     "/home/jemarch/.poke.d:.:/home/jemarch/.local/share/poke:..."

   The default value of 'load_path' contains both user-specific
directories and system-wide directories.  This assures that all the
pickles installed by poke are available, and that the user can load her
own pickles in her scripts.

   Once a pickle is loaded in a script the types, functions and
variables defined in it (either directly or indirectly by loading its
own pickles) become available.

7.5 elfextractor
================

As an example, in this section we will be hacking a very simple utility
called elfextractor, that extracts the contents of the sections of an
ELF file, whose name is provided as an argument in the command line,
into several output files.  This is the synopsis of the program:

     elfextractor FILE [SECTION_NAME]

   Where FILE is the name of the ELF file from which to extract
sections, and an optional SECTION_NAME specifies the name of the section
to extract.

   Say we have a file 'foo.o' and we would like to extract its text
section.  We would use elfextractor like:

     $ elfextractor foo.o .text

   Provided 'foo.o' indeed has a section named '.text', the utility will
create a file 'foo.o.text' with the section's contents.  Note how the
names of the output files are derived concatenating the name of the
input ELF file and the name of the extracted section.

   If no section name is specified, then all sections are extracted.
For example:

     $ elfextractor foo.o
     $ ls foo.o*
     foo.o          foo.o.eh_frame       foo.o.shstrtab  foo.o.symtab
     foo.o.comment  foo.o.rela.eh_frame  foo.o.strtab    foo.o.text

   This is a possible implementation of elfextractor:

     #!/usr/bin/poke -L
     !#

     /* elfextractor - Extract sections from ELF64 files. */

     load elf;

     if (!(argv'length in [1,2]))
       {
         print "Usage: elfextractor FILE [SECTION_NAME]\n";
         exit (1);
       }

     var file_name = argv[0];
     var section_name = (argv'length > 1) ? argv[1] : "";

     try
       {
         var fd = open (file_name, IOS_M_RDONLY);
         var elf = Elf64_File @ fd : 0#B;

         for (shdr in elf.shdr where shdr.sh_type != 0x0)
           {
             var sname = elf.get_string (shdr.sh_name);

             if (section_name == "" || sname == section_name)
               save :ios elf'ios :file file_name + sname
                    :from shdr.sh_offset :size shdr.sh_size;
           }

         close (fd);
       }
     catch (Exception e)
       {
         if (e == E_constraint)
           printf ("error: `%s' is not a valid ELF64 file\n", file_name);
         else if (e == E_io)
           printf ("error: couldn't open file `%s'\n", file_name);
         else
           raise e;

         exit (1);
       }

   First the command line arguments are handled.  The script checks
whether the right number of arguments have been passed (either 1 or 2)
exiting with an error code otherwise.  The file name and the section
name are then extracted from the 'argv' array.

   Once we have the file name and the optional desired section name, it
is time to do the real work.  The code is enclosed in a try-catch block
statement, because some of the operations may result on exceptions being
raised.

   First, the ELF file whose name is specified in the command line is
opened for reading:

     var fd = open (file_name, IOS_M_RDONLY);

   The built-in function 'open' returns a file descriptor that can be
subsequently used in mapping operations.  If the provided file name
doesn't identify a file, or if the file can't be read for whatever
reason, an 'E_io' exception is raised.  Note how the exception is
handled in the 'catch' block, emitting an appropriate diagnostic message
and exiting with an error status.

   Once the ELF file is open for reading, we map an 'Elf64_File' on it,
at the expected offset (zero bytes from the beginning of the file):

     var elf = Elf64_File @ fd : 0#B;

   If the file doesn't contain valid ELF data, this map will fail and
raise an 'E_constraint' exception.  Again, the 'catch' block handles
this situation.

   At this point the variable 'elf' contains an 'Elf64_File'.  Since we
want to extract the sections contained in the file, we need to somehow
iterate on them.  The section header table is available in 'elf.shdr'.
A for-in-where loop is used to iterate on all the headers, skipping the
"null" ELF sections which are always empty, and are characterized by a
'shdr.sh_type' of 0.  An inner conditional filters out sections whose
name do not match the provided name in the command line, if it was
specified at all.

   For each matching section we then save its contents in a file named
after the input ELF file, by calling a function 'save', which is
provided by poke:

     save :ios elf'ios :file file_name + sname
          :from shdr.sh_offset :size shdr.sh_size;

   The above is exactly what we would have written at the poke REPL!
(modulus trailing semicolon).  How is this supposed to work?  Thing is,
GNU poke commands are implemented as Poke functions.  Let's consider
'save', for example.  It is defined as a function having the following
prototype:

     fun save = (int ios = get_ios,
                   string file = "",
                   off64 from = 0#B,
                   off64 size = 0#B,
                   int append = 0,
                   int verbose = 0) void:
     { ... }

   Once a Poke function is defined in the environment, it becomes
available as such.  Therefore, in a poke session we could call it like:

     (poke) save (get_ios, "filename", 0#B, 12#B, 0, 1)

   However, this is cumbersome and error prone.  To begin with, we
should remember the name, position and nature of each argument accepted
by the command.  What is even more annoying, we are forced to provide
explicit values for them, like in the example above we have to pass the
current IOS (the default), and 0 for 'append' (the default) just to
being able to set 'verbose'.  Too bad.

   To ease commanding poke, the Poke language supports an alternative
syntax to call functions, in which the function arguments are referred
by name, can be given in any order, and can be omitted.  The command
above can be thus written like:

     (poke) save :from 0#B :size 12#B :verbose 1

   This syntax is mostly intended to be used interactively, but nothing
prevents to use it in Poke programs and scripts whenever it is deemed
appropriate, like we did in elfextractor.  We could of course have used
the more conventional syntax:

     if (section_name == "" || sname == section_name)
       save (elf'ios, file_name + sname,
             shdr.sh_offset, shdr.sh_size, 0, 0);

What style to use is certainly a matter of taste.

   Anyhow, once the sections have been written out, the file descriptor
is closed and the program exits with the default status, which is
success.  Should the 'save' function find any problem saving the data,
such as a full disk, not enough permissions or the like, exceptions will
be raised, caught and maybe handled by our 'catch' block.

   And this is it!  The complete program is 44 lines long.  This is a
good example that shows how, given a pickle providing a reasonable
description of some binary-oriented format (ELF in this case) poke can
be leveraged to achieve a lot in a very concise way, free from the many
details involved in the encoding, reading and writing of binary data.

7.6 Filters
===========

The classic notion of "filter" in Unix-like systems and other Operating
Systems is of a program that reads certain input using its standard
input, realized some transformations on it, and writes the result on its
standard output.  The filter doesn't need to operate on all the data at
once, nor to buffer it: it reads input in chunks once it becomes
available and is needed, and write output as necessary.

   GNU poke provides the so called "stream IO spaces" in order to
implement binary utilities that act like filters.

7.6.1 Stream IO Spaces
----------------------

GNU poke provides support for opening and operating on several kinds of
IO spaces: files, memory buffers, process memory, etc.  An IO space can
be opened for reading, for writing or both, and they all can be accessed
randomly, i.e.  to read or write some data from/to a particular offset
in the IO space.

   Writing a filter would involve to peek data from the standard input
and then poke the data (maybe transformed) to the standard output or the
standard error output.  However, we know that these devices are not
random oriented devices that can be accessed at any offset, but rather
stream-like devices whose special characteristics are:

   - They are either read-only or write-only.  You cannot write to an
     input stream and you cannot read from an output stream.
   - Once data is read from an input stream, it can't be read again.  It
     is not buffered.
   - Once data is written to an output stream, it can't be altered
     again.

Given the above characteristics of streams like the standard input and
output, it is clear we cannot use the File IO streams on them.  It is
also not clear how could we perform a map operation on a stream device:
in poke these operations always require an explicit offset.

   GNU poke solves this by providing three special IO spaces that the
user can open using the following handlers:

'<stdin>'
     Read-only stream IOS that is connected to the standard input of the
     poke process.
'<stdout>'
     Write-only stream IOS that is connected to the standard output of
     the poke process.
'<stderr>'
     Write-only stream IOS that is connected to the standard error of
     the poke process.

Opening these IO spaces is done exactly like with any other IO space.
Example:

     var stdin = open ("<stdin>");

7.6.2 Reading from Streams
--------------------------

Unlike normal Unix file descriptors, read-only poke streams are
buffered.  Let's supposed we open the standard input:

     var stdin = open ("<stdin>");

Then we map some data (one byte) at some particular offset:

     var b = byte @ stdin : 28#B;

The mapping operation above causes poke to read 28 bytes from the
standard input and to set the variable 'b' to the 28-th byte.  The bytes
read from the stream are still available (buffered) and the offsets are
still relative from the "beginning" of the standard input:

     var c = byte @stdin : 0#B;

The byte in 'c' is the byte that appeared in the standard input 27 bytes
before the byte in 'b'.

   This buffering is nice, and it is what allows us to access the
standard input like if it were a random oriented device.  However, it is
obvious we would be in trouble if we filter big amounts of data, like in
a network interface: we would likely use all available memory.

   To allow filtering big amount of data, poke allows to "flush" the
read-only streams.  Flushing means that the buffered data in the
read-only stream is "forgotten", and trying to access it will result in
an exception:

     var stdin = open ("<stdin>");

     var b = byte @ stdin : 28#B;
     var c = byte @ stdin : 29#B;
     flush (stdin, 29#B);
     var d = byte @ stdin : 30#B;
     var e = byte @ stdin : 20#B; /* Exception.  */

7.6.3 Writing to Streams
------------------------

Similarly, unlike normal Unix file descriptors, write-only poke streams
are also buffered and can be accesses randomly:

     var stdout = open ("<stdout>");

We can write to the output stream by mapping as usual:

     byte @ stdout : 28#B = 0xff;

The mapping operation above writes 27 null bytes, i.e.  it fills until
reaching the requested offset in the output stream.  Then it writes the
byte '0xff' at offset 28 bytes.  If we then write to one of the buffered
bytes:

     byte @ stdout : 27#B = 0xab;

This results in updating the byte that will be written right before the
'0xff' once the output stream is flushed.

   Again, we cannot buffer ad-infinitum: we would exhaust all available
output.  Therefore, XXX

7.6.4 pk-strings
----------------

Below you can find a very simple Poke program that works like the
standard Unix utility 'strings'.

     #!/usr/local/bin/poke -L
     !#

     /* Printable ASCII characters: 0x20..0x7e */

     var stdin = open ("<stdin>");
     var stdout = open ("<stdout>");

     var offset = 0#B;

     try
     {
       flush (stdin, offset);

       var b = byte @ stdin : offset;
       if (b >= 0x20 && b <= 0x7e)
         byte  stdout : iosize (stdout) = b;

       offset = offset + 1#B;
     }
     until E_eof;

     close (stdin);
     close (stdout);

8 Configuration
***************

8.1 '.pokerc'
=============

Upon invocation poke will read and execute the commands of an
initialization file, if it exists: the 'pokerc' file.  There are two
ways to keep an initialization file.

   The simple, traditional way is to have a file named '.pokerc' in your
home directory.  GNU poke will look for a file like that first.

   If '.pokerc' is not found your home directory, poke looks in the
locations specified by the "XDG Base Directory Specification"(1) for a
file named 'poke/pokerc.conf'.  For example, you could use
'~/.config/poke/pokerc.conf'.

   Which way is better depends on your specific requirements and taste.

   GNU poke can be insturcted to not read the initialization file by
passing '-q' or '--no-init-file' in the command line.

   Example of initialization file:

     # My poke configuration.
     .set endian host
     .set obase 16
     .set pretty-print yes
     pk_dump_cluster_by = 4
     .load ~/.poke.d/mypickles.pk

   ---------- Footnotes ----------

   (1) 
https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html

8.2 Load Path
=============

The 'load_path' Poke variable contains a list of directories separated
by the colon character (':').  When a module is loaded using the 'load'
construction, these directories are searched in sequential order for the
file corresponding to the requested module.

   Empty directory names and entries that do not name existing
directories are ignored.

   Some entries have special meanings:

'%DATADIR%'
     This is interpreted as the system-wide datadir directory, that
     depends on the prefix where poke is installed.

   For example, say you want to maintain '.pk' files in your '~/.poke.d'
directory.  You will probably want to add that directory to the
'load_path', when poke initializes.  A way to do that is to add a
command like this to your 'pokerc' file:

     load_path = getenv ("HOME") + "/.poke.d:" + load_path

   If the environment variable 'POKE_LOAD_PATH' is defined in the
environment, its value is added to 'load_path' at poke startup time.

8.3 Styling
===========

XXX

9 Time
******

Systems encode time-stamps or dates in several ways.  GNU poke provides
support for some of them.

9.1 POSIX Time
==============

In POSIX systems time is encoded as a certain number of seconds elapsed
since some origin date, namely the first of January 1970.  The 'time'
pickle provides two types for 32-bit and 64-bit POSIX time encodings:

     POSIX_Time32
     POSIX_Time64

   A function 'ptime' is also provided, that prints a human-readable
representation of a POSIX date, given a number of seconds.

10 Colors
*********

Colors are often found in binary data, encoded in several different
ways.  GNU poke provides several pickles that makes it easier to work
with these colors.

10.1 The Color Registry
=======================

The 'color' pickle provides a registry of "standard colors", organized
as a space of integers.  Each integer identifies a standard color, which
are accessible as variables named 'color_*' after the pickle is loaded.
Example:

     (poke) load color
     (poke) color_tomato
     114

   The purpose of having this register is to have a global namespace for
colors that can be used in different pickles.  The position of each
color in the registry is totally unrelated to how the color may be
encoded.  Other pickles, as we shall see below, contain tables
associating standard colors with their encoding, such as RGB.

   If you want to add a new color that is not in the standard
collection, you can use the 'color_register' function, which gets no
arguments:

     (poke) var mycolor = color_register
     (poke) mycolor
     490

   The total number of registered colors is recorded in the variable
'color_num_colors'.  You can use it to iterate on all the colors in the
register, from '0' to 'color_num_colors - 1'.

   The index in the registry of the first user-defined color is hold in
the variable 'color_LAST'.  For example, if you wanted to store a frob
per standard color, you would do it like:

     (poke) type Frobs = Frob[color_LAST]

   The pickle also provides a function 'color_name' that, given a color
code, returns a printable name for the color, i.e.  a string describing
it.  For user defined colors, this string is fixed:

     (poke) color_name (23)
     "lavender blush"
     (poke) color_name (color_register)
     "user-defined color"

If a non-existent color code is passed to 'color_name' the function
raises 'E_out_of_bounds':

     (poke) color_name (color_num_colors)
     unhandled out of bounds exception

10.2 RGB24 Encoding
===================

The "RGB24" encoding encodes each color as a triplet of "color beans",
each beam indicating a level of red, green and blue respectively.

   Types are provided for both the color beams, and the triplets:

     type RGB24_Color_Beam = uint<8>;
     type RGB24_Color = RGB24_Color_Beam[3];

The indexes 'RGB24_RED', 'RGB24_GREEN' and 'RGB24_BLUE' can be used to
access to a specific beam of a given color:

     (poke) rgb24_color[color_tomato][RGB24_GREEN]
     99UB

   The 'rgb24' pickle also provides a table associating poke standard
colors (*note The Color Registry::) with their RGB24 encodings.  Once
the pickle is loaded, this table is available in the variable
'rgb24_color'.  The table is to be indexed by color codes:

     (poke) load rgb24
     (poke) rgb24_color[color_tomato]
     [255UB,99UB,71UB]

11 Audio
********

11.1 MP3
========

11.1.1 ID3V1 Tags
-----------------

The 'id3v1' pickle provides abstractions in order to edit the metadata
stored in a MP3 file.

11.1.1.1 Song Genres
....................

The ID3V1 tags support the notion of "song genre".  The space for genres
is from 0 to 254.  The genre code 255 is reserved to mean "no genre".

   The table 'id3v1_genres' can be indexed with a code in order to get
the corresponding genre name:

     (poke) id3v1_genres[14]
     "rhythm and blues"

   Conversely, the function 'id3v1_search_genre' gives us the code of a
genre, given its name.  The prototype of this function is:

     fun id3v1_search_genre = (string name) uint<8>:

For example:

     (poke) id3v1_search_genre ("rock")
     17UB

   If 'id3v1_search_genre' is given a name that doesn't correspond with
any genre in the genres table, then 'E_inval' is raised.

11.1.1.2 The ID3V1_Tag Type
...........................

The main data structure defined in the 'id3v1' pickle is 'ID3V1_Tag',
which corresponds to a ID3V1 tag (surprise!).

   Tags comprise the following information:

   - The title of the song, which is limited to 30 bytes.
   - The artist, which is limited to 30 bytes.
   - The album, which is limited to 30 bytes.
   - The year, which is encoded as text in 4 bytes, each byte containing
     the ASCII code for the corresponding digit.
   - A comment, which is limited to either 28 or 30 bytes, depending
     whether the tag contains track information or not.
   - An optional track, which is an unsigned 8-bit number.

   The specification does not mention any specific encoding for the
entries that store text (such as title or artist), but it is safe to
assume some ASCII-compatible encoding is used.

   The text entries are stored as arrays of characters, and they are
_not_ finished by NULL characters.  Instead the arrays of characters are
filled with whitespaces (ASCII code 0x20) at the right.  For example,
the artist name Picasso encoded in ID3V1 would be:

     ['P','i','c','a','s','s','o',' ',' ', ..., ' ']

   In order to ease the manipulation of the text fields, setters and
getters are provided in order to handle these values as strings and not
as whitespace-filled arrays of characters.  Example:

     (poke) tag.title
     [48UB,49UB,32UB,45UB,32UB,...]
     (poke) tag.get_title
     "01 - Eclipse De Mar"

Also for setters:

     (poke) tag.set_title ("Join us Now")
     (poke) tag.title
     [74UB,111UB,105UB,110UB,32UB,...]

   Setters and getters are also provided in order to manipulate the year
as an integer value:

     (poke) tag.year
     [0x31UB,0x39UB,0x38UB,0x30UB]
     (poke) tag.get_year
     1980
     (poke) tag.set_year (1988)

12 Object Formats
*****************

This chapter is of special interest to toolchain developers, people
doing reverse-engineering, and similar low-level development activities.

12.1 ELF
========

XXX

12.2 Dwarf
==========

XXX

13 Programs
***********

This chapter describes pickles whose purpose is to facilitate the
writing of binary utilities in Poke.

13.1 argp
=========

Like any other program, Poke scripts often need to handle options passed
in the command line.  The 'argp' pickle provides a convenient and easy
way to handle these arguments.

   The main entry point of the pickle is the function 'argp_parse', with
the following prototype:

     fun argp_parse = (string PROGRAM,
                       string VERSION = "",
                       string SUMMARY = "",
                       Argp_Option[] OPTS = Argp_Option[](),
                       string[] ARGV = string[](),
                       int ALLOW_UNKNOWN = 0) string[]

Where PROGRAM is the name of the program providing the command-line
options.  VERSION is either a string identifying the version of the
program, or the empty string.  SUMMARY is a short summary of what the
program does.  OPTS is an array of 'Argp_Option' structs, each of which
describe a command line option and, in particular, a handler for that
option.  ARGV is an array of string containing the command line elements
to process.  Finally, ALLOW_UNKNOWN specifies whether unknown options
constitute an error or not.  This last option is useful in order to
support sub-parsers.

   Once invoked, 'argp_parse' analyzes the command line provided in
ARGV, recognizes options, performs checks making sure that all specified
options are well formed, and that every option requiring an argument
gets one, invokes the handlers registered for each option, and finally
returns an array of non-option arguments in ARGV, for further processing
by the user.

   Each 'Argp_Option' struct describes an option.  They have this form:

     type Argp_Option =
       struct
       {
         string NAME;
         string LONG_NAME;
         string SUMMARY;
         string ARG_NAME;
         int ARG_REQUIRED;
         Argp_Option_Handler HANDLER;
       };

Where NAME is a string of size one character specifying the short name
for the option.  LONG_NAME is a non-empty string of any size specifying
the long name for the option.  SUMMARY is a short paragraph with a
summary of that the option does.  ARG_NAME, if specified, is a string to
be used to describe the argument of the option in the '--help' output.
If ARG_NAME is not specified then '"ARG"' is used.  ARG_REQUIRED is a
boolean specifying whether the option accepts and expects an argument.
Defaults to 0.  Finally, HANDLER is a function that will be invoked to
process the option.  The function has the following prototype:

     type Argp_Option_Handler = (string)void

   The 'argp_parse' function provides default handlers for several
standard options:

'--help'
     The default handler for this option prints out an usage message in
     the standard GNU way, and exits.  In particular, this output is
     compatible with the 'help2man' utility.
'--version'
     The default handler for this option prints out the name and the
     version of the program, and exits.

If the user provides handlers for the options above, these take
precedence wrt the default handlers.

   Several short options can be accumulated this way:

     foo -qyz

   The special option '--', if found in the command line, stops the
processing of options.  Everything found about it is considered
non-option arguments.

14 Programming Emacs Modes
**************************

GNU poke ships with a bunch of Emacs major modes that eases writing Poke
and PVM assembly.  This chapter documents these modes.

14.1 poke-mode
==============

'poke-mode' is a major mode for editing Poke source files, i.e.  '.pk'
files.  It provides font-lock, auto-completion and indentation features.

14.2 poke-map-mode
==================

'poke-map-mode' is a major mode for editing poke map-files.  It provides
font-lock.  *Note Maps and Map-files::.

14.3 poke-ras-mode
==================

'poke-ras-mode' is a major mode for editing PVM assembly.  It is used in
the development of GNU poke.  See the file 'libpoke/ras' in the source
distribution for more information about RAS, our Retarded Assembler.

15 Vim Syntax Highlighting
**************************

GNU poke ships with a vim syntax highlighter for *.pk files.  Normally
this should be installed as part of your existing poke installation and
work out of the box.  If that is not the case this chapter explains how
to install it manually.

15.1 'poke.vim'
===============

'etc/vim/syntax/poke.vim' is a syntax highlighter for Poke source i.e.
'.pk' files.  To use it, first place the file in your '~/.vimrc/syntax/'
folder on Unix based systems, or in '$HOME/vimfiles/syntax/' on Windows.
There are a few ways to tell Vim to use 'poke.vim' for '.pk' files, but
the quickest is to add the following line to your '.vimrc':

     au BufRead,BufNewFile *.pk set filetype=poke

16 Dot-Commands
***************

16.1 '.load'
============

The '.load' command loads a file containing Poke code and compiles and
executes it.  These files usually have the extension '.pk'.

   If a relative path is provided, then 'PREFIX/share/poke' is tried
first as a base directory to find the specified file.  If it is not
found, then the current directory is tried next.

   If the environment variable 'POKEDATADIR' is defined, it replaces
'PREFIX/share/poke'.  This is mainly intended to test a poke program
before it gets installed in its final location.

   If an absolute path is provided, it is used as-is.

16.2 '.source'
==============

The '.source' command executes command lines from some given file.  The
syntax is:

     .source PATH

where PATH is a path to the file containing the commands.  *Note Command
Files::.

16.3 '.file'
============

The '.file' command opens a new IO space backed by a file, or switches
to a previously opened file.  The syntax is:

     .file PATH

where PATH is a path to a file to open, which can be relative to poke's
current working directory or absolute.

   Tilde expansion is performed in PATH, much like it's done in the
shell.  This means you can include special characters like '~' (which
will expand to your home directory) delimit the file name with '"' in
case it includes leading or trailing blank characters, etc.

   When a new file is opened it becomes the current IO space.  From that
point on, every map executed in the REPL or while loading a Poke program
will operate on that IO space:

     (poke) .file foo.o
     The current file is now `foo.o'.

   When '.file' opens a file, it is opened in whatever mode makes more
sense: if the file allows to be written and read then the IO space is
open in read/write mode, for example.

   If the specified file path doesn't exist then poke emits an error.
However, the flag '/c' ( for "create") can be passed to the command to
tell poke it should create a new file.

16.4 '.mem'
===========

The '.mem' command opens a new IO space backed by a memory buffer.  The
syntax is:

     .mem NAME

where NAME is the name of the buffer to create.  Note that poke adds
prefix and trailing asterisk characters, to differentiate file names
from buffer names.

   When a new memory buffer IOS is opened it becomes the current IO
space.  *Note file command::.

16.5 '.nbd'
===========

The '.nbd' command opens a new IO space backed by an external NBD
server.  The syntax is:

     .nbd URI

where URI is the name of the newly created buffer, matching the NBD URI
specification
(https://github.com/NetworkBlockDevice/nbd/blob/master/doc/uri.md).

   When a new NBD IOS is opened, it becomes the current IO space.  *Note
file command::.

   NBD support in GNU poke is optional, depending on whether poke was
compiled against libnbd.

   For an example of connecting to the guest-visible content of a qcow2
image, with the default export name as exposed by using qemu as an NBD
server:

     $ qemu-nbd --socket=/tmp/mysock -f qcow2 image.qcow2
     $ poke
     (poke) .nbd nbd+unix:///socket=?/tmp/mysock
     The current file is now `nbd+unix:///socket=?/tmp/mysock'.

16.6 '.proc'
============

The '.proc' command opens a new IO space backed by the memory mapped for
some live process, or switches to a previously opened process.  The
syntax is:

     .proc PID

where PID is a process identifier.  In most systems this is a number.

16.7 '.sub'
===========

The '.sub' command opens a new IO sub-space.  The syntax is:

     .sub IOS, BASE, SIZE[, NAME]

where IOS is the tag of some existing IO space, BASE and SIZE are
integers denoting the base and size of the range covered by the
sub-space (in bytes) and NAME is an optional name.

16.8 '.ios'
===========

A list of open files, and their corresponding tags, can be obtained
using the '.info ios' command.  Once a tag is known, you can use the
'.ios' command to switch back to that file:

     (poke) .ios #1
     The current IOS is now `foo.o'.

16.9 '.close'
=============

The '.close' command closes the selected IO space.  The syntax is:

     .close #TAG

where #TAG is a tag identifying an open IO space.

   Note that closing an IO space with this dot-command implies closing
any sub IO space having it as a base.

16.10 '.doc'
============

The '.doc' command is used to display this manual in poke's REPL. The
syntax is:

     .doc [NODE]

where NODE is an optional parameter which indicates the chapter or
section at which the manual should be opened.

   This command uses whatever documentation viewer configured using
'.set doc-viewer' whose valid options are either 'info' and 'less'.

   Unless 'doc-viewer' is set to 'less', this command uses the info
program (*note (info)Top::) to interactively present the manual.  If
info is not installed, then less is tried next.

   When using less to display the documentation, the entry in the Table
of Contents corresponding to the requested entry is highlighted.  Just
press </> and then <RET> to jump to the corresponding section in the
manual.

   If neither info nor less are installed, the '.doc' command will fail.
If poke is not running interactively then '.doc' does nothing.

16.11 '.editor'
===============

The '.editor' command (usually abbreviated as '.edit') invokes an
external text editor on a temporary file.  You can then put contents on
that file, save it and exit the editor.  At that point poke will read
the file contents, turn them into a single line and execute them in the
repl.  If poke is not running interactively, then '.editor' does
nothing.

   The editor used is identified by the 'EDITOR' environment variable.

16.12 '.info'
=============

The '.info' command provides information about several kinds of
entities.  The recognized sub commands are:

'.info ios'
     Display a list of open files.

          (poke) .info ios
            Id	Type	Mode	Bias		Size		Name
            #1	FILE	r	0x00000000#B	0x0000df78#B	foo.o
          * #0	FILE	rw	0x00000000#B	0x00000022#B	foo.bson

     The file acting as the current IO space is marked with an asterisk
     character '*' at the beginning of the file.  The mode in which the
     file is open is also specified.  The 'Id' field is the tag of the
     file that can be passed to the '.file' command in order to switch
     to it as the new current IO space:

          (poke) .ios #1
          The current file is now `foo.o'.
          (poke) .info ios
            Id	Type	Mode	Bias		Size		Name
          * #1	FILE	r	0x00000000#B	0x0000df78#B	foo.o
            #0	FILE	rw	0x00000000#B	0x00000022#B	foo.bson

'.info variables [REGEXP]'
     Shows a list of defined variables along with their current values
     and the location where the variables were defined.  If a regular
     expression is provided then only the types whose name match the
     expression are listed.  If a regular expression is provided then
     only the types whose name match the expression are listed.
'.info functions [REGEXP]'
     Shows a list of defined functions along with their prototypes and
     the location where the functions were defined.  If a regular
     expression is provided then only the types whose name match the
     expression are listed.  If a regular expression is provided then
     only the types whose name match the expression are listed.
'.info types [REGEXP]'
     Shows a list of defined types along with the locations where the
     types were defined.  If a regular expression is provided then only
     the types whose name match the expression are listed.
'.info type NAME'
     Prints a description of the type with name NAME.

16.13 '.set'
============

The '.set' command allows you to inspect and set the value of global
settings.  The following invocations are valid:

'.set'
     Prints all settings along with their values.
'.set SETTING'
     Prints the value of the given SETTING.
'.set SETTING VALUE'
     Sets the value of SETTING to VALUE.

The following settings can be handled with '.set':

'pretty-print'
     This setting determines whether poke should use pretty-printers
     while printing out the written representation of struct values.

     Pretty-printers are defined as methods named '_print'.

'auto-map'
     This setting determines whether to automatically load map files
     when opening IO spaces.

     The decision of what maps to load is driven by the contents of the
     array 'auto_map'.  Each entry in the array is an array whose first
     element is a regular expression, and the second element is the name
     of a map.

     Example:

     auto_map = [["\.o", "elf"], ["\.o", "dwarf"]];

     The entry above makes poke to automatically load the maps 'elf.map'
     and 'dwarf.map' when opening any file whose name ends in '.o',
     provided 'auto-map' is set to 'yes'.

'prompt-maps'
     This setting determines whether a list of open maps is displayed
     before the poke prompt.

     Example of a prompt with map information and 'prompt-maps' set to
     'yes':

     (poke) .file foo.o [dwarf,elf](poke)

     Which indicates that the opening of the IO space 'foo.o' resulted
     in the 'elf.map' and 'dwarf.map' maps to be automatically loaded.

'omaps'
     This sort of misnamed setting determines whether to show the mapped
     offsets and element/field offsets of composite values like arrays
     and structs.

     Example:

     (poke) .set omaps yes (poke) [1,2,3] [0x1 @ 0x0UL#b,0x2 @
     0x20UL#b,0x3 @ 0x40UL#b] @ 0x0UL#b

     This setting defaults to 'no'.

'obase'
     This setting determines the output numerical base to use when
     printing integral values like integers, offset magnitudes, and the
     like.  The supported bases are 2 for binary, 8 for octal, 10 for
     decimal and 16 for hexadecimal.

'oacutoff'
     This setting determines the maximum number of elements to output
     when printing out array values.  Once the maximum number of
     elements is reached, an ellipsis '...'  is printed to represent the
     rest of the elements.

     Example:

     (poke) [1,2,3,4,5] [1,2,3,4,5] (poke) .set oacutoff 3 [1,2,3,...]

     The default value is 0, which means no limit.

'odepth'
     This setting determines the maximum number of nested structs to
     output when printing struct and union values.  Once the maximum
     number of nested values is reached, a collapsed form 'Type {...}'
     is printed.

     Example:

     (poke) type Foo = struct { struct { int i; } bar; } (poke) Foo {}
     Foo { bar=struct {...} }

     The default value for 'odepth' is 0, which means infinite depth.

'oindent'
     This setting determines the width (in blank characters) of each
     indentation level when printing out struct values in 'tree' mode.

     Example:

     (poke) .set oindent 4 (poke) Foo {} Foo { i=0x0, l=0x0L }

     This setting defaults to 2.  See also ".help omode".

'omode'
     This setting determines the way binary struct data is displayed.

     In 'flat' mode data is not formatted in any special way.  Each
     value, be it simple or composite, is printed on one line.

     In 'tree' mode composite values like structs and arrays are
     displayed in an hierarchical way, using newlines and indentation.

     Examples:

     (poke) .set omode flat (poke) Foo {} Foo {i=0x0,l=0x0L}

     (poke) .set omode tree (poke) Foo {} Foo { i=0x0, l=0x0L }

     The default value of 'omode' is 'flat'.

'endian'
     This setting determines the byte endianness used when accessing IO
     spaces.  Valid values are 'big', 'little', 'host' and 'network'.

     The meaning of 'big' and 'little' is the obvious one: the
     endianness is set to big-endian and little-endian respectively.

     The 'host' endianness is the endianness used by the computer and
     operating system that is running poke in order to handle it's own
     memory and files.

     The 'network' endianness is the endianness used by the host machine
     in order to communicate with the network.

'doc-viewer'
     This setting determines what mechanism to use to open the online
     user manual.

     If 'info' is selected, the GNU info standalone program is launched
     to show the info version of the user manual.

     If 'less' is selected, the less utility is launched ot show a plain
     text version of the user manual.

     See also ".help .doc".

16.14 '.vm'
===========

The Poke Virtual Machine (PVM) executes the programs that are the result
of the compilation of what you write in the REPL or the pickles you
load.  The '.vm' command provides sub-commands to interact with the PVM.

16.14.1 '.vm disassemble'
-------------------------

The '.vm disassemble' command provides access to the PVM disassembler.
It supports the following subcommands:

'.vm disassemble expression EXPR'
     Dumps the assembler corresponding to the Poke expression EXPR.
'.vm disassemble function FUNCTION'
     Dumps the assembler corresponding to the Poke function called
     FUNCTION.  The function shall be reachable from the top-level.
'.vm disassemble mapper EXPR'
     If EXPR is a mapped value, dumps the assembler corresponding to its
     mapper function.
'.vm disassemble writer EXPR'
     If EXPR is a mapped value, dumps the assembler corresponding to its
     writer function.

   The disassembler will provide a PVM disassembly by default, but it
can be passed the flag '/n' to do a native disassembly instead in
whatever architecture running poke.

16.14.2 '.vm profile'
---------------------

The '.vm profile' command provides access to the PVM profiler.  This
profiler is only available if poke has been configured with PVM
profiling support.  This command supports the following subcommands:

'.vm profile reset'
     Resets the profiling counts in the virtual machine.
'.vm profile show'
     Outputs a summary with both counts and sample information.

16.15 '.exit'
=============

The '.exit' command exits poke.  The syntax is:

     .exit [STATUS]

Poke will terminate, returning the exit status STATUS.  If STATUS is
omitted, then the exit status zero will be returned.

16.16 '.quit'
=============

The '.quit' command behaves exactly like '.exit'.  *Note exit command::.

17 Commands
***********

17.1 'dump'
===========

At the most basic level, memory can be examined byte by byte.  To do
this, use the 'dump' command.

   This command has the following synopsis.

     dump [:from OFFSET] [:size OFFSET] [:ios INT] [:ruler BOOL]
          [:ascii BOOL] [:group_by INT] [:cluster_by INT]

All arguments are optional, which means the simplest use of the command
is to simply type 'dump':

     (poke) dump
     76543210  0011 2233 4455 6677 8899 aabb ccdd eeff  0123456789ABCDEF
     00000000: 9b07 5a61 4783 f306 4897 f37c fe39 4cd3  ..ZaG...H..|.9L.
     00000010: b6a2 a578 8d82 7b7f 2076 374c 3eab 7150  ...x..{. v7L>.qP
     00000020: 31df 8ecb 3d33 ee12 429b 2e13 670d 948e  1...=3..B...g...
     00000030: 86f1 2228 ae07 d95c 9884 cf0a d1a8 072e  .."(...\........
     00000040: f93c 5368 9617 6c96 3d61 7b92 9038 a93b  .<Sh..l.=a{..8.;
     00000050: 3b0d f8c9 efbd a959 88d0 e523 fd3b b029  ;......Y...#.;.)
     00000060: e2eb 51d5 cb5b 5ba9 b890 9d7a 2746 72ad  ..Q..[[....z'Fr.
     00000070: 6cbd 6e27 1c7f a554 8d2e 77f9 315a 4415  l.n'...T..w.1ZD.

The first row is the "ruler" which serves as a heading for each
subsequent row.  On the left hand side is the offset of the io space
under examination.  The centre block displays the hexadecimal
representation of each byte, and on the right hand side is their ascii
representation.  If a byte is not representable in ascii, then the byte
will be displayed as a dot.

   By default the 'dump' command reads from the currently selected IO
space.  However, it is possible to specify an explicit IO space using
the 'ios' option:

     (poke) var myfile = open ("/path/to/file")
     (poke) dump :ios myfile

17.1.1 Information 'dump' shows
-------------------------------

By default 'dump' displays 128 bytes of memory starting at offset 0#B.
You can change the quantity and starting offset by using the 'size' and
'from' arguments.  For example:

     (poke) dump :from 0x10#B :size 0x40#B
     76543210  0011 2233 4455 6677 8899 aabb ccdd eeff  0123456789ABCDEF
     00000010: b6a2 a578 8d82 7b7f 2076 374c 3eab 7150  ...x..{. v7L>.qP
     00000020: 31df 8ecb 3d33 ee12 429b 2e13 670d 948e  1...=3..B...g...
     00000030: 86f1 2228 ae07 d95c 9884 cf0a d1a8 072e  .."(...\........
     00000040: f93c 5368 9617 6c96 3d61 7b92 9038 a93b  .<Sh..l.=a{..8.;

Note that both the 'size' and 'from' arguments are offsets.  As such,
both must be specified using '#' and an appropriate unit.  (*note Offset
Literals::).

   The other arguments change the appearance of the dump.  If the
'ruler' argument is zero, then the ruler will be omitted:

     (poke) dump :ruler 0 :size 0x40#B
     00000000: b1fd 1608 2346 759c 46a6 aa94 6fcd 846a  ....#Fu.F...o..j
     00000010: e39f 473f 3247 415f 174d a32b ed89 a435  ..G?2GA_.M.+...5
     00000020: d2c6 2c52 bc82 e0a7 e767 31ea 84de 41e5  ..,R.....g1...A.
     00000030: 2add 2869 e9c2 226b e222 8c74 4b94 af24  *.(i.."k.".tK..$

   To omit the ascii representation of the memory, call 'dump' with the
'ascii' argument set to zero:

     (poke) dump :ruler 0 :size 0x40#B :ascii 0
     00000000: 4393 85e7 0b0c 3921 5a26 39ec 2f5f 5f15
     00000010: cc46 e6f3 d50f 6ae6 8988 d50e f8c4 d1c6
     00000020: 5a2f 7c3e 490b 18d8 d867 4b6f 2549 1f6c
     00000030: 34a9 a0d7 24d2 e9ac 9240 8247 10cb 4ba1

17.1.2 Presentation options for 'dump'
--------------------------------------

By default, the hexadecimal display shows two bytes grouped together,
and then a space.  You can alter this behaviour using the 'group_by'
parameter.

     (poke) dump :ascii 0 :size 0x40#B :group_by 4#B
     00000000: 68f19a63 df2a8886 c466631c a7fdd5c7  h..c.*...fc.....
     00000010: 3075746a 0adb03ca f5b1ff14 6166fa07  0utj........af..
     00000020: 0dd3cfbd 8eff46a2 4152a81d 471beddf  ......F.AR..G...
     00000030: a0501cae 8bfcec6f 7a4f5701 45ba9fc3  .P.....ozOW.E...

   Another parameter is the 'cluster_by' argument.  By setting
'cluster_by' to N, this causes 'dump' to display an additional space
after the Nth group has been displayed, and also in the corresponding
position in the ascii display:

     (poke) dump :size 0x40#B :group_by 2#B :cluster_by 4
     76543210  0011 2233 4455 6677  8899 aabb ccdd eeff  01234567 89ABCDEF
     00000000: 91b8 540d d4dc 49ae  3320 ba7d efd1 16ab  ..T...I. 3 .}....
     00000010: b1a8 5ea0 5846 8bea  f741 3f80 42bc 201f  ..^.XF.. .A?.B. .
     00000020: 6e5e fa50 23fb f16a  d380 be8c fc98 d195  n^.P#..j ........
     00000030: 7bbf fa3e 3fc2 43a4  2a1e 9763 2bd6 5d24  {..>?.C. *..c+.]$

   Another variable that the user can customize is
'pk_dump_nonprintable_char'.  It defaults to ''.'', and contains the
character to use in the ASCII dump to represent a non-printable
character.

   Not all bytes from an IO space can be read, depending on the kind of
IO device.  When 'dump' cannot read a byte it will print
'pk_dump_unknown_byte' instead, which is a string.

   If the variable 'pk_dump_addr_hyperlinks' is 1 then the 'dump'
command will emit hyperlinks in the printed bytes.  Clicking on a byte
value will result on the address of that byte to be inserted at the
prompt.

   If you have a personal preference on how memory dumps should appear,
you can set the relevant 'pk_dump_*' variables.  These global variables
serve as the defaults for 'dump', so this way, you will not need to
explicitly pass them when you call the function.

17.2 'copy'
===========

The command 'copy' allows to copy regions of data inside an IO space, or
between different IO spaces.

   This command has the following synopsis.

     copy [:from OFFSET] [:to OFFSET] [:size OFFSET]
          [:from_ios IOS] [:to_ios IOS]

All arguments are optional.  When invoked with no arguments, 'copy' does
nothing.

   The arguments 'from' and 'size' determine the region to copy.  By
default, this region is taken from the current IO space, but this can be
overwritten using the optional 'from_ios' argument.

   The argument 'to' tells 'copy' where to copy the stuff.  It defaults
to 'from'.  Again, this offset is applied to the current IO space by
default, but this can be overwritten using the 'to_ios' argument.

   Note that it is allowed for the source and destination ranges to
overlap.

17.3 'save'
===========

Use the 'save' command in order to write a region from an IO space into
a file in your file system.

   This command has the following synopsis.

     save [:ios IOS] [:from OFFSET] [:size OFFSET] [:file STRING]
          [:append BOOL] [:verbose BOOL]

All arguments are optional.  When invoked with no arguments, 'save' does
nothing.

   The arguments 'from' and 'size' are used to determine the region of
the IO space to save.  This is how you would extract and save the
contents of an ELF section to a file 'out.text':

     (poke) var s = elf_section_by_name (Elf64_Ehdr @ 0#B, ".text")
     (poke) save :file "out.text" :from s.sh_offset :size s.sh_size

Both 'from' and 'size' are arbitrary offsets.  You should keep in mind
however that files are byte oriented.  Therefore saving, say, nine bits
to a file will actually write two bytes.

   By default 'save' will extract the data from the "current IO space".
However, it is possible to specify an alternative IOS by using the 'ios'
argument.

   By default 'save' will truncate the output file, if it exists, before
starting writing.  If the argument 'append' is set as true, however, it
will append to the existing contents of the file.  In this case, the
file should exist.

17.4 'extract'
==============

Use the 'extract' command in order to create a temporary memory IO space
with the contents of a mapped value.

   This command the has the following synopsis.

     extract :val VAL :to IOS_NAME

Where ':val' is a mapped value, and :TO is the name of the memory IOS to
create (or use).  The contents of VALUE are copied to the beginning of
the memory IOS '*IOS_NAME*'.

17.5 'scrabble'
===============

The 'scrabble' command rearranges portions of an IO space based on a
transformation defined by a pair of patterns.

   This command has the following synopsis:

     scrabble [:from OFFSET] [:size OFFSET]
              [:from_pattern STRING] [:to_pattern STRING]
              [:ent_size OFFSET]
              [:from_ios IOS] [:to_ios IOS]

The optional arguments are documented below.

':from OFFSET'
     Beginning of the range in the origin IO space from where to start
     rearranging units.  Defaults to '0#B'.
':size OFFSET'
     Size of the range to scrabble in the origin IO space.  Defaults to
     '0#B'.
':from_pattern STRING'
     String specifying a sequence of units.  Each character is an unit.
     Defaults to '""'.
':to_pattern STRING'
     String specifying the scrabbled units.  Each character is an unit.
     If a character that doesn't appear in the 'from_pattern' is found
     in this string, it is ignored.  Defaults to 'from_pattern'.
':ent_size OFFSET'
     Size of the entities to scrabble around.  Defaults to '1#B'.
':from_ios IOS'
     IO space from where to obtain the data to scrabble.  Defaults to
     the current IO space.
':to_ios IOS'
     The IO space to where write the scrabbled data.  Defaults to
     'from_ios'.

18 The Poke Language
********************

18.1 Integers
=============

Most of the values manipulated in Poke programs are whole numbers, also
typically known as "integers" in computing parlance.  This is because
integers are pervasive in binary formats, often featuring unusual
characteristics in terms of size and/or alignment.  Single bits denoting
flags or packed small integers are good examples of this.  In order to
ease the manipulation of such entities, and unlike most programming
languages, Poke provides integer types of any number of bits and a rich
set of accompanying operators.

18.1.1 Integer Literals
-----------------------

Integers literals can be expressed in several numeration bases.

   _Decimal numbers_ use the usual syntax '[1-9][0-9]*'.  For example,
'2345'.

   _Octal numbers_ are expressed using a prefix '0o' (or '0O') followed
by one or more digits in the range '[0-7]'.  Examples are '0o0', '0o100'
and '0o777'.

   _Hexadecimal numbers_ are expressed using a prefix '0x' (or '0X')
followed by one or more hexadecimal digits in the range '[0-f]'.
Examples are '0x0' and '0xfe00ffff'.  Note that both the 'x' in the
prefix and the letters in the hexadecimal number are case insensitive.
Thus, '0XdeadBEEF' is a valid (but ugly as hell) literal.

   _Binary numbers_ are expressed using a prefix '0b' (or '0B') followed
by one or more binary digits in the range '[0-1]'.  Examples of binary
literals are '0b0' and '0B010'.

   Negative numbers, of any numeration base, are constructed using the
minus operator as explained below.  Therefore the minus symbol '-' in
negative numbers is not part of the literal themselves.

18.1.1.1 The digits separator '_'
.................................

The character '_' can appear anywhere in a numeric literal except as the
first character.  It is ignored, and its purpose is to make it easier
for programmers to read them:

     0xf000_0000_0000_0000
     0b0000_0001_0000_0001

18.1.1.2 Types of integer literals
..................................

The type of a numeric literal is the smallest signed integer capable of
holding it, starting with 32 bits, in steps of powers of two and up to
64 bits.(1)

   So, for example, the value '2' has type 'int<32>', but the value
'0xffff_ffff' has type 'int<64>', because it is out of the range of
signed 32-bit numbers.

   A set of suffixes can be used to construct integer literals of
certain types explicitly.  'L' or 'l' is for 64-bit integers.  'H' or
'h' is for 16-bit integers (also known as "halves"), 'B' or 'b' is for
8-bit integers (also known as "bytes") and 'n' or 'N' is for 4-bit
integers (also known as "nibbles").

   Thus, '10L' is a 64-bit integer with value '0x0000_0000_0000_000A',
'10H' is a 16-bit integer with value '0x000A' and '10b' is a 8-bit
integer with value '0x0A'.

   Similarly, the signed or unsigned attribute of an integer can be
explicitly specified using the suffix 'u' or 'U' (the default are signed
types).  For example '0xffff_ffffU' has type 'uint<32>' and '0ub' has
type 'uint<8>'.  It is possible to combine width-indicating suffixes
with signedness suffixes: '10UL' denotes the same literal as '10LU'.

   The above rules guarantee that it is always possible to determine the
width and signedness of an integer constant just by looking at it, with
no ambiguity.

   ---------- Footnotes ----------

   (1) Rationale: the width of a C "int" is 32 bits in most currently
used architectures, and binary data formats are usually modelled after
C.

18.1.2 Characters
-----------------

8-bit unsigned integers can use an alternative literal notation that is
useful when working with _ASCII character codes_.  Printable character
codes can be denoted with ''c''.

   Non-printable characters can be expressed using escape-sequences.
The allowed sequences are:

'\n'
     New-line character (ASCII 012).
'\t'
     Tab character (ASCII 011).
'\\'
     The backslash character.
'\[0-9][0-9]?[0-9]?'
     Character whose ASCII code is the specified number, in octal.

   Examples:

     'o'
     '\n'
     '\t'
     '\\'
     '\0'

   The type of a character literal is always 'char', aka 'uint<8>'.

18.1.3 Booleans
---------------

Like in C, truth values in Poke are encoded using integers.  Zero ('0')
denotes the logical value "false", and any integer other than zero
denotes the logical value "true".

18.1.4 Integer Types
--------------------

Most general-purpose programming languages provide a small set of
integer types, each featuring a range corresponding to strategic storage
sizes: basically, signed and unsigned variants of 8, 16, 32, 64 bits.
As we have seen in the previous sections, suffixes like 'H' or 'L' are
used in Poke for that purpose.

   However, in conventional programming languages when integers having
an "odd" width (like 13 bits, for example) get into play for whatever
reason, the programmer is required to use the integer arithmetic
operators (and sometimes bit-wise operators) herself, in a clever way,
in order to achieve the desired results.

   Poke, on the contrary, provides a rich set of integer types featuring
different widths, in both signed and unsigned variants.  The language
operators are aware of these types, and will do the right thing when
operating on integer values having different widths.

   Unsigned integer types are specified using the type constructor
'uint<N>', where N is the number of bits.  N should be an integer
literal in the range '[1,64]'.  Examples:

     uint<1>
     uint<7>
     uint<64>

   Similarly, signed integer types are created using the type
constructor 'int<N>', where N is the number of bits.  N should be an
integer literal in the range '[1,64]'.  Examples:

     int<1>
     int<8>
     int<64>

   Note that expressions are not allowed in the type integral
constructor parameters.  Not even constant expressions.  Thus, things
like 'int<foo>' and 'uint<2+3>' are not allowed.

18.1.5 Casting Integers
-----------------------

The right-associative unary operator cast 'as' can be used to derive a
new integer value having a different type from an existing value.

   For example, this is how we would create a signed 12-bit integer
value holding the value 666:

     (poke) 666 as int<12>
     (int<12>) 666

   Note that the '666' literal is originally a 32-bit signed integer.
The cast performs the conversion.

   Casts between integer types are always allowed, and never fail.  If
the new type is narrower than the existing type, truncation may be
performed to accommodate the value in its new type.  For example, the
expression '0x8765_4321 as uint<16>' evaluates to '0x4321'.  If the new
type is wider than the existing type, either zero-extension or
sign-extension is performed depending on the signedness of the operand.

   The semantics of the sign-extension operation depends on the
signedness of the value being converted, and on the currently selected
encoding for negative numbers.

   When using two's complement encoding, converting a signed value will
always sign-extend regardless of the signedness of the target type.
Thus:

     (poke) -2H as uint<32>
     0xfffffffeU
     (poke) -2H as int<32>
     0xfffffffe

   Likewise, converting an unsigned value will always zero-extend
regardless of the signedness of the target type.  Thus:

     (poke) 0xffffUH as uint<32>
     0xffffU
     (poke) 0xffffUH as int<32>
     0xffff

18.1.6 Relational Operators
---------------------------

The following binary relational operators are supported on integer
values, in descending precedence order:

   * Equality '==' and inequality '!='.
   * Less than '<' and less or equal than '<='.
   * Greater than '>' and greater or equal than '>='.

   When applied to integer and character values, these operators
implement an arithmetic ordering.

   These operators resolve in boolean values encoded as 32-bit integers:
'0' meaning false and '1' meaning true.

18.1.7 Arithmetic Operators
---------------------------

The following left-associative binary arithmetic operators are
supported, in descending precedence order:

   * Exponentiation '**', multiplication '*', integer division '/',
     integer ceil-division '/^' and modulus '%'.
   * Addition '+' and subtraction '-'.

   In all the binary arithmetic operations automatic promotions
(coercions) are performed in the operands as needed.  The rules are:

   * If one of the operands is unsigned and the other operand is signed,
     the second is converted to an unsigned value.
   * If the size in bits of one of the operands is bigger than the size
     of the other operand, the second is converted to the same number of
     bits.

   The following right-associative unary arithmetic operators are
supported:

   * Unary minus '-' and unary plus '+'.

   Finally, pre-increment, pre-decrement, post-increment and
post-decrement operators '++' and '--' are supported.

18.1.8 Bitwise Operators
------------------------

The following left-associative bitwise binary operators are supported,
in descending precedence order:

   * Bitwise shift left '<<.' and bitwise shift right '.>>'.
   * Bitwise AND '&'.
   * Bitwise exclusive OR '^'.
   * Bitwise inclusive OR '|'.
   * Bitwise concatenation ':::'.

   Both '<<.' and '.>>' operators perform logical shifting.  Unlike in
many other programming languages, arithmetic right-shifting operators
are not provided.  This means that right shifting always inserts zeroes
at the most-significant side of the value operand, whereas left shifting
always inserts zeroes at the least-significant side of the value
operand.

   Left shifting by a number of bits equal or bigger than the size of
the value operand is an error, and will trigger either a compile-time
error or a run-time 'E_out_of_bounds' exception.  This does not apply to
right shifting.

   Bitwise concatenation works with any integral type, of any bit
length.

   The following right-associative unary bitwise operators are
supported:

   * Bitwise complement '~'.

18.1.9 Boolean Operators
------------------------

The following left-associative, short-circuited binary logical operators
are supported, in descending precedence order:

   * Logical implication: '=>'.
   * Logical AND: '&&'.
   * Inclusive OR: '||'.

   The following right-associative unary logical operators are
supported:

   * Logical negation '!'.

18.1.10 Integer Attributes
--------------------------

The following attributes are defined for integer values.

'size'
     Gives an offset with the storage occupied by the string.  This
     includes the terminating null.  Examples:

          (poke) 10'size
          0x20UL#b
          (poke) 10N'size
          0x4UL#b
          (poke) (10 as int<1>)'size
          0x1UL#b

'signed'
     Gives 1 if the value is a signed integer, 0 otherwise.  Examples:

          (poke) 10'signed
          1
          (poke) 10UL'signed
          0
'mapped'
     Always 0 for integers.  (*note Mapping::).

18.2 Offsets
============

Poke uses united values to handle offsets and data sizes.  This is a
very central concept in poke.

18.2.1 Offset Literals
----------------------

Poke provides a convenient syntax to provide united values, which are
called "offsets" (because in a binary editor you mostly use them to
denote offsets in the file you are editing):

     12#B
     7#b
     1024#KB

   The offsets above denote twelve bytes, seven bits and one thousand
twenty four kilobytes, respectively.  The unit can be separated from the
magnitude by blank characters, so you can write the following instead if
you are so inclined:

     12 #B
     7 #b
     (1024 * 1024) #Kb

   Note how the magnitude part of an offset doesn't need to be constant.
If the variable 'a' contains an integer, this is how you would denote "a
bytes":

     a#B

   In the offset syntax units are specified as '#UNIT', where UNIT is
the specification of an unit.  See the next section for details.

18.2.2 Offset Units
-------------------

There are several ways to express the unit of an offset, which is always
interpreted as a multiple of the basic unit, which is the bit (one bit).

18.2.2.1 Named Units
....................

The first way is to refer to an unit by name.  For example, '2#B' for
two bytes.  Units are defined using the 'unit' construction:

     unit NAME = CONSTANT_EXPRESSION [, NAME = CONSTANT_EXPRESSION]...;

where NAME is the name of the unit, and CONSTANT_EXPRESSION is a
constant expression that should evaluate to an integral value.  The
resulting value is always coerced into an unsigned 64-bit integer.

   Note that unit names live in a different namespace than variables and
types.  However, when a given name is both a type name and an unit name
in an unit context, the named unit takes precedence:

     (poke) type xx = int
     (poke) unit xx = 2
     (poke) 1#xx
     1#2

   Many useful units are defined in the standard library.  *Note
Standard Units::.

18.2.2.2 Arbitrary Units
........................

It is also possible to express units in multiples of the base unit,
which is the bit.  Using this syntax, it becomes possible to express
offsets in any arbitrary unit, as disparate as it may seem:

     17#3
     0#12
     8#1

   That's it: 17 units of 3 bits each, zero units of 12 bits each, and
eight units of 1 bit each.  Note that the unit should be greater than 0.

18.2.2.3 Types as Units
.......................

But then, why stop there?  Poking is all about defining data structures
and operating on them... so why not use these structures as units as
well?  Consider the following struct:

     type Packet = struct { int i; long j; };

   The size of a 'Packet' is known at compile time (which is not
generally true for Poke structs).  Wouldn't it be nice to use it as a
unit in offsets?  Sure it is:

     23#Packet

   The above is the size occupied by 23 packets.  Any type whose size is
known at compile time can be specified as an offset unit.

   Expressing offsets as united values also relieves the programmer from
doing many explicit unit conversions: poke can do them for you.
Consider for example an ELF section header.  One of its fields is the
size of the described section, in bytes:

     type Elf64_Shdr =
       struct
       {
        ...
        offset<Elf64_Xword,B> sh_size;
        ...
       };

   If a given section is to contain, say, relocations with addends, we
can set its size doing something like this:

     shdr.sh_size = 10#Elf64_Rela;

Instead of doing the conversion to bytes explicitly.

   If the magnitude of an offset is 1 then it is allowed to omit it
entirely.  To denote one kilobyte, for example, we can write '#KB'.

18.2.3 Offset Types
-------------------

Offset types are denoted as 'offset<BASE_TYPE,UNIT>', where BASE_TYPE is
an integer type and UNIT the specification of an unit.

   The offset base type is the type of the magnitude part of the united
value.  It can be any integer type, signed or unsigned, of any size.

   The unit specification should be one of the unit identifiers that are
allowed in offset literals (see above), a constant positive integer or
the name of a Poke type whose size is known at compile time.

   Let's see some examples.  A signed 32-bit offset expressed in bytes
has type 'offset<int<32>,B>'.  An unsigned 12-bit offset expressed in
kilobits has type 'offset<uint<12>,Kb>'.  The latter type can also be
written using an explicit integer unit like in 'offset<uint<12>,1024>'.
Finally, a signed 64-bit offset in units of "packets", where a packet is
denoted with a Poke type 'Packet' has type 'offset<uint<64>,Packet>'.

18.2.4 Casting Offsets
----------------------

The right-associative unary operator cast 'as' can be used to derive a
new offset value having a different type from an existing value.

   For example, this is how we would create a signed 12-bit offset in
units bytes:

     (poke) 1024#b as offset<int<12>,B>
     (int<12>) 128#B

   The same rules governing conversion of integers apply for the
magnitude part.  Depending on the unit, there can be truncation, like
in:

     (poke) 9#b as offset<int,B>
     1#B

18.2.5 Offset Operations
------------------------

Poke supports a little algebra for offsets.

18.2.5.1 Addition and subtraction
.................................

The addition or subtraction of two offsets results in another offset.
Examples:

     (poke) 1#B + 1#b
     9#b
     (poke) 2#KB - 1024#B
     1024#B

   The unit of the result is the greatest common divisor of the units of
the operands.

   The operators '++' and '--', in their prefix and suffix versions, can
be applied to offsets as well.  The step used in the increment/decrement
is one unit.

18.2.5.2 Multiplication by a scalar
...................................

Multiplying an offset by a magnitude gives you another offset.
Examples:

     (poke) 8#b * 2
     16#b
     (poke) 16#MB * 0
     0#MB

   The unit of the result is the same as the unit of the offset operand.

   Note that multiplying two offsets is not supported.  This makes
sense, since computer memory is linear, and therefore it wouldn't make
any sense to have units like '#B^{2}'.

18.2.5.3 Division
.................

Dividing two offsets gives you a magnitude.  Examples:

     (poke) 16#b / 1#B
     2
     (poke) 1024#MB / 512#Mb
     16

   Dividing offsets is the Pokish way of converting memory magnitudes
between different units: just use units like you do when doing physics
or working with units in other contexts.

   For example, using the syntactic trick of omitting the magnitude (in
which case it is assumed to be 1) it is very natural to write something
like the following to convert from kilobits to bytes:

     (poke) 24 #Kb/#B
     3072

   There is also a ceil-division operator for offsets, with the same
semantics as the ceil-division for integers:

     (poke) 10#B /^ 3#B
     4

18.2.5.4 Division by an integer
...............................

Dividing an offset by an integer gives you an offset.  Example:

     (poke) 8#B / 2
     4#B

18.2.5.5 Modulus
................

The modulus of two offsets gives you another offset with the expected
semantics.  Examples:

     (poke) 9#b % 1#B
     1#b
     (poke) 1#B % 9#b
     8#b

   The unit of the result is the greatest common divisor of the units of
the operands.

18.2.6 Offset Attributes
------------------------

The following attributes are defined for offset values.

'size'
     Gives an offset with the storage occupied by the offset.  Examples:

          (poke) 10#B'size
          0x20UL#b
          (poke) 10N#B'size
          0x4UL#b
'magnitude'
     Gives the magnitude part of the offset.  Examples:

          (poke) 10#B'magnitude
          10
          (poke) 2H#b'magnitude
          2H
'unit'
     Gives a number with the unit of the offset, expressed in bits.
     Examples:

          (poke) 10#B'unit
          8UL
          (poke) 2H#b'unit
          1UL
'mapped'
     Always 0 for offsets.  (*note Mapping::).

18.3 Strings
============

Poke supports a notion of "strings" which is very similar to the C
programming language: a string value is a sequence of characters that is
terminated by the so-called "null character".

   The standard library provides functions which process strings.  *Note
String Functions::.

18.3.1 String Literals
----------------------

NULL-terminated sequences of ASCII codes can be denoted using the
following syntax:

     "foo"

   Poke string values are very similar to C strings.  They comprise a
sequence of 8-bit character codes, terminated by the value '0UB'.

   The following escape sequences are supported inside string literals:

'\n'
     Denotes a new line character.
'\t'
     Denotes an horizontal tab.
'\\'
     Denotes a backlash '\' character.
'\"'
     Denotes a double-quote '"' character.
'\NUM'
     NUM is a number in the range 1 to 255 (inclusive) in base 8.
'\xNUM'
     NUM is a number in the range 1 to 255 (inclusive) in base 16.

   Strings cannot contain '0UB' character, and inserting one using
escape sequences ('\0' or '\x0') leads to compilation error.

18.3.2 String Types
-------------------

Every string value in Poke is of type 'string'.

18.3.3 String Indexing
----------------------

Poke supports accessing the characters in a string using the array
indexing notation.  The indexes are in the '[0,N]' range, where N is the
length of the string minus one.  Note the length doesn't include the
null character, i.e.  it is not possible to access the terminating null.
Examples:

     (poke) "foo"[0]
     0x66UB
     (poke) "foo"[1]
     0x6fUB

   If the passed index is less than zero or it is too big, an
'E_out_of_bounds' exception is raised:

     (poke) "foo"[-1]
     unhandled out of bounds exception
     (poke) "foo"[3]
     unhandled out of bounds exception

18.3.4 String Concatenation
---------------------------

Strings can be concatenated using the '+' operator.  This works like
this:

     (poke) "foo" + "bar"
     "foobar"

   Note how the null character terminating the first string is removed.
Therefore, the length of the concatenation of two given strings of
lengths 'N' and 'M' is always 'N+M-1'.

   Concatenation and indexing are useful together for building strings.
A string can be created empty, and additional characters added to it by
means of concatenation:

     (poke) var bytes = "";
     (poke) bytes = bytes + 'x' as string;

   Then, we can retrieve characters from the string we built using
indexing:

     (poke) bytes[0]
     0x78UB

   Additionally, the '*' operator allows to "multiply" a string by
concatenating it with itself a given number of times.  This works like
this:

     (poke) "foo" * 3
     "foofoofoo"
     (poke) "foo" * 0
     ""

   This is useful for building strings whose length is not known at
compile time.  For example:

     fun make_empty_string = (int length) string:
     {
        return " " * length;
     }

18.3.5 String Attributes
------------------------

The following attributes are defined for string values.

'length'
     Gives the number of characters composing the string, not counting
     the terminating null.  Examples:

          (poke) "foo"'length
          3UL
          (poke) ""'length
          0UL

'size'
     Gives an offset with the storage occupied by the string.  This
     includes the terminating null.  Examples:

          (poke) "foo"'size
          32UL#b
          (poke) ""'size
          8UL#b
'mapped'
     Always 0 for strings.  (*note Mapping::).

18.3.6 String Formatting
------------------------

Poke provides a built-in function 'format' that can be used in order to
format, or build, a string value that is derived from a set of zero or
more Poke values:

     format (FMT[, VALUE...])

Where FMT is a format string that interpreted verbatim but for
predefined "format tags" which start with '%'.  These format tags are
interpreted especially.  *Note printf::.

   Example:

     var s = format ("Age is %i32d.", 66);

The value of the variable 's' is '"Age is 66."'.

18.4 Arrays
===========

Arrays are homogeneous collections of values.

18.4.1 Array Literals
---------------------

Array literals are constructed using the following syntax:

     [EXP,EXP...]

where EXP is an arbitrary expression.

   For example, '[1,2,3]' constructs an array of three signed 32-bit
integers.  Likewise, '['a','b','c']' constructs an array of three
unsigned 8-bit integers (characters).  For convenience, a trailing comma
is accepted but not required.

   The type of the array literal is inferred from the type of its
elements.  Accordingly, all the elements in an array literal must be of
the same type.  Examples of invalid array literals, that will raise a
compilation-time error if evaluated, are:

     [1,2u,3]
     [1,0xffff_ffff,3]
     ['a',"b",'c']

   Array literals must contain at least one element.  Accordingly, '[]'
is not a valid array literal.

   This is how a '3x3' matrix could be constructed using an array of
arrays:

     [[1,2,3],[4,5,6],[7,8,9],]

   It is possible to refer to specific elements when constructing array
literals.  For example, '[1,2,3,.[3] = 4]' denotes the same array as
'[1,2,3,4]'.

   This allows creating arrays without having to specify all its
elements; every unspecified element takes the value of the first
specified element to its right.  For example, '[.[2] = 2]' denotes the
same array as '[2,2,2]'.

   Note that an array element can be referenced more than once.  When
that happens, the final value of the element is the last specified.  For
example, '[1,2,3,.[1]=10]' denotes the array '[1,10,3]'.

18.4.2 Array Types
------------------

There are three different kind of array types in Poke.

   "Unbounded" arrays have no explicit boundaries.  Examples are 'int[]'
or 'Elf64_Shdr[]'.  Arrays can be "bounded by number of elements"
specifying a Poke expression that evaluates to an integer value.  For
example, 'int[2]'.  Finally, arrays can be "bounded by size" specifying
a Poke expression that evaluates to an offset value.  For example,
'int[8#B]'.

18.4.2.1 Writing unbounded array literals
.........................................

The type of an array literal is always bounded by number of elements.
For example, the type of '[1,2,3]' is 'int[3]'.  If what we want is an
unbounded array literal we can obtain it with a case like '[1,2,3] as
int[]'.

18.4.2.2 Array boundaries and closures
......................................

Poke arrays are rather peculiar.  One of their seemingly bizarre
characteristics is the fact that the expressions calculating their
boundaries (when they are bounded) evaluate in their own lexical
environment, which is captured.  In other words: the expressions
denoting the boundaries of Poke arrays conform closures.  Also, the way
they evaluate may be surprising.  This is no capricious.

   When an array type is bounded, be it by number of elements or by
size, the expression indicating the boundary doesn't need to be constant
and it can involve variables.  For example, consider the following type
definition:

     var N = 2;
     type List = int[N*2];

Let's map a 'List' at some offset:

     (poke) List @ 0#B
     [0x746f6f72,0x303a783a,0x723a303a,0x3a746f6f]

   As expected, we get an array of four integers.  Very good, obviously
the boundary expression 'N*2' got evaluated when defining the type
'List', and the result of the evaluation was '4', right?.  Typical
semantics like in my garden variety programming language...?  Right?!?

   Well, not really.  Let's modify the value of 'N' and map a 'List'
again...

     (poke) N = 1
     (poke) List @ 0#B
     [0x746f6f72,0x303a783a]

   Yes, The boundary of the array type changed... on, this is Poke, was
you *really* expecting something typical?  ;)

   What happens is that at type definition time the lexical environment
is captured and a closure is created.  The body of the closure is the
expression.  Every time the type is referred, the closure is
re-evaluated and a new value is computed.

   Consequently, if the value of a variable referred in the expression
changes, like in our example, the type itself gets updated
auto-magically.  Very nice but, why is Poke designed like this?  Just to
impress the cat?  Nope.

   In binary formats, and also in protocols, the size of some given data
is often defined in terms of some other data that should be decoded
first.  Consider for example the following definition of a 'Packet':

     type Packet =
       struct
       {
         byte size;
         byte[size] payload;
       };

   Each packet contains a 8-bit integer specifying the size of the
payload transported in the packet.  The payload, a sequence of 'size'
bytes, follows.

   In struct types like the above, the boundaries of arrays depend on
fields that have been decoded before and that exist, like variables, in
the lexical scope captured by the struct type definition (yes, these are
also closures, but that's for another article.)  This absolutely depends
on having the array types evaluate their bounding expressions when the
type is used, and not at type definition time.

To show this property in action, let's play a bit:

     (poke) var data = byte[4] @ 0#B
     (poke) data[0] = 2
     (poke) data[1] = 3
     (poke) data[2] = 4
     (poke) data[3] = 5
     (poke) dump
     76543210  0011 2233 4455 6677 8899 aabb ccdd eeff
     00000000: 0203 0405 0000 0000 0000 0000 0000 0000
     00000010: 0000 0000 0000 0000 0000 0000 0000 0000
     (poke) var p1 = Packet @ 0#B
     (poke) var p2 = Packet @ 1#B
     (poke) p1
     Packet {size=0x2UB,payload=[0x3UB,0x4UB]}
     (poke) p2
     Packet {size=0x3UB,payload=[0x4UB,0x5UB,0x0UB]}

   Now, let's change the data and see how the sizes of the payloads are
adjusted accordingly:

     (poke) data[0] = 1
     (poke) data[1] = 0
     (poke) p1
     Packet {size=0x1UB,payload=[0x0UB]}
     (poke) p2
     Packet {size=0x0UB,payload=[]}

   So, as we have seen, Poke's way of handling boundaries in array types
allows data structures to adjust to the particular data they contain, so
usual in binary formats.  This is an important feature, that gives Poke
part of its feel and magic.

18.4.3 Casting Arrays
---------------------

There are two kinds of casts for array values:

   * Cast to array types that may have a different bounding.
   * Cast of array of integers or integral structs to an integral type.

   For example, we have seen that the type of an array literal is always
bounded by a constant number of elements.  Thus the type of '[1,2,3]' is
'int[3]'.

   We could cast the value above to an unbounded array type by doing:

     [1,2,3] as int[]

Or to an array bounded by size:

     [1,2,3] as int[12#B]

   Of course trying to cast an array value to a type for which the
bounding restrictions will fail.  This will give you a compile-time
error:

     [1,2,3] as int[4]

And this will result in a run-time error:

     (poke) [1,2,3] as int[13#B]
     unhandled conversion error exception

   Now let's see how Poke reinterprets integral arrays as integers:

     (poke) [0xdeUB,0xadUB] as uint16
     0xdeadUH
     (poke) [0xdeUB,0xadUB,0UB] as int
     0xdead00

   Trying to cast an array with a size larger than 64-bit will result in
an error:

     (poke) [1,2,3] as int
     unhandled conversion error exception
     arrays bigger than 64-bit cannot be casted to integral types

   It also works for nested arrays and arrays of integral structs (*Note
Integral Structs::):

     (poke) type Foo = struct uint { int16 hi; int16 lo; };
     (poke) [[Foo{ hi=0xdeadH, lo=0xbeefH}],[Foo{}]] as ulong
     0xdeadbeef00000000UL

   The way it works is like putting all array elements in an 'uint<64>'
number and then casting that number to the destination integral type.

18.4.4 Array Constructors
-------------------------

Array literals can only specify arrays which a constant number of
elements.  They also require initial values for their elements, and it
is also not possible to construct an array literal denoting an empty
array of some given type.

   Array constructors provide a suitable way to build such array values.
The syntax is:

     ARRAY_TYPE ([VALUE])

Where ARRAY_TYPE is an array type, or the name of an array type, and
VALUE is an optional argument that specify the value to which the
elements of the new array are initialized.  If no value is provided in
the constructor then default values are calculated.

   Examples:

     (poke) int[]()
     []
     (poke) int[2]()
     [0,0]
     (poke) int[2](10)
     [10,10]
     (poke) offset<int,B>[2](16#b)
     [2#B,2#B]
     (poke) string[3]()
     ["","",""]
     (poke) int[][3](int[2](666))
     [[666,666],[666,666],[666,666]]
     (poke) Packet[3]()
     [Packet {...}, Packet {...}, Packet {...}]

18.4.5 Array Comparison
-----------------------

The equality operator ('==') and the inequality operator ('!=') can be
applied to arrays.  Examples:

     (poke) [1,2,3] == [1,2,3]
     1
     (poke) [[1,2],[3,4]] != [[5,6],[7,8]]
     1

   Note that the array elements are compared recursively.

18.4.6 Array Indexing
---------------------

Arrays are indexed using the usual notation, providing an index enclosed
between square brackets with '[' and ']':

     (poke) [1,2,3][0]
     1
     (poke) [1,2,3][1]
     2

   The index should be an expression that evaluates to an integer value,
and it is promoted to an unsigned 64-bit integer when needed.

   The valid range for the index is '[0,N]' where N is the number of
elements stored in the array minus one.  If the passed integer is out of
that range, an 'E_out_of_bounds' exception is raised:

     (poke) [1,2,3][-1]
     unhandled out of bounds exception
     (poke) [1,2,3][3]
     unhandled out of bounds exception

18.4.7 Array Trimming
---------------------

Indexing is used to fetch elements from arrays.  Another operation,
called "trimming", allows you to extract a subset of the array, as
another array.

   Trims use the following notation, where a range is specified between
square brackets.  The left sides of the range are closed, whereas the
right sides of the range are open:

     (poke) [1,2,3][0:2]
     [1,2]
     (poke) [1,2,3][1:2]
     [2]
     (poke) [1,2,3][0:3]
     [1,2,3]

   If the minimum side of the range is omitted, it is assumed to be
zero.  If the maximum side of the range is omitted, it is assumed to be
the length of the trimmed array:

     (poke) [1,2,3][:2]
     [1,2]
     (poke) [1,2,3][1:]
     [2,3]
     (poke) [1,2,3][:]
     [1,2,3]

   The elements of the base array and the trimmed sub-array are copied
by shared value, exactly like when passing arguments to functions.  This
means that for simple types, copies of the elements are done:

     (poke) var a = [1,2,3]
     (poke) var s = a[1:2]
     (poke) s[0] = 66
     (poke) a
     [1,2,3]

   However, for complex types like arrays and structs, the values are
shared:

     (poke) var a = Packet[] @ 0#B
     (poke) var s = a[1:2]
     (poke) s[0].field = 66
     (poke) a[1].field
     66

   There is an alternative syntax that can be used in order to denote
trims, in which the length of the trim is specified instead of the index
of the second element.  It has this form:

     (poke) [1,2,3][0+:3]
     [1,2,3]
     (poke) [1,2,3][1+:2]
     [2,3]
     (poke) [1,2,3][1+:0]
     []

18.4.8 Array Elements
---------------------

The 'in' operator can be used to determine whether a given element is
stored in an array.  Examples:

     (poke) 2 in [1,2,3]
     1
     (poke) 10#B in [2#b,10*8#b,3#b]
     1
     (poke) 30#B in [2#b,10*8#b,3#b]
     0

18.4.9 Array Concatenation
--------------------------

The operator '+' can be used to build new arrays resulting from the
concatenation of the elements of two arrays given as operands:

     (poke) [1,2] + [3,4,5]
     [1,2,3,4,5]

The two arrays given as operands shall have the same elements of the
same type.  The resulting array is always unbounded.

18.4.10 Array Attributes
------------------------

The following attributes are defined for array values.

'size'
     Gives an offset with the storage occupied by the complete array.
     Example:

          (poke) [1,2,3]'size
          96UL#b
'length'
     Gives the number of elements stored in the array.  Example:

          (poke) [1,2,3]'length
          3
'mapped'
     Boolean indicating whether the array is mapped.  Examples:

          (poke) var a = [1,2,3]
          (poke) var b = int[3] @ 0#B
          (poke) a'mapped
          0
          (poke) b'mapped
          1
'strict'
     Boolean indicating whether the array is mapped in strict mode.
     Examples:

          (poke) var a = int[3] @ 0#B
          (poke) a'strict
          1
          (poke) var a = int[3] @! 0#B
          (poke) a'strict
          0

18.5 Structs
============

Structs are the main abstraction that Poke provides to structure data.
They contain heterogeneous collections of values.

18.5.1 Struct Types
-------------------

A simple struct type definition in Poke looks like:

     type Packet =
       struct
       {
         byte magic;
         uint<16> length;
         byte[length] data;
       }

   The above defines a "Packet", which consists on a magic number
encoded in a byte, a length encoded in an unsigned 16-bit integer, and
an array of 'length' bytes, which is the payload of the packet.

   Each entry in the struct type above defines a "struct field".

   Each field has a type, which is mandatory, and a name, which is
optional.  Fields without names are not accessible (not even within the
struct itself) but they are handy for expressing esoteric padding.
*Note Padding and Alignment::.  Example:

     type Imm64 =
       struct
       {
         uint<32> lo;
         uint<32>;
         uint<32> hi;
       }

   It is not allowed to have several fields with the same name in the
same struct.  The compiler will complain if it finds such an occurrence.

18.5.2 Struct Constructors
--------------------------

Once a struct type gets defined, there are two ways to build struct
values from it.  One is "mapping".  *Note Mapping Structs::.  The other
is using a "struct constructor", which is explained in this section.

   Struct constructors have the following form:

     TYPE_NAME { [FIELD_INITIALIZER,]... }

where each FIELD_INITIALIZER has the form:

     [FIELD_NAME=]EXP

   Note how each field has an optional name FIELD_NAME and a value EXP.
The expression for the value can be of any type.  For convenience, a
trailing comma is accepted but not required.

   Suppose for example that we have a type defined as:

     type Packet =
       struct
       {
         uint<16> flags;
         byte[32] data;
       }

   We can construct a new packet, with all its fields initialized to
zero, like this:

     (poke) Packet {}
     Packet {
       flags=0x0UH,
       data=[0x0UB,0x0UB,0x0UB,0x0UB,0x0UB,...]
     }

   In the constructor we can specify initial values for some of the
fields:

     (poke) Packet { flags = 0x8 }
     Packet {
       flags=0x8UH,
       data=[0x0UB,0x0UB,0x0UB,0x0UB,0x0UB,...]
     }

   It is not allowed to specify initializers that are not part of the
type being constructed:

     (poke) Packet { foo = 10 }
     <stdin>:1:10: error: invalid struct field `foo' in constructor
     Packet { foo = 10 };
              ^~~

   As we shall see later, many struct types define constraints on the
values their fields can hold.  This is a way to implement data
integrity.  While building struct values using constructors, these
constraints are taken into account.  For example, given the following
struct type:

     type BPF_Reg =
       struct
       {
        uint<4> code : code < 11;
       };

we will get a constraint violation exception if we try to construct an
'BPF_Reg':

     (poke) BPF_Reg { code = 20 }
     unhandled constraint violation exception

18.5.3 Struct Comparison
------------------------

The equality operator ('==') and the inequality operator ('!=') can be
applied to struct values.  Examples:

     (poke) E_eof == E_eof
     1
     (Poke) Packet { payload=0x3 } != Packet { payload=0x4 }
     1

   The struct values are compares recursively.

18.5.4 Field Endianness
-----------------------

By default fields are accessed in IO space using the current default
endianness.  However, it is possible to annotate fields with an explicit
endianness, like in:

     type Foo =
       struct
       {
         little int a;
         big int b;
         int c;
       };

   In the example above, the field 'a' will be stored using
little-endian, the field 'b' will be stored using big-endian, and the
field 'c' will be stored using whatever current endianness.

   The endianness annotations can be used in any kind of fields, like
offsets, arrays and structs, and will impact the storage ofintegral
fields defined in them.

   When endianness annotations are used in struct fields which are
themselves structs, they effectively change the default endianness in
the contained struct.  Therefore, in the following struct type:

     type Bar =
       struct
       {
         little struct
         {
           big int a;
           int b;
         } f;
         int c;
       }

'a' is big endian, but 'b' is little endian.

   However, note that the scope of the 'big' and 'little' annotations is
lexical.  Therefore, in these types:

     type Foo =
       struct
       {
         int a;
         int b;
       };

     type Bar =
       struct
       {
         big Foo f;
         int c;
       };

The endianness of the 'a' and 'b' fields stored in 'f' is the default
endianness, not 'big'.

18.5.5 Accessing Fields
-----------------------

Poke uses the usual dot-notation to provide access to struct fields.
Examples:

     (poke) var s = struct { i = 10, l = 20L }
     (poke) s.i
     10
     (poke) s.l
     20L

   Writing to fields is achieved by having the field reference in the
left side of an assignment statement:

     (poke) s.i = 100
     (poke) s.l = 200
     (poke) s
     struct {i=100,l=200L}

18.5.6 Field Constraints
------------------------

It is common for struct fields to be constrained to their values to
satisfy some conditions.  Obvious examples are magic numbers, and
specification-derived constraints.

   In Poke you can specify a field's constraint using the following
syntax:

     FIELD_TYPE FIELD_NAME : EXPRESSION ;

where EXPRESSION is an arbitrary Poke expression, that should evaluate
to an integer value.  The result is interpreted as a boolean.  As an
example, this is how the ELF magic number is checked for:

     type Ctf_Preamble =
       struct
       {
         uint<16> ctp_magic : ctp_magic == CTF_MAGIC;
         byte ctp_version;
         byte ctp_flags;
       };

   The constraint expression will often include the field where it is
installed, but that's not mandatory.

   Field constraints play an important role in mapping.  On one side, a
map will fail if there is some constraint that fails.  On the other,
they guide the mapping of unbounded arrays.  *Note Mapping Arrays::.

   Another common usage of constraint expressions is to alter the global
state after decoding a field.  For example, this is how mapping an ELF
header sets the current endianness, depending on the value of 'ei_data':

     byte ei_data : ei_data == ELFDATA2LSB       \
                    ? set_endian (ENDIAN_LITTLE) \
                    : set_endian (BIG);

   Note that 'set_endian' always returns '1'.

18.5.7 Field Initializers
-------------------------

We saw that field constraints are useful to express magic numbers, which
are pretty common in binary formats.  Imagine a package has a marker
byte at the beginning, which should always be '0xff'.  We could use a
constraint like in:

     type Packet =
       struct
       {
         byte marker : marker == 0xff;
         byte length;
         byte[length] payload;
       };

   This works well when mapping packages.  The constraint is checked and
a constraint violation exception is raised if the first byte of the
alleged package is not '0xff'.

   However, suppose we want to construct a new 'Package', with no
particular contents.  We would use a constructor, but unfortunately:

     (poke) Packet { }
     unhandled constraint violation exception

   What happened?  Since we didn't specify a value for the marker in the
struct constructor, a default value was used.  The default value for an
integral type is zero, which violates the constraint associated with the
field.  Therefore, we would need to remember to specify the marker,
every time we construct a new packet:

     (poke) Packet { marker = 0xff }
     Packet {
       marker=0xffUB,
       length=0x0UB,
       payload=[]
     }

   Unfortunately, such markers and magic numbers are not precisely very
memorable.  To help with this, Poke has the notion of "type field
initializers".  Let's use one in our example:

     type Packet =
       struct
       {
         byte marker = 0xff;
         byte length;
         byte[length] payload;
       };

   Note how the syntax is different than the one used for constraints.
When a field in a struct type has an initializer, the struct constructor
will use the initializer expression as the initial value for the field.
For example:

     (poke) Packet {}
     Packet {
       marker=0xffUB,
       length=0x0UB,
       payload=[]
     }

   It is possible to specify both a constraint and an initializer in the
same field.  Suppose we want to support several kinds of packets,
characterized by several markers.  The supported markers are however a
closed set.  We could do it like this:

     type Packet =
       struct
       {
         byte marker = 0xff : marker in [0xffUB, 0xfeUB];
         byte length;
         byte[length] payload;
       };

   Note however that struct mappers do not make use of field
initializers, since the mapped IO space provides values for all the
fields in the struct type.  If we mapped the 'Packet' type above, we
would need to add also a constraint to make sure the value of 'marker'
is the right one.  This also applies to constructing 'Packet' types
where an initial value is explicitly specified.  We could do it by
specifying both an initial value, and a constraint expression:

     type Packet =
       struct
       {
         byte marker = 0xff : marker == 0xff;
         byte length;
         byte[length] payload;
       };

   This idiom is to common that Poke provides a more compact syntax to
denote it, that avoids verbosity and replicated logic:

     type Packet =
       struct
       {
         byte marker == 0xff;
         byte length;
         byte[length] payload;
       };

   It is considered good practice to design struct types in a way that a
constructor with no arguments will result in something usable.

18.5.8 Field Labels
-------------------

In structs, each field is associated with an offset, which is relative
to the beginning of the struct.  This is the offset used when reading
and writing the field from/to the IO space when mapping.

   The offset is reset to zero bits at the beginning of the struct type,
and it is increased by the size of the fields:

     struct
     {
                     /* Offset: 0#b */
         uint<1> f1; /* Offset: 1#b */
         sint<7> f2; /* Offset: 1#B */
         int     f3; /* Offset: 5#B */
         Bar     f4; /* Offset: 23#B */
     }

   It is possible to specify an alternative offset for a field using a
"field label".

   Consider for example an entry in an ELF symbol table.  Each entry has
a 'st_info' field which is 64-bits long, that in turn can be interpreted
as two fields 'st_bind' and 'st_type'.

   The obvious solution is to encode 'st_info' as a sub-struct that is
integral, like this:

     struct
     {
       elf32_word st_name;
       struct uint<64>
       {
         uint<60> st_bind;
         uint<4> st_type;
       } st_info;
     }

   This makes the value of 'st_info' easily accessible as an integral
value.  But we may prefer to use labels instead:

     struct
     {
       elf32_word st_name;
       elf64_word st_info;
       uint<60> st_bind @ 4#B;
       uint<4> st_type @ 4#B + 60#b;
     }

   The resulting struct has fields 'st_info', 'st_bind' and 'st_type',
with the last two overlapping the first.

18.5.9 Pinned Structs
---------------------

Pinned structs is a convenient way to write struct types where the
offset of all its fields is zero.  They are equivalent to C unions.

   For example, consider the '_u' field below in a CTF type description:

     type Ctf_Stype_V1 =
       struct
       {
         Ctf_Name ctt_name;
         Ctf_Info_V1 ctt_info;
         pinned struct
         {
           uint32 _size;		/* Size of entire type in bytes.  */
           uint32 _type;		/* Reference to another type.  */
         } _u;
       };

   Note that integral structs cannot be pinned.  *Note Integral
Structs::.  And field labels are not allowed in pinned structs.

18.5.10 The OFFSET variable
---------------------------

The offset of the current field being mapped or constructed, relative to
the beginning of the struct, is at all times available in a variable
called 'OFFSET'.  This is useful in situations where we need that
information in a constraint expression, the boundary of an array, an
initialization value, or the like.

   For example, what follows is a typical usage of OFFSET in order to
introduce some padding in a struct:

     type Elf64_Note =
       struct
       {
         [...]

         Elf64_Xword name;
         byte[alignto (OFFSET, 4#B)] padding;

         [...]
       };

   The value of this variable changes every time a new field is mapped
or constructed, so writing to it will only have effect until the next
field is processed: at that time the value will be overwritten.

18.5.11 Integral Structs
------------------------

Integral structs are useful to cover cases where data is stored in
composited integral containers, i.e.  where data is structured within
stored integers.

   Basically, when we structure data using Poke structs, arrays and the
like, we often use the same structure than a C programmer would use.
For example, to model ELF RELA structures, which are defined in C like:

     type struct
       {
         Elf64_Addr   r_offset;  /* Address */
         Elf64_Xword  r_info;    /* Relocation type and symbol index */
         Elf64_Sxword r_addend;  /* Addend */
       } Elf64_Rela;

we could use something like this in Poke:

     type Elf64_Rela =
       struct
       {
         Elf64_Addr r_offset;
         Elf64_Xword r_info;
         Elf64_Sxword r_addend;
       };

   Here the Poke struct type is pretty equivalent to the C incarnation.
In both cases the fields are always stored in the given order,
regardless of endianness or any other consideration.

   However, there are situations where stored integral values are to be
interpreted as composite data.  This is the case of the 'r_info' field
above, which is a 64-bit unsigned integer ('Elf64_Xword') which is
itself composed by several fields, depicted here:

      63                                          0
     +----------------------+----------------------+
     |       r_sym          |      r_type          |
     +----------------------+----------------------+
      MSB                                       LSB

   In order to support this kind of composition of integers, C
programmers usually resort to either bit masking (most often) or to the
often obscure and undefined behaviour-prone C bit fields.  In the case
of ELF, the GNU implementations define a few macros to access these
"sub-fields":

     #define ELF64_R_SYM(i)         ((i) >> 32)
     #define ELF64_R_TYPE(i)        ((i) & 0xffffffff)
     #define ELF64_R_INFO(sym,type) ((((Elf64_Xword) (sym)) << 32) + (type))

   Where 'ELF64_R_SYM' and 'ELF64_R_TYPE' are used to extract the fields
from an 'r_info', and 'ELF64_R_INFO' is used to compose it.  This is
typical of C data structures.

   We could of course mimic the C implementation in Poke:

     fun Elf64_R_Sym = (Elf64_Xword i) uint<32>:
        { return i .>> 32; }
     fun Elf64_R_Type = (Elf64_Xword i) uint<32>:
        { return i & 0xffff_ffff; }
     fun Elf64_R_Info = (uint<32> sym, uint<32> type) Elf64_Xword:
        { return sym as Elf64_Xword <<. 32 + type; }

   However, this approach has a huge disadvantage: since we are not able
to encode the logic of these "sub-fields" in proper Poke fields, they
become second class citizens, with all that implies: no constraints on
their own, can't be auto-completed, can't be assigned individually, etc.

   This is where "integral structs" come to play.  These are structs
that are defined exactly like your garden variety Poke structs, with a
small addition:

     type Elf64_RelInfo =
       struct uint<64>
       {
         uint<32> r_sym;
         uint<32> r_type;
       };

   Note the 'uint<64>' addition after 'struct'.  This can be any integer
type (signed or unsigned).  The fields of an integral struct should be
integral themselves (this includes both integers and offsets) and the
total size occupied by the fields should be the same size than the one
declared in the struct's integer type.  This is checked and enforced by
the compiler.

   The Elf64 RELA in Poke can then be encoded like:

     type Elf64_Rela =
       struct
       {
         Elf64_Addr r_offset;
         struct Elf64_Xword
         {
           uint<32> r_sym;
           uint<32> r_type;
         } r_info;
         Elf64_Sxword r_addend;
       };

   When an integral struct is mapped from some IO space, the total
number of bytes occupied by the struct is read as a single integer
value, and then the values of the fields are extracted from it.  A
similar process is using when writing.  That is what makes it different
with respect a normal Poke struct.

   Consider for example we have the following sequence of bytes in our
IO space (like a file):

     0x10 0x20 0x30 0x40  0x50 0x60 0x70 0x80

   Let's see what happens when we map the integral struct above, in both
big and little endian:

     (poke) .set endian big
     (poke) Elf64_RelInfo @ 0#B
     Elf64_RelInfo {
       r_sym=0x10203040U,
       r_type=0x50607080U
     }
     (poke) .set endian little
     (poke) Elf64_RelInfo @ 0#B
     Elf64_RelInfo {
       r_sym=0x80706050U,
       r_type=0x40302010U
     }

   Looks good.  For comparison, this is what happens when we do the same
with an "equivalent" (not really) non-integral struct operating on the
same data:

     type Elf64_RelInfoBogus =
       struct
       {
         uint<32> r_sym;
         uint<32> r_type;
       };

   We would get:

     (poke) .set endian big
     (poke) Elf64_RelInfoBogus @ 0#B
     Elf64_RelInfoBogus {
       r_sym=0x10203040U,
       r_type=0x50607080U
     }
     (poke) .set endian little
     (poke) Elf64_RelInfoBogus  0#B
     Elf64_RelInfoBogus {
       r_sym=0x40302010U,
       r_type=0x80706050U
     }

   In this case, and unlike with integral structs, the endianness
impacts the bytes of the individual fields, not of the whole struct.

   As you can see, integral structs can be used to denote a lot of
commonly found idioms in data structures and this includes a lot of what
is sometimes denoted in C bit field.  However, one should be cautious
when "translating" C structures to Poke, especially when the C
programmer has not been careful and incurres in sometimes obscure
implementation-defined behavior.  An integral struct is not always the
right abstraction to use when we see a C bit field!

   As an example of the above, consider the following C struct:

     struct regs
     {
      __u8 dst_reg:4;
      __u8 src_reg:4;
     };

   Certain virtual architecture uses that data layout to store registers
in instructions (no comment.)  Thing is, in bit fields like the above
with sub-byte field sizes, the ordering of the fields is not clearly
defined, and ultimately what order to use is up to the compiler, i.e.
to lore and tradition.  As it happens, GCC encodes 'src_reg' in the most
significant nibble of the byte and 'dst_reg' in the least significant
nibble of the byte when compiling for a little-endian target, and the
other way around when compiling for a little-endian target.  (I may have
had that wrong, this always confuses me.)

   How could we encode the C struct regs in Poke?  Let's see.

   A normal Poke struct clearly won't do it:

     type RegsBogus1 =
       struct
       {
         uint<4> src;
         uint<4> dst;
       };

   The reason being, the ordering of src and dst does not change when
you switch endianness (since this is Poke, we can in fact talk about
real ordering of bits)...  remember, poke is WYPIWIG (what you poke is
what you get) ;)

   What about an integral struct?

     type RegsBogus2 =
       struct uint<8>
      {
         uint<4> src;
         uint<4> dst;
      };

   This won't work either.  In fact, the net effect of the normal
decoding of the normal struct type RegsBogus1 and the
map-an-integer-and-extract-fields decoding of the integral struct
RegsBogus2 is in this case totally equivalent.

   A solution is to use a normal struct, and field labels:

     type RegsBogus =
       struct
       {
         var little_p = (get_endian == ENDIAN_LITTLE);

         uint<8> src @ !little_p * 4#b;
         uint<8> dst @ little_p * 4#b;
       };

   At this point, you may be wondering: is there anything particular in
a field defined in an integral struct?  The answer is: no, not at all.
These are regular, first-class fields.  Likewise, integral structs are
perfectly regular structs.  And of course, since this is poke, you can
have integral structs of say, 11 bits, or 3 bits, map them at offsets
not aligned to bytes, and all the typical poke-atrocities that we enjoy
so much.

   However, there exist a few restrictions, some of them fundamental,
the others to be lifted eventually:

   - There are no integral unions.  This is a fundamental limitation and
     will most likely stay like that.
   - Integral structs can only have integral fields.  This includes
     offsets.
   - No labels are allowed in the fields of integral structs.  This is
     not a fundamental limitation, and may be supported at some point.
   - No integral structs are supported inside other integral structs.
     This is purely because of lazyness on my part.  This will be
     eventually supported.
   - No optional fields are supported in integral structs.  Support for
     this is actually partially implemented (the mapper supports them
     but not the writer) and most probably will be completed one of
     these days.

   Integral structs can be converted from/to integral structs to/from
integers, so we can do things like:

     rel.r_info as uint<64>;

   And also automatic promotions in arithmetic operators, like:

     rel.r_info + 20 * rel.r_info.r_type

   Integers can also be "structured" into integral structs:

     (poke) 0xdeadbeef as Elf32_RelInfo
     Elf32_RelInfo {
       r_sym=(uint<24>) 0xdeadbe,
       r_type=0xefUB
     }

18.5.12 Unions
--------------

Union types are defined much like struct types, using a very similar
syntax:

     union
     {
       [... elements ...]
     }

Like in structs, the main kind of elements are fields.  However, unlike
in structs, the fields in an union type denote "alternatives".  At any
given time, only one alternative is selected, and therefore union
_values_ have only one field.

   Which of the several alternatives is selected is determined by the
data integrity defined by means of constraint expressions and other
means.  Alternatives are considered in turn, in written order, and the
first alternative that can be used without triggering a constraint
violation exception is selected.

   It makes little sense to mark union types as pinned, since whatever
alternative is chosen will always be located at the beginning of the
union.  To avoid unneccesary confusion, the Poke compiler will emit an
error if you try to tag an union as pinned.

   Similarly, labels are not allowed in union alternatives.

18.5.13 Union Constructors
--------------------------

Constructing union values is very similar to constructing struct values:

     TYPE_NAME { [FIELD_INITIALIZER] }

   The main difference is that union constructors require either exactly
one field initializer, or none.  Specifying initialization values for
more than one field doesn't make sense in the case of unions.

   Suppose we have the following union type:

     type Packet =
       union
       {
         byte b : b > 0;
         int k;
       };

   If we try to construct a 'Packet' with an invalid 'b', we get a
constraint error:

     (poke) Packet {b = 0}
     unhandled constraint violation exception

The reason why 'k' is not chosen is because we specified 'b' in the
union constructor: we explicitly as for a 'Packet' of that kind, but the
data we provide doesn't accommodate to that.  If we wanted another kind
of value, we could of course specify an initial value for 'k' instead:

     (poke) Packet {k = 10}
     Packet {
       k=10
     }

   If no field initializer is specified in an union constructor then
each alternative is tried assuming all fields have default values, which
in the case of integral types is a zero:

     (poke) Packet {}
     Packet {
       k=0
     }

Therefore the alternative 'b' is considered invalid (it has to be bigger
than 0) and 'k' is chosen instead.

18.5.14 Optional Fields
-----------------------

Sometimes a field in a struct is optional, i.e.  it exists or not
depending on some criteria.  A good example of this is the "extended
header" in a MP3 id3v2 tag.  From the specification:

     The second bit (bit 6) indicates whether or not the header is followed
     by an extended header. The extended header is described in section
     3.2. A set bit indicates the presence of an extended header.

   In order to express this in a Poke struct type, we could of course
use an union, like:

     type ID3V2_Tag =
       struct
       {
         ID3V2_Hdr hdr;
         union
         {
           ID3V2_Ext_Hdr hdr if hdr.extended_hdr_present;
           struct {};
         } ext;
         ...
       }

   That's it, we use an union with an empty alternative.  However, this
is a bit cumbersome.  Therefore Poke provides a more convenient way to
denote this:

     type ID3V2_Tag =
       struct
       {
          ID3V2_Hdr hdr;
          IDV2_Ext_Hdr ext_hdr if hdr.extended_hdr_present;
          ...
       }

   If both a constraint and an optional field expression are specified,
the second should follow the first.

   One important characteristic of optional fields to keep in mind is
that they are always constructed or mapped, even when they are omitted.
This is because, generally speaking, the value of an optional field may
be necessary in order to determine whether the optional field is present
or not.

   This may result on unexpected results for the unaware pokist, like in
the example below:

     struct
     {
       Header hdr;
       byte[hdr.len - hdr.padding] data if hdr.len >= hdr.padding;
     }

where the array index may underflow if 'hdr.len' is less or equal than
'hdr.padding'.

   For obvious reasons, optional fields are not allowed in unions.

18.5.15 Casting Structs
-----------------------

It is possible to cast a struct of some particular type into another
struct type.  Examples:

     (poke) type Foo = struct { int i; int j; };
     (poke) type Bar = struct { int k; int j; };
     (poke) Bar {j=2} as Foo {}
     Foo {i=0,j=2}

   The semantics of the cast are exactly the same than constructing a
struct of the target type using the struct provided as an expression to
the cast.

   This means that some fields in the original struct may be "lost" when
the struct is converted to the new type.  Consider the following two
definitions:

     type Foo = struct { int i; long j; };
     type Bar = struct { int i; }

   Let's cast some values and see what we obtain:

     (poke) Bar {}
     Bar {
       i=0x0
     }
     (poke) Bar {} as Foo
     Foo {
       i=0x0,
       j=0x0L
     }
     (poke) Foo {} as Bar
     Bar {
       i=0x0
     }

   It is possible to cast an integral struct into an integral type.  The
value to cast is extracted from the struct fields, and then it is
converted to the target type.

   Given the following integral struct type:

     type Foo =
       struct int<64>
       {
         int<32> f1;
         uint<32> f2;
       };

We can operate:

     (poke) Foo { f1 = 1, f2 = 1 } as uint<64> + 2
     0x100000003UL

   Conversely, it is possible to cast an integral value to an integral
struct:

     (poke) 0x100000003UL as Foo
     Foo {
       f1=0x1,
       f2=0x3U
     }

18.5.16 Declarations in Structs
-------------------------------

Type declarations, variable declarations and function declarations are
all allowed inside struct and union types.  The scope of the entity
being declared spans from the declaration until the end of the struct or
union type being defined.

   Functions can't set fields defined in the struct type.

18.5.17 Methods
---------------

Methods in structs can be defined using the 'method' keyword in
definitions like this:

     method IDENTIFIER = FUNCTION_BODY

where FUNCTION_BODY is the body of a function.  *Note Function
Declarations::.

   When invoked, a method gets an implicit _last_ argument which is the
struct value whose method is being used.

   Methods can refer to the fields of the containing struct type that
have been defined before the method itself, by name.

18.5.18 Struct Attributes
-------------------------

The following attributes are defined for struct values.

'size'
     Gives an offset with the storage occupied by the complete struct.
     Example:

          (poke) type Packet = struct { byte s; byte[s] d; }
          (poke) (Packet { s = 3 })'size
          32UL#b
'length'
     Gives the number of fields stored in the struct.  Note that, due to
     absent fields, this doesn't always corresponds to the number of
     fields that appear in the definition of a struct type.

     For example, for the following struct type:

          type Packet =
            struct
            {
              byte magic: magic in [0xff,0xfe];
              byte control if magic == 0xfe;
              byte[128] payload;
            };

     We get the following results:

          (poke) (Packet { magic = 0xff })'length
          2UL
          (poke) (Packet { magic = 0xfe })'length
          3UL
'mapped'
     Boolean indicating whether the struct is mapped.
'strict'
     Boolean indicating whether the struct is mapped in strict mode.

18.6 Types
==========

18.6.1 'type'
-------------

The 'type' directive allows you to declare named types.  The syntax is:

     type NAME = TYPE [, NAME = TYPE]...;

where NAME is the name of the new type, and TYPE is either a type
specifier or the name of some other type.

   The supported type specifiers are:

'int<N>, uint<N>'
     Integral types.  *Note Integer Types::.
'string'
     The string type.  *Note String Types::.
'TYPE[BOUNDARY]'
     Array types.  *Note Array Types::.
'struct { ... }'
     Struct types.  *Note Struct Types::.
'(TYPE,...)TYPE:'
     Function types.  *Note Function Types::.
'any'
     The 'any' type is used to implement polymorphism.  *Note The any
     Type::.

18.6.2 The 'any' Type
---------------------

Poke supports polymorphism with the 'any' type.  This type is used in
contexts where a value of any type is allowed.  For example, this is how
you would declare a function that prints the size of any given value:

     fun print_size = (any value) void:
     {
       printf "%v\n", any'size;
     }

   The rules for handling 'any' values are simple:
   - Everything coerces to 'any'.
   - Nothing coerces from 'any'.

   This means that using any operator that require certain types with
'any' values will fail: you have to cast them first.  Example:

     (poke) fun foo = (any v) int: { return v as int; }

   Arrays of 'any', 'any[]', are also supported:

18.6.3 The 'isa' Operator
-------------------------

The binary operator 'isa' allows you to check for the type of a given
value:

     (poke) 10 isa int
     1
     (poke) "foo" isa string
     1
     (poke) (Packet  0#B) isa Packet
     1
     (poke) 2 as int<3> isa uint<4>
     0

18.7 Assignments
================

The assignment statement has the form:

     LVALUE = EXP;

where LVALUE is either:

   * A variable.
   * A field reference like 'foo.bar'.
   * An index reference like 'foo[30]'.
   * A map of a type, like 'int @ 0#B' or 'int[3] @ 0#B' or 'Packet @
     12#B'.

   In all cases, the type of EXP should match the type of the referred
entity.

   Examples:

     (poke) foo = 10
     (poke) packet.length = 4
     (poke) packet.data = [1,2,3,4]
     (poke) packet.data[2] = 666
     (poke) int @ 23#B = 23
     (poke) string @ str.offset = "foo"
     (poke) int[2] @ 23#B = [1,2]
     (poke) Packet @ 0#B = Packet { ... }

18.8 Compound Statements
========================

Compound statements have the form

     { STMT... }

where STMT... is a list of statements, which can be themselves compound
statements.  Compound statements are primarily used to sequence
instructions like:

     {
       do_a;
       do_b;
       do_c;
     }

   A compound statement introduces a new lexical scope.  Declarations in
the compound statements are local to that statement.

   Finally, compound statements can be empty: '{ }'.

18.9 Conditionals
=================

Poke provides several conditional statements, and a ternary conditional
operator, which are discussed in the sections below.

18.9.1 'if-else'
----------------

The 'if-else' statement has the form

     if (EXP) IF_STMT [else ELSE_STMT]

where EXP is an expression that should evaluate to a boolean value (i.e.
to an integer), IF_STMT is a statement that will be executed if EXP
holds true, and ELSE_STMT is a statement that will be executed if EXP
holds false.  The 'else' part of the statement is optional.

18.9.2 Conditional Expressions
------------------------------

Poke supports a ternary conditional expression that has the form:

     CONDITION ? TRUE_EXPRESSION : FALSE_EXPRESSION

where CONDITION is an expression that should evaluate to a boolean, and
TRUE_EXPRESSION and FALSE_EXPRESSION are expressions that have exactly
the same type.

18.10 Loops
===========

Poke supports several iteration statements, which are discussed in the
sections below.

18.10.1 'while'
---------------

The 'while' statement has this form:

     while (EXP) STMT

where EXP is an expression that should evaluate to a boolean (i.e.  to
an integer) and STMT is an statement that will be executed until EXP
holds false.

   It is possible to leave the loop from within STMT using the 'break'
statement.  Example:

     while (1)
     {
       [...]
       if (exit_loop)
         break;
     }

   It is also possible to jump to the next iteration of the loop from
within STMT using the 'continue' statement.  Example:

     while (1)
     {
        [...]
        if (continue_loop)
          continue;
     }

18.10.2 'for'
-------------

The 'for' statement has this form:

     for ([DECLS];[EXPR];[STMTS]) STMT

where DECLS is an optional declaration or chain of declarations
separated by commas, EXPR is an optional boolean expression, STMTS is an
optional comma-separated list of statements, and STMT is a statement.

18.10.3 'for-in'
----------------

The 'for-in' statement has this form:

     for (FORMAL in CONTAINER [where EXP]) STMT

where, in each iteration, the name FORMAL will be associated with
consecutive values of CONTAINER, which shall be an expression evaluating
to an array or a string.  FORMAL is available in STMT, which is the
statement executed in each iteration.

   If the WHERE part is specified, then only iteration in which EXP
holds true are processed.  FORMAL can be referred in EXP.  Note that
this doesn't mean the loop will stop after processing the first "not
selected" element.  See the following example:

     (poke) for (c in [1,2,3,4] where c % 2) printf " %v", c
      1 3

   It is possible to leave the loop from within STMT using the 'break'
statement.

   It is also possible to jump to the next iteration of the loop from
within STMT using the 'continue' statement.

18.11 Expression Statements
===========================

Poke is one of these languages where there is a clear separation between
"expressions" and "statements".  However, it is often useful to use an
expression in the place of an statement, in order to benefit from its
side effects.

   For that purpose Poke allows you to expressions as statements using
the following syntax:

     EXP;

   The value computed by the expression will be discarded.

18.12 Functions
===============

18.12.1 Function Declarations
-----------------------------

A function is declared using the following syntax:

     fun NAME = [(FORMAL,...)] RET_TYPE:
     {
        ... body ...
     }

where NAME is the name of the function, which uses the same namespace as
variables and types and RET_TYPE is the type of the value returned by
the function.  If the function returns no value then it is 'void'.

   Each FORMAL argument has the form:

     TYPE NAME [= EXP]

where TYPE is the type of the formal, NAME its name, and EXP is an
optional expression that will be used to initialize the argument in case
it is not specified when the function is called.

   The last formal argument can take the form 'NAME...', meaning the
function is variadic.  *Note Variadic Functions::.

   If the function takes no arguments, it is possible to omit the list
of arguments entirely:

     fun hello = void: { print "Hello!\n"; }

   The 'return' statement is used to return values in functions that
return a value.  Example:

     fun gcd = (uint<64> a, uint<64> b) uint<64>:
       {
        if (b == 0)
          return a;
        else
          return gcd (b, a % b);
       }

   Note that reaching the end of a non-void function will trigger a
run-time error.

18.12.2 Optional Arguments
--------------------------

Optional function arguments are specified like:

     fun atoi = (string s, int b = 10) long: { ... }

   Which means that if the base argument is not specified when passed to
'atoi' then it is initialized to 10.

   Optional arguments should not appear before any non-optional argument
in function declarations.  The following is not valid Poke:

     fun foo = (int i = 10, int j) int: { return i + j; }

   Note that arguments declared before an optional argument can be used
in its initialization expression.  This is valid Poke:

     fun foo = (int n, int[n] array = init_array (n)) void: { ... }

18.12.3 Variadic Functions
--------------------------

Functions getting an arbitrary number of arguments are denoted like
this:

     fun printf (string fmt, args...) void: { ... }

   The variadic argument shall be the last argument in the function, and
it is of type 'any[]'.

18.12.4 Calling Functions
-------------------------

To call a function, write its name followed by the arguments in
parentheses.  Examples:

     foo (1,2,3)
     bar ()

   If the function takes no arguments then it is not necessary to write
the empty list of arguments.  Therefore the following two calls are
equivalent:

     bar ()
     bar

   If the closure of function that takes no arguments is needed, the
call can be avoided by putting the variable immediately inside
parenthesis, like this:

     (poke) (foo)
     #<closure>

   There is an alternate syntax that can be used in both expressions and
expression-statements.  This alternate syntax is:

     FUNCTION_NAME :ARG1 VAL1...

where ARG1 is the name of an argument and VAL1 the value to pass for
that argument.  This is useful when using functions as commands in the
REPL:

     (poke) dump :from 12#B :size 16#B :ascii 0

   Note that the named arguments can appear in any order.  The following
two calls are equivalent:

     dump :from 12#B :size 16#B
     dump :size 16#B :from 12#B

   If this alternate syntax for function calls is to be used in an
expression, it is necessary to surround it with parenthesis:

     2 + (gcd :a 1024 :b 8)

18.12.5 Function Types
----------------------

Function types are denoted like:

     (TYPE,...)RET_TYPE:

where TYPE are the types of the arguments and RET_TYPE is the type of
the value returned by the function.

   Optional arguments are marked with a '?' after the type.  For
example, the type of the 'atoi' function with declaration:

     fun atoi = (string s, int b = 10) long: { ... }

is '(string,int?)long:'.

   If the function has variadic arguments, the position of the variadic
argument in the function type specifier contains '...'.  For example,
the type of a 'printf' function with declaration:

     fun printf (string fmt, args...) void: { ... }

is '(string,...)void:'.

18.12.6 Lambdas
---------------

Poke support "lambda expressions" using the following syntax:

     lambda FUNCTION_SPECIFIER

Where FUNCTION_SPECIFIER is any function specifier.  Examples:

     lambda void: {}
     lambda (int i) int: { return i * 2; }

Lambdas can be manipulated exactly like any other Poke value, and can be
stored in variables.  Therefore, this is an alternative way of defining
a named function (which can't be recursive for obvious reasons):

     var double = lambda (int i) int: { return i * 2 };

18.12.7 Function Comparison
---------------------------

The equality operator ('==') and the inequality operator ('!=') can be
applied to functions.  They evaluate to true if the operands are the
same function value.  Examples:

     (poke) lambda void: {} == lambda void: {}
     0
     (poke) lambda void: {} != lambda void: {}
     1
     (poke) var f = void: {}
     (poke) f == f
     1
     (poke) f != f
     0

18.12.8 Function Attributes
---------------------------

The following attributes are defined for function values.

'size'
     Gives an offset '0#B', by convention.

18.13 Endianness
================

Byte endianness is an important aspect of encoding data.  As a good
binary editor poke provides support for both little and big endian, and
will soon acquire the ability to encode exotic endianness like PDP
endian.  Endianness control is integrated in the Poke language, and is
designed to be easily used in type descriptions.

18.13.1 '.set endian'
---------------------

GNU poke maintains a global variable that holds the current endianness.
This is the endianness that will be used when mapping integers whose
types do not specify an explicit endianness.

   Like other poke global state, this global variable can be modified
using the '.set' dot-command:

     .set endian little
     .set endian big
     .set endian host

The current endianness can be obtained like this:

     (poke) .set endian
     little

   We can easily see how changing the current endianness indeed impacts
the way integers are mapped:

     (poke) dump :from 0#B :size 4#B :ruler 0 :ascii 0
     00000000: 8845 4c46
     (poke) .set endian little
     (poke) int @ 0#B
     0x464c4588
     (poke) .set endian big
     (poke) int @ 0#B
     0x88454c46

18.13.2 Endian in Fields
------------------------

It is possible to set the endianness of integral fields in struct type
descriptors.  *Note Field Endianness::.

18.13.3 Endian built-ins
------------------------

As handy as the '.set endian' dot-command may be, it is also important
to be able to change the current endianness programmatically from a Poke
program.  For that purpose, the PKL compiler provides a couple of
built-in functions: 'get_endian' and 'set_endian'.

   Their definitions, along with the specific supported values, look
like:

     var ENDIAN_LITTLE = 0;
     var ENDIAN_BIG = 1;

     fun get_endian = int: { ... }
     fun set_endian = (int endian) int: { ... }

   Accessing the current endianness programmatically is especially
useful in situations where the data being poked features a different
structure, depending on the endianness.

   A good (or bad) example of this is the way registers are encoded in
eBPF instructions.  eBPF is the in-kernel virtual machine of Linux, and
features an ISA with ten general-purpose registers.  eBPF instructions
generally use two registers, namely the source register and the
destination register.  Each register is encoded using 4 bits, and the
fields encoding registers are consecutive in the instructions.

   Typical.  However, for reasons we won't be discussing here the order
of the source and destination register fields is switched depending on
the endianness.

In big-endian systems the order is:

     dst:4 src:4

Whereas in little-endian systems the order is:

     src:4 dst:4

   In Poke, the obvious way of representing data whose structure depends
on some condition is using an union.  In this case, it could read like
this:

     type BPF_Insn_Regs =
       union
       {
         struct
         {
           BPF_Reg src;
           BPF_Reg dst;
         } le : get_endian == ENDIAN_LITTLE;

         struct
         {
           BPF_Reg dst;
           BPF_Reg src;
         } be;
       };

   Note the call to the 'get_endian' function (which takes no arguments
and thus can be called Algol68-style, without specifying an empty
argument list) in the constraint of the union alternative.  This way,
the register fields will have the right order corresponding to the
current endianness.

   Nifty.  However, there is an ever better way to denote the structure
of these fields.  This is it:

     type BPF_Insn_Regs =
       struct
       {
         var little_p = (get_endian == ENDIAN_LITTLE);

         BPF_Reg src @ !little_p * 4#b;
         BPF_Reg dst @ little_p * 4#b;
       };

   This version, where the ordering of the fields is implemented using
field labels, is not only more compact, but also has the virtue of not
requiring additional "intermediate" fields like 'le' and 'be' above.  It
also shows how convenient can be to declare variables inside structs.

Let's see it in action:

     (poke) BPF_Insn_Regs @ 1#B
     BPF_Insn_Regs {src=#<%r4>,dst=#<%r5>}
     (poke) .set endian big
     (poke) BPF_Insn_Regs @ 1#B
     BPF_Insn_Regs {src=#<%r5>,dst=#<%r4>}

   Changing the current endianness in constraint expressions is useful
when dealing with binary formats that specify the endianness of the data
that follows using some sort of tag.  This is the case of ELF, for
example.  The first few bytes in an ELF header conform what is known as
the 'e_ident'.  One of these bytes is called 'ei_data' and its value
specifies the endianness of the data stored in the ELF file.

This is how we handle this in Poke:

     fun elf_endian = (int endian) byte:
      {
        if (endian == ENDIAN_LITTLE)
          return ELFDATA2LSB;
        else
          return ELFDAT2MSB;
      }

     [...]

     type Elf64_Ehdr =
       struct
       {
         struct
         {
           byte[4] ei_mag : ei_mag[0] == 0x7fUB
                            && ei_mag[1] == 'E'
                            && ei_mag[2] == 'L'
                            && ei_mag[3] == 'F';
           byte ei_class;
           byte ei_data : (ei_data != ELFDATANONE
                           && set_endian (elf_endian (ei_data)));
           byte ei_version;
           byte ei_osabi;
           byte ei_abiversion;
           byte[6] ei_pad;
           offset<byte,B> ei_nident;
         } e_ident;

         [...]
       };

   Note how 'set_endian' returns an integer value... it is always '1'.
This is to facilitate its usage in field constraint expressions.

18.14 Mapping
=============

The purpose of poke is to edit "IO spaces", which are the files or
devices, or memory areas being edited.  This is achieved by mapping
values.  Mapping is perhaps the most important concept in Poke.

18.14.1 IO Spaces
-----------------

GNU poke supports the abstract notion of "IO space", which is an
addressable space of Poke objects: integers, strings, arrays, structs,
etc.  This underlying storage for the IO spaces (which we call "IO
devices") can be heterogeneous: from a file your file system to the
memory of some process.

18.14.1.1 'open'
................

The 'open' builtin allows you to create new IO spaces, by opening an IO
device.  It has the following prototype:

     fun open = (string HANDLER, uint<64> flags = 0) int<32>

where HANDLER is a string identifying the IO device that will serve the
IO space.  This handler can be:

'*NAME*'
     An auto growing memory buffer.
'pid://[0-9]+'
     The process ID of some process.
'/path/to/file'
     An either absolute or relative path to a file.
'nbd://HOST:PORT/EXPORT'
'nbd+unix:///EXPORT?socket=/PATH/TO/SOCKET'
     A connection to an NBD server.  *Note nbd command::

   FLAGS is a bitmask that specifies several aspects of the operation,
including the mode in which the IO space is opened.  Its value is
usually built by ORing a set of flags that are provided by the compiler.
These are:

'IOS_F_READ'
     The IO space is intended to be read.
'IOS_F_WRITE'
     The IO space is intended to be written.
'IOS_F_CREATE'
     If the IO device doesn't exist, then create it, usually empty.

Note that the specific behaviour of these flags depend on the nature of
the IO space that is opened.  For example a memory buffer is always
truncated at creation by default.

   In order to ease the usage of 'open', a few pre-made bitmaps are
provided to specify opening "modes":

'IOS_M_RDONLY'
     This is equivalent to 'IOS_F_READ'.
'IOS_M_WRONLY'
     This is equivalent to 'IOS_F_WRITE'.
'IOS_M_RDWR'
     This is equivalent to 'IOS_F_READ | IOS_F_WRITE'.

   The 'open' builtin returns a signed 32-bit integer.  This number will
identify the just opened IOS until it gets closed.

   If there is a problem opening the specified IO device then 'open'
will raise an 'E_no_ios' exception.

18.14.1.2 'opensub'
...................

The 'opensub' standard function allows you to create IO spaces that show
a sub-region of some other IO space.  The prototype is:

     fun opensub = (int<32> IOS,
                    offset<uint<64>,B> BASE, offset<uint<64>,B> SIZE,
                    string NAME = "",
                    uint<64> FLAGS = 0) int<32>

where IOS is the ID of the base IOS, BASE is the offset in IOS where the
sub-range starts and SIZE is the size of the range.

   The argument NAME is a descriptive name of the contents of the range,
and it is empty by default.

   The argument FLAGS is an ORed value of 'IOS_F_*' flags.  If no
explicit FLAGS are specified then the sub space inherits the flags of
the underlying base IOS. If explicit FLAGS are provided and they
contradict the flags of the underlying IOS then 'E_ios_flags' is raised.

   Trying to access a sub space whose base IOS has been closed results
in a 'E_io' exception.

18.14.1.3 'openproc'
....................

The 'openproc' standard function allows you to create IO spaces to
access the memory of some running program, given its process ID. The
prototype is:

     fun openproc = (uint<64> PID, uint<64> FLAGS = 0) int<32>

where PID is the process ID whose memory we want to poke and FLAGS is a
set of open flags.

18.14.1.4 'close'
.................

The 'close' builtin allows you to destroy IO spaces, closing the
underlying IO device.  The prototype is:

     fun close = (int<32> IOS) void

where IOS is some previously created IO space.  All pending data is
written to the underlying IO device.

   If the IO space specified to 'close' doesn't exist then an 'E_no_ios'
exception is raised.

   If errors occured while closing the IO space, then 'E_ios' exception
is raised.

18.14.1.5 'flush'
.................

The builtin 'flush' performs a "flushing" operation on a given IO space.
The prototype is:

     fun flush = (int<32> IOS, offset<uint<64>,1> OFFSET) void

   Where IOS is the IOS identifier where to perform the flush.  The
semantics associated with the "flushing" operation depends on the kind
of IO space:

   * Read-only stream IOS will discard already mapped input up to
     OFFSET.  Any further attempt of mapping data at that area will
     cause an E_EOF exception (for Early Of File ;)).
   * Flushing is a no-operation for other kind of IO spaces.

18.14.1.6 'get_ios'
...................

GNU poke maintains a "current IO space", which is the last created IO
space (this includes IO spaces opened and selected using a dot-command).
The builtin 'get_ios' returns this space.  It has the following
prototype:

     fun get_ios = int<32>

   If there is no IO space, 'get_ios' will raise the 'E_no_ios'
exception.

18.14.1.7 'set_ios'
...................

The 'set_ios' builtin allows you to set a specific IO space as the new
current IO space.  It has the following prototype:

     fun set_ios = (int<32> IOS) int<32>

where IOS is the IO space that will become the current IO space.  If the
IO space specified to 'set_ios' doesn't exist, 'E_no_ios' will be
raised.

   Note that 'set_ios' always returns '1'.  This is to ease its usage in
struct fields constraint expressions.

18.14.1.8 'iosize'
..................

The 'iosize' builtin returns the size of a given IO space, as an offset.
It has the following prototype:

     fun iosize = (int<32> ios = get_ios) offset<uint<64>,1>

   If the IO space specified to 'iosize' doesn't exist, 'E_no_ios' will
be raised.

18.14.1.9 'ioflags'
...................

The 'ioflags' builtin returns the flags active in a given IO space,
encoded in an unsigned 64-bit integer.  It has the following prototype:

     fun ioflags = (int<32> ios = get_ios) uint<64>

   If the IO space specified to 'ioflags' doesn't exist, 'E_no_ios' will
be raised.

18.14.2 The Map Operator
------------------------

Poke values reside in memory, and their in-memory representation is not
visible from Poke programs.  For example, '32' is a 32-bit signed
integer value, and it happens to not be boxed in the Poke Virtual
Machine.  Therefore, it occupies exactly 32-bit in the memory of the
machine running poke.  Other values, like arrays for example, are boxed,
and they need to store various meta-data.

   Regardless of the internal representation, we say these values live
"in memory".  Now, it is also possible to "map" a value to some area in
some underlying IO space.  This is done with the map operator '@', which
has two alternate syntax:

     TYPE @ OFFSET
     TYPE @ IOS : OFFSET

   The ternary version creates a new value using the data located at the
offset OFFSET in the specified IO space IOS, which shall be an
expression evaluating to a signed 32-bit integer.

   The binary version uses the current IO space.

   If there is no IO space, or the specified IO space doesn't exist, an
'E_no_ios' exception is raised:

     (poke) int @ 0#B
     unhandled no IOS exception

   The value created in a map can be either mapped or not mapped.
Mapping simple types produces not mapped values, whereas mapping
non-simple types create mapped values.

   The value attributes 'mapped' and 'offset' can be used to check
whether a value is mapped or not, and in that case the offset where it
is mapped:

     (poke) var p = Packet @ 0#B
     (poke) p'mapped
     0x1
     (poke) p'offset
     0x0UL#b

   Using the 'offset' attribute in a not mapped value results in the
'E_no_map' exception being raised:

     (poke) [1,2,3]'mapped
     0x0
     (poke) [1,2,3]'offset
     unhandled no map exception

   If the type specified in the map is not a simple type, like an array
or a struct, the resulting value is said to be mapped in the IO space:

     (poke) type Packet = struct { int i; long l; }
     (poke) Packet @ 0#B
     Packet {i=0x464c457f,l=0x10102L}
     (poke) uint<8>[2] @ 0#B
     [0x7fUB,0x45UB]

   A very important idea on Poke mapping is that it should be possible
to manipulate mapped and non-mapped values in a transparent way.  For
example, consider the quick sort implementation in poke's standard
library.  The prototype is:

     fun qsort = (any[] array, Comparator cmp_f,
                    long left = 0, long right = array'length - 1) void

'qsort' works with both mapped and not-mapped arrays:

     (poke) var a = [2,3,1]
     (poke) var b = int[3] @ 0#B
     (poke) b
     [1179403647,65794,0]
     (poke) qsort (a, IntComparator)
     (poke) a
     [1,2,3]
     (poke) qsort (b, IntComparator)
     (poke) b
     [0,33620224,1179403647]
     (poke) dump :from b'offset :size b'size :ascii 0
     76543210  0011 2233 4455 6677 8899 aabb ccdd eeff
     00000000: 0000 0000 0001 0102 7f45 4c46

   Similarly, you can write functions that operate on abstract entities
and data structures such as ELF relocations and sections, DWARF DIEs,
etc, and the same code will work with non mapped and mapped values.

18.14.3 Mapping Simple Types
----------------------------

Simple values (i.e.  integers, offsets, strings) cannot be mapped.
Therefore, if the type specified in the map is a simple type, the
resulting value will be a regular non-mapped value.  Examples:

     (poke) uint<8> @ 0#B
     0x7fUB
     (poke) string @ 0#B
     "ELF"

18.14.4 Mapping Structs
-----------------------

Struct and union types can be referred by name in a mapping ('@')
operation.  For example, given a struct type 'Foo' defined as:

     type Foo =
       struct
       {
         int i;
         long l;
       }

we could get a mapped struct value of that type like this:

     var f = Foo @ 128#B;

18.14.5 Mapping Arrays
----------------------

Arrays can be mapped in IO space in three different ways, depending on
the characteristics of the type provided to the mapping operator.

18.14.5.1 Array maps bounded by number of elements
..................................................

When an array type bounded by number of elements is used in a mapping
operation, the resulting mapped array is also bounded by number of
elements.

   For example, this is how we would map an array of four 32-bit signed
integers in the current IO space:

     (poke) int[4] @ 0#B
     [10,20,30,40]

   Since you can also provide a dynamic array type to the map operator,
the number of elements doesn't need to be constant.  For example, given
the variable 'nelems' has a value of '2':

     (poke) var nelems = 2
     (poke) int[nelems + 1] @ 0#B
     [100,222,333]

   If an end-of-file condition happens while mapping the array, because
the number of elements specified in the array type, at the given offset,
exceeds the capacity of the underlying IO device, an exception is raised
and the mapping is not completed:

     (poke) int[99999999999] @ 0#B
     unhandled EOF exception

   Likewise, if a constraint fails while performing the mapping (while
mapping an array of structs, for example) an exception is raised and the
map is aborted.

18.14.5.2 Array maps bounded by size
....................................

While dealing with binary formats, it often happens that the number of
entities in a collection is given by the space they occupy, rather than
the count itself.

   For example, consider ELF sections holding relocations.  These
sections contain a collection of zero or more relocations.  The layout
of each relocation is specified by the following type:

     type Elf64_Rela =
       struct
       {
         offset<Elf64_Addr,B> r_offset;
         Elf64_Xword r_info;
         Elf64_Sxword r_addend;
       };

   The section is described by an entry in the ELF sections header
table:

     type Elf64_Shdr =
       struct
       {
         Elf_Word sh_name;
         Elf_Word sh_type;
         Elf64_Xword sh_flags;
         Elf64_Addr sh_addr;
         Elf64_Off sh_offset;
         offset<Elf64_Xword,B> sh_size;
         Elf_Word sh_link;
         Elf_Word sh_info;
         Elf64_Xword sh_addralign;
         offset<Elf64_Xword,b> sh_entsize;
       };

   The relevant elements of 'Elf64_Shdr' are 'sh_offset' and 'sh_size',
which indicate the offset of the beginning of the section's contents,
and its size, respectively.  At this point, if we wanted to get an array
with all the relocations in the section, we could map an array bounded
by number of elements like we saw in the previous section, like this:

     (poke) Elf64_Rela[sh_size / 1#Elf64_Rela]
     [... relocs ...]

   However, this approach adoleces from two problems.  First, it doesn't
work with any entity type.  For an offset like '1#Elf64_Rela' to work,
it is required to know the size of the type specified as the unit at
compile time.  In the particular case of 'Elf64_Rela', that condition is
satisfied, but too often that's not the case.  For example, think about
a section containing 'NULL' terminated strings: you can't know the
number of strings contained in the section until you actually read it.

   Another problem is when the data in the header is corrupt.  Using the
mapping bounded by number of elements, we wouldn't realize it.  It would
be good if the tool would tell us whether the specified size actually
holds an exact number of the requested elements.

   A mapping bound by size is what we need.  Fortunately, as we saw when
discussing array types, Poke allows you to specify an offset instead of
an integral value, in the array type specification.  The right amount of
entities (in this case relocations) to strictly satisfy the provided
size will be mapped in the IO space.  So, in order to obtain an array
containing all the relocations in the section, we simply write:

     (poke) Elf64_Rela[ehdr.sh_size] @ ehdr.sh_offset
     [... relocs ...]

   The strictness mentioned above is important.  GNU poke will complain
(and abort the mapping) if it is not possible to map an exact number of
elements.  Thus the following mapping would not be successful:

     (poke) int[33#b] @ 0#B
     unhandled out of map bounds exception

   Like in mappings bounded by number of elements, if a constraint fails
while performing the mapping, an exception is raised and the map is
aborted.

18.14.5.3 Unbounded array maps
..............................

We mentioned above that if an end-of-file condition happens while
performing a mapping (be it bounded by number of elements or bounded by
size) an EOF exception is raised, and the mapping operation is aborted.

   Unbounded array mappings are performed by using an unbounded array
type in the mapping operation, like in:

     TYPE[] @ 0#B

   The above construction will map values of type TYPE in the IO space
until there is an end-of-file condition, or a constraint fails, whatever
happens first.  When it is a constraint expression that fails, that last
element is not included in the mapped array.

   Let's assume a binary file contains a series of blocks, located one
after the other, of a kind described by the following struct type:

     type Block =
       struct
       {
          byte magic[2] : magic[0] == 'B' && magic[1] == 'K';
          ... other data ...
       };

   we can map the blocks using an unbounded array map:

     (poke) Block[] @ 0#B
     [ ... blocks ... ]

   If the blocks extend up to the end of the IO space, that many blocks
will be mapped.  If there is some other content in the file following
the blocks, the constraint in the 'magic' field will fail and will
delimit the map that way (provided the binary format is well designed.)

18.14.5.4 Mapped bounds in bounded arrays
.........................................

When an array map is bounded, be it by number of elements or by size,
the bounding value can be mapped itself.  To illustrate how this works,
let's go back to our ELF file and the section containing relocations.
First, we map an 'Elf64_Shdr' to get the section header:

     (poke) var shdr = Elf64_Shdr @ OFFSET
     (poke) shdr.sh_offset
     120#B
     (poke) shdr.sh_size
     24#B

   Now we map an array with the relocations themselves, using a map
bounded by size, as we learned in the last section:

     (poke) var relocs = ELF64_Rela[shdr.sh_size] @ shdr.sh_offset
     (poke) relocs'length
     3

   Now, observe that 'shdr.sh_size' is mapped itself!  This means that,
should the section size be modified (to accommodate an extra relocation,
for example) the mapping of 'relocs' will reflect that automatically:

     (poke) shdr.sh_size = shdr.sh_size + 1#Elf64_Rela
     (poke) relocs'length
     4

   This is certainly an useful idiom, that is often used while poking
around.  However, sometimes this is _not_ what we want.  If we don't
want the mapping bounds of 'relocs' to be tied to 'shdr', we can just
use a temporary for the size:

     (poke) var s = shdr.sh_size
     (poke) var relocs = Elf64_Rela[s] @ shdr.sh_offset

   Since simple values (such as the size above) are not mapped, this
trick works as intended.

18.14.6 Mapping Functions
-------------------------

Struct fields can be of any type, and that includes function types or
closures.  This is especially useful to build interfaces.

   For orthogonality, it is possible to map function types.  The result
of the mapping is not very useful though: mapping a function type
results in a function with an empty body.  If the function returns a
value, a default value (zero for integral types, empty arrays for array
types, etc) is returned.

   Example:

     (poke) type Fun = (int) int
     (poke) Fun @ 0#B
     #<closure>
     (poke) (Fun @ 0#B) (10)
     0x0

18.14.7 Non-strict Mapping
--------------------------

The map operator '@' performs what we call a "strict mapping".  This
means that the data integrity of the mapped data is checked for.  Often
this is what we want.

   However, sometimes we have to work with incorrect or incomplete data.
In these cases, we want to inspect that data and complete it using poke.

   For this purpose poke provides an alternate map operator '@!' that
behaves exactly like the normal operator, but inhibits the control of
data integrity.

   The attribute ''strict' can be used to query whether a given value is
strict or non-strict.  Non-mapped values are strict by definition.

18.14.8 Unmapping
-----------------

The unary 'unmap' operator has the form:

     unmap VALUE

   It gets any value and produces the same value, making it not mapped
in case it is a mapped value.

   This is useful when we want to read a data structure from the IO
space (say, an array of integers) and then use it for storage without
changing the underlying IO space.  We would do something like:

     (poke) var a = unmap (int[3] @ 10#B)
     (poke) a[2] = 100

18.15 Exception Handling
========================

Sometimes an error or some other unexpected situation arises.  Poke
provides an exceptions mechanism to deal with these situations.

18.15.1 Exceptions
------------------

Exceptions in Poke are values of type 'Exception', which is a struct
defined like this:

     type Exception =
       struct
       {
         int<32> code;
         string msg;
       };

where 'code' identifies the type of the exception, and 'msg' is supposed
to be a textual description of the exceptional situation.

   You can use codes '255' and higher for your own exceptions.  For
example:

     raise Exception { code = 255; msg = "double upset event" };

   User-defined exceptions should be registered using 'exception_code'
function.  For example:

     var E_my_exception = Exception {
       code = exception_code,
       msg = "double upset event",
     };

where the 'E_my_exception.code' is a unique number greater than or equal
to '255'.

   Exception codes in the range '0..254' are reserved for poke.  These
are used in predefined exceptions which are standard, and have specific
meanings:

'E_generic'
     Generic error.
'E_out_of_bounds'
     Out of bounds exception.  This can be raised when accessing
     containers, like arrays and strings.
'E_eof'
     End of file exception.  This can be raised when mapping in the IO
     space.
'E_elem'
     Invalid element exception.  This is raised when attempting to
     access union fields that do not exist.
'E_constraint'
     Constraint violation exception.  This is raised when a constraint
     exception fails while mapping or constructing a struct.
'E_conv'
     Conversion exception.  This can be raised while casting values.
'E_map_bounds'
     Out of map bounds exception.  This can be raised while modifying a
     mapped value in a way it would violate its declared boundary (like
     the size of a mapped array.)
'E_map'
     No map exception.  This is raised when trying to map a not mapped
     value.
'E_div_by_zero'
     Division by zero exception.
'E_no_ios'
     No IOS exception.  This is raised when the IO space is accessed but
     there is no IO space.
'E_no_return'
     No return exception.  This is raised when the end of a void
     function is reached.
'E_io'
     Generic IO exception.
'E_io_flags'
     Invalid flags were tried while opening an IO device.
'E_assert'
     Assertion failure exception.  This is raised by 'assert' statement.
'E_overflow'
     This exception is raised when signed overflow is detected in an
     arithmetic operation.
'E_perm'
     This exception is raised when some operation can't be performed due
     to incorrect (lack of) permissions or capabilities.  An example is
     writing to a read-only IO space.

   The exception codes of the standard exceptions are available in the
form of 'EC_*' variables.  For example, this how you would raise an IO
error with a particular message:

     raise Exception { code = EC_io,
                        msg = "fluzo capacitator overheat impedes IO" }

18.15.2 'try-catch'
-------------------

The 'try-catch' statement provides a way to catch exceptions and handle
them.

   The simplest form of the statement is:

     try STMT catch COMPOUND_STMT

where STMT is any statement and COMPOUND_STMT is a compound statement.
First, STMT is executed.  If during its execution an exception is
raised, then COMPOUND_STMT is executed.

   The second form of the statement allows you to catch just one type of
exception:

     try STMT catch if EXP COMPOUND_STMT

where EXP is an expression that should evaluate to an 'Exception'.  The
handler COMPOUND_STMT will only be executed if that specific exception
is caught.  Any other exception will be re-raised.

   The third form of the statement is the most generic:

     try STMT catch (Exception FORMAL) COMPOUND_STMT

where FORMAL is a formal argument that contains the exception when
COMPOUND_STMT is executed.

18.15.3 'try-until'
-------------------

The 'try-until' statement allows you to execute instructions until some
exception is caught.  The syntax is:

     try STMT until EXP

where STMT is the statement that will be executed repeatedly until some
exception is raised.  If the raised exception has type EXP then
execution continues normally.  EXP should be an expression that
evaluates to an 'Exception'.

   This statement is particularly useful for mapping IO spaces until an
'eof' condition occurs.  For example, this is how we would compute with
every integer in the current IO space:

     var o = 0#B;
     try
     {
       compute (int @ o);
       o = o + 1#B;
     } until E_eof;

   It is possible to leave the loop from within STMT using the 'break'
statement.

18.15.4 'raise'
---------------

In previous sections we saw how exceptions are usually the side-product
of performing certain operations.  For example, a division by zero.

   However, it is also useful to explicitly raise exceptions.  The
'raise' statement can be used for that purpose.  Its syntax is:

     raise EXCEPTION;

where EXCEPTION is an integer.  This integer can be any number, but most
often is one of the 'E_*' codes defined in Poke.

18.15.5 exception-predicate
---------------------------

It is often needed to determine whether the execution of some given code
raised some particular exception.  A possibility is to use a 'try-catch'
statement like in the following example, a pretty-printer in an union
type:

     method _print = void:
     {
       try print "#<%u32d>", simple_value;
       catch if E_elem { print "%v\n", complex_value; }
     }

   This works as intended, but it is somewhat verbose and not that easy
to read for anyone not accustomed to the idiom.  It is better to use the
"exception predicate" operator, '?!', that has two forms:

     EXP ?! EXCEPTION
     { [STATEMENTS...] } ?! EXCEPTION

   In the first form, the expression EXP is executed.  If the execution
raises the exception EXCEPTION then the result of the predicate is '0'
(false), otherwise it is '1' (true).  The value of the expression is
discarded.

   In the second form, the compound statement passed as the first
operand is executed.  If the exception EXCEPTION is raised, then the
result of the predicate is '0' (false), otherwise it is '1' (true).

   Using exception predicates the pretty-printer above can be written in
a much more readable way:

     method _print = void:
     {
       if (simple_value ?! E_elem)
         print "#<%u32d>", simple_value;
       else
         print "%v\n", complex_value;
     }

Or, alternatively (and slightly more efficiently):

     method _print = void:
     {
       ({ print "<%u32d>", simple_value; } ?! E_elem)
         || print "%v\n", complex_value;
     }

18.15.6 'assert'
----------------

The 'assert' statement allows you test if a condition is true, if not,
the program will raise an exception with code 'EC_assert'.

     assert (CONDITION)
     assert (CONDITION, MESSAGE)

   The optional MESSAGE will be part of the 'msg' field of raised
'Exception' to explain the situation.

     assert (1 == 1);
     assert (0 == 0, ``Zero is equal to zero'');

   'assert' is useful for writing unit tests.

18.16 Terminal
==============

Poke programs have access to a text-oriented terminal, that can be used
to communicate with the user.  The system provides several primitive
functions to manipulate the terminal, which are described in the
sections below.

18.16.1 Terminal Colors
-----------------------

At every moment the text terminal is configured to use certain
foreground and background colors.  Initially, these are the "default
colors", which depend on the underlying terminal capabilities, and maybe
also on some system wide configuration setting.

   Terminal colors are encoded in triplets of integers signifying RGB
colors, i.e.  levels of beams for red, green and blue.  These triplets
are implemented as values of type 'int<32>[3]'.

   The special value '[-1, -1, -1]' means the default color, for both
foreground and background.  There are two predefined variables for this:

     var term_default_color = [-1,-1,-1];
     var term_default_bgcolor = [-1,-1,-1];

   Getting and setting the foreground and background colors is performed
using the 'term_get_color', 'term_set_color', 'term_get_fgcolor' and
'term_set_fgcolor' functions, with signatures:

     fun term_get_color = int<32>[3]
     fun term_set_color = (int<32>[3] color) void
     fun term_get_bgcolor = int<32>[3]
     fun term_set_bgcolor = (int<32>[3] color) void

18.16.2 Terminal Styling
------------------------

Styling of the output sent to the terminal is supported in the form of
"styling classes".  Each class is characterized by a name, a string.

   A class is activated by using the function 'term_begin_class', and
deactivated by using the function 'term_end_class'.  Several classes can
be activated simultaneously, but they should be deactivated in reverse
order.  It is down to the implementation to decide how to honor some
given class, i.e.  to decide how to characterize it physically
(visually, or audibly, or using an electrical discharge on the keyboard,
or whatever) The class may very well be ignored all together.  Poke
programs should never rely on how a specific class is implemented.

   The 'term_begin_class' and 'term_end_class' functions have the
following signatures:

     fun term_begin_class = (string class) void
     fun term_end_class = (string class) void

Styling classes can overlap, but trying to deactivate them in the wrong
order will cause an error in the form of an 'E_inval' exception.

18.16.3 Terminal Hyperlinks
---------------------------

The poke terminal supports terminal hyperlinks.  Each hyperlink is
characterized by an URL, and an ID. The 'term_begin_hyperlink' and
'term_end_hyperlink' functions have the following signatures:

     fun term_begin_hyperlink = (string url, string id) void
     fun term_end_hyperlink = void

Hyperlinks can overlap, but trying to end an hyperlink when there is no
t a currently open one will result in an 'E_generic' exception raised.

18.17 Printing
==============

Poke programs can print text to the standard output in two ways: simple
unformatted output, and formatted output.

18.17.1 'print'
---------------

The 'print' statement prints the given string to the standard output.
'print' outputs text strings verbatim.  It can be invoked using two
alternative syntaxes, which are equivalent:

     print (STR);
     print STR;

   'print' is simple, but fast.  It is good to use it in simple cases
where the information to print out doesn't require any kind of
formatting and styling.

18.17.2 'printf'
----------------

The 'printf' statement gets a format string and, optionally, a list of
values to print.  It can be invoked using two alternative syntaxes,
which are equivalent:

     printf (FMT[, VALUE...])
     printf FMT[, VALUE...]

   The format string FMT is printed verbatim to the standard output, but
for "format tags" which start with '%'.  These format tags are
interpreted especially.  Most of the format tags "consume" one of the
specified values.  Every value in the list shall be described by a tag,
or the compiler will signal an error.  Likewise, values without a
corresponding describing tag is an error.  These tags are:

'%s'
     Print the argument as a string.
'%iBITS(d|x|o|b|c)'
     Print the argument as a signed integer of size BITS.  The last
     letter determines how the argument is printed.
     'd'
          Print the integer in decimal.
     'x'
          Print the integer in hexadecimal.
     'o'
          Print the integer in octal.
     'b'
          Print the integer in binary.
     'c'
          Print the integer as an ASCII character.  This only works with
          8 bit integers.
'%u'
     Same as '%i', but for unsigned integers.
'%c'
     A shorter way to write '%u8c'.
'%v'
     Print the value printed representation of the argument, which can
     be of any type including complex types like arrays and structs.
     This is similar to the 'write' operation available in many Lisp
     systems.

     This tag supports an optional numerical argument (restricted to a
     single digit) that specifies the maximum depth level when printing
     structures.  Example:

          (poke) printf ("%1v\n", struct { s = struct { i = 10 },  l = 20L });
          struct {s=struct {...},l=0x14L}

     By default, the depth level is '0', which means no limit.

     This tag also support an optional flag that specifies how the value
     is printed.  The supported tags are 'F' (for "flat") and 'T' (for
     "tree").  The default is flat.  Example:

          (poke) printf ("%Tv\n", struct { s = struct { i = 10 },  l = 20L });
          struct {
            s=struct {
              i=0xa
            },
            l=0x14L
          }
          (poke) printf ("%Fv\n", struct { s = struct { i = 10 },  l = 20L });
          struct {s=struct {i=0xa},l=0x14L}

     This tag is mainly intended to be used in pretty-printers.

   The following format tags do not consume arguments.  They support
emitting styled text using the libtextstyle approach of having styling
classes that you can customize in a '.css' file.

'%<CLASSNAME:'
     Start the styling class with name CLASSNAME.  The class name cannot
     be empty.
'%>'
     End the last opened styling class.  All styling classes should be
     ended before finishing the format string.
'%%'
     Print '%' character.

   Note that styling classes can be nested, but all classes should be
ended before finishing the format string.

   If you use a NAME class, you can define how to style it in the '.css'
file (poke installs and uses poke-default.css but you can set the
'POKE_STYLE' environment variable to point to another css) like this:

     .NAME { text-decoration: blink; color : pink; }

   Examples:

     (poke) printf "This is a NAME: %<NAME:xxx%>"
     This is a NAME: xxx
     (poke) printf "Name: %<string:%s%> Age: %<integer:%i32d%>, "Jose", 39
     Name: Jose Age: 39

18.18 Comments
==============

There are several ways to document your Poke programs: comments of
several types and support for separator characters.

18.18.1 Multi-line comments
---------------------------

Poke supports C-like multi-line, comments, which is text enclosed
between '/*' and '*/' sequences.  These comments cannot be nested.

18.18.2 Single line comments
----------------------------

C++-like single line comments are supported.  Everything after the '//'
sequence is interpreted as a comment, until the end of the line or the
end of the file, whatever comes first.

18.18.3 Vertical separator
--------------------------

Poke ignores form feed characters (ASCII code 12, often visualized as
^L). In GNU software, this character is traditionally used to separate
conceptually different entities in source files.

18.19 Modules
=============

It is common for pickles to depend on stuff defined in other pickles.
In such cases, the 'load' language construction can be used to load
pickles from your Poke program.  The syntax is:

     load MODULE;

where MODULE is the name of the pickle to load.

   For example, your Poke program may want to access some ELF data
structures.  In that case, we can just do:

     /* Pickle to poke the contents of some ELF section.  */

     load elf;

     /* ... code  ... */

   When asked to open a module, poke assumes it is implemented in a file
named 'MODULE.pk'.  In the example above, it will try to load 'elf.pk'.

   The pickles are searched first in the current working directory.  If
not found, then 'PREFIX/share/poke/MODULE.pk' is tried next.

   If the environment variable 'POKEDATADIR' is defined, it replaces
'PREFIX/share/poke'.  This is mainly intended to test a poke program
before it gets installed in its final location.

   Nothing prevents you to load the same pickle twice.  This will work
if the pickle doesn't include definitions, but just executes statements.
Otherwise, you will likely get an error due to trying to define stuff
twice.

   There is an alternate syntax of the 'load' construction that is
useful when the module is implemented in a file whose name doesn't
conform to a Poke identifier.  This happens, for example, when the file
name contains hyphens.  Example:

     load "my-pickle.pk";

   Note that if you use this variant of 'load', you must specify the
full file name, including whatever extension it uses (usually '.pk').

18.20 System
============

Poke provides several constructions to interact with the operating
system.  These are described in the subsections below.

18.20.1 getenv
--------------

The 'getenv' built-in function has the form:

     fun getenv (string name) string:

Given the name of an environment variable, it returns a string with its
value.  If the variable is not defined in the environment, then the
'E_inval' exception is raised.

18.20.2 rand
------------

The 'rand' built-in function has the form:

     fun rand = (uint<32> seed = 0) int<32>:

It returns a pseudo-random number in the range of the unsigned 32-bit
integers.  If a SEED different to zero is provided, it is used to
initialize a new sequence of pseudo-random numbers (which includes the
one returned.)

   The 'get_time' builtin function can be used in order to seed the
pseudo-random number generator very easily:

     srand (get_time[1])

18.21 VM
========

The Poke language provides a set of pre-defined functions which can be
used to access and modify properties of the underlying implementation
running the Poke programs.

18.21.1 'vm_obase'
------------------

The pre-defined function 'vm_obase' returns the current output base in
use in the underlying VM. It has the following prototype:

     fun vm_obase = int<32>:

18.21.2 'vm_set_obase'
----------------------

The pre-defined function 'vm_set_obase' sets the output base in the
underlying VM. It has the following prototype:

     fun vm_set_obase = (int<32> OBASE) void:

Where OBASE is the output base to set.  The base must be one of 2 (for
binary), 8 (for octal), 10 (for decimal) or 16 (for hexadecimal).  If
the provided base is not contained in that set, 'vm_set_obase' raises
'E_inval'.

18.21.3 'vm_opprint'
--------------------

The pre-defined function 'vm_opprint' returns a boolean indicating
whether pretty-printers are used when printing struct values, or not.
It has the following prototype:

     fun vm_opprint = int<32>:

18.21.4 'vm_set_opprint'
------------------------

The pre-defined function 'vm_set_opprint' sets a flag in the underlying
VM, indicating whether pretty-printers are used when printing struct
values.  It has the following prototype:

     fun vm_set_opprint = (int<32> PRETTY_PRINT_P) void:

18.21.5 'vm_oacutoff'
---------------------

The pre-defined function 'vm_oacutoff' returns an integer indicating the
current cutoff limit used when printing array values.  It has the
following prototype:

     fun vm_oacutoff = int<32>:

18.21.6 'vm_set_oacutoff'
-------------------------

The pre-defined function 'vm_set_oacutoff' sets the current cutoff limit
used when printing array values.  It has the following prototype:

     fun vm_set_oacutoff = (int<32> CUTOFF) void:

where CUTOFF is an integer specifying the maximum number of array
elements that get printed.

18.21.7 'vm_odepth'
-------------------

The pre-defined function 'vm_odepth' returns an integer indicating the
maximum depth level used when printing nested structures.  It has the
following prototype:

     fun vm_odepth = int<32>:

18.21.8 'vm_set_odepth'
-----------------------

The pre-defined function 'vm_set_odepth' sets the maximum depth level to
be used when printing nested structures.  It has the following
prototype:

     fun vm_set_odepth = (int<32> DEPTH_LEVEL) void:

where DEPTH_LEVEL specifies the number of depth levels in nested structs
to consider when printing nested structures.

18.21.9 'vm_oindent'
--------------------

The pre-defined function 'vm_oindent' returns an integer indicating the
indentation step.  It has the following prototype:

     fun vm_oindent = int<32>:

18.21.10 'vm_set_oindent'
-------------------------

The pre-defined function 'vm_set_oindent' sets the indentation level
used when printing nested structures.  It has the following prototype:

     fun vm_set_oindent = (int<32> STEP) void:

where STEP specifies how much to indent when printing nested structures.
This value typically translates into the number of white spaces (or
tabs) used in terminals, but this must not be relied on since other
printing media may interpret it differently.

18.21.11 'vm_omaps'
-------------------

The pre-defined function 'vm_omaps' returns a boolean indicating whether
offset information is included when printing out values.  It has the
following prototype:

     fun vm_omaps = int<32>:

18.21.12 'vm_set_omaps'
-----------------------

The pre-defined function 'vm_set_omaps' sets whether offset information
is included when printing out values.  It has the following prototype:

     fun vm_set_omaps = (int<32> OMAPS_P) void:

18.21.13 'vm_omode'
-------------------

The pre-defined function 'vm_omode' returns an integer indicating the
currently enabled output mode.  It has the following prototype:

     fun vm_omode = int<32>:

There are currently two supported output modes:

     var VM_OMODE_PLAIN = 0;
     var VM_OMODE_TREE = 1;

18.21.14 'vm_set_omode'
-----------------------

The pre-defined function 'vm_set_omode' sets the output mode to use by
default when printing values.  It has the following prototype:

     fun vm_set_omode = (int<32> OMODE) void:

Where OMODE is one of the 'VM_OMODE_*' values documented above.

18.22 Debugging
===============

18.22.1 '__LINE__' and '__FILE__'
---------------------------------

When printing traces it is often useful to include a description of the
location of the trace.  The poke compiler provides two builtins for this
purpose.

'__LINE__'
     Expands to an unsigned 64-bit integer containing the current line
     of the program being compiled.
'__FILE__'
     Expands to a string with the name of the file currently being
     compiled.  If the program is read from the standard input (like in
     the REPL) then this is '"<stdin>"'.

18.22.2 strace
--------------

The 'strace' built-in function has the prototype:

     fun strace = void:

It prints out the current contents of the PVM stack.  This is especially
useful to debug the code generated by the compiler.

19 The Standard Library
***********************

GNU poke ships with a standard library for Poke programs that is
available to Poke programs.  The following sections detail the
facilities provided by the library.

19.1 Standard Integral Types
============================

The Poke standard library provides the following standard integral
types.

'bit'
     1-bit unsigned integer.
'nibble'
     4-bit unsigned integer.
'uint8'
'byte'
'char'
     8-bit unsigned integer.
'uint16'
'ushort'
     16-bit unsigned integer.
'uint32'
'uint'
     32-bit unsigned integer.
'uint64'
'ulong'
     64-bit unsigned integer.
'int8'
     8-bit signed integer.
'int16'
'short'
     16-bit signed integer.
'int32'
'int'
     32-bit signed integer.
'int64'
'long'
     64-bit signed integer.

19.2 Standard Offset Types
==========================

The Poke standard library provides the following standard offset types.

'off64'
     64-bit signed offset in bits.
'uoff64'
     64-bit unsigned offset in bits.

19.3 Standard Units
===================

The following list of units are defined in the standard library.

Base units:

'b'
     bits.
'N'
     nibbles.
'B'
     bytes.

Base 10 units:

'Kb'
     kilo bits (1000 bits.)
'KB'
     Kilo bytes (1000 bytes.)
'Mb'
     Mega bits.
'MB'
     Mega bytes.
'Gb'
     Giga bits.
'GB'
     Giga bytes.

Base 2 units:

'Kib'
     kilo bits (1000 bits.)
'KiB'
     Kilo bytes (1000 bytes.)
'Mib'
     Mega bits.
'MiB'
     Mega bytes.
'Gib'
     Giga bits.
'GiB'
     Giga bytes.

19.4 Conversion Functions
=========================

The Poke standard library provides the following functions to do useful
conversions.

19.4.1 'catos'
--------------

It is often useful to convert arrays of characters into strings.  The
standard function 'catos' provides the following interface:

     fun catos = (char[] CHARS) string: { ... }

   It builds a string containing the characters in CHARS, and returns
it.  Examples:

     (poke) catos (['a','b','c'])
     "abc"
     (poke) catos (['\0'])
     ""

   Note that if the passed array contains a 'NULL' character ''\0'' then
no further characters are processed.  For example:

     (poke) catos (['f','o','o','\0','b','a','r'])
     "foo"

19.4.2 'stoca'
--------------

Sometimes we want to store strings in character arrays.  The standard
function 'stoca' provides the following interface:

     fun stoca = (string s, char[] ca, char fill = 0) void: { ... }

   It fills the given array CA with the contents of the string S.  If
the string doesn't fit in the array, then 'E_out_of_bounds' is raised.
If the length of the string is less than the length of the array, the
extra characters are set to the FILL character, which defaults to '\0'.

19.4.3 'atoi'
-------------

The standard function 'atoi' provides the following interface:

     fun atoi = (string STR, int BASE = 10) long: { ... }

   It parses a signed integral number in the given BASE in the string
STR and returns it as a signed 64-bit integer.

   The accepted values for BASE are '2', '8', '10' (the default) and
'16'.  If any other base is requested an 'E_inval' exception is raised.

   Note that atoi allows for extra information to be stored in STR after
the parsed integer.  Thus, this works:

     (poke) atoi ("10foo")
     10L

19.4.4 'ltos'
-------------

The 'ltos' standard function converts a given long integer into its
printed representation in some given base.  It has the following
prototype:

     fun ltos = (long I, uint BASE = 10) string:

where I is the number for which to calculate the printed representation,
and BASE is a number between 0 and 16.

19.5 Array Functions
====================

The Poke standard library provides the following functions to do work on
arrays:

19.5.1 'reverse'
----------------

The standard function 'reverse' provides the following interface:

     fun reverse = (any[] a) void:

It reverses the elements of the given array.

19.6 String Functions
=====================

The Poke standard library provides the following functions to do work on
strings:

19.6.1 'ltrim'
--------------

The standard function 'ltrim' provides the following interface:

     fun ltrim = (string s, string cs = " \t") string: { ... }

   It returns a copy of the input string 's' with any leading character
that also appears in CS removed.

19.6.2 'rtrim'
--------------

The standard function 'rtrim' provides the following interface:

     fun rtrim = (string s, string cs = " \t") string: { ... }

   It returns a copy of the input string 's' with any trailing character
that also appears in CS removed.

19.6.3 'strchr'
---------------

The standard function 'strchr' provides the following interface:

     fun strchr = (string s, uint<8> c) int<32>: { ... }

   It returns the index of the first occurrence of the character C in
the string S.  If the character is not found in the string, this
function returns the length of the string.

19.7 Sorting Functions
======================

19.7.1 'qsort'
--------------

The standard function 'qsort' has the following prototype:

     fun qsort = (any[] ARRAY, Comparator CMP_F,
                    long LEFT = 0,
                    long RIGHT = array'length - 1) void

where ARRAY is the array to sort, CMP_F is a comparator function, LEFT
is the index of the first array element to include in the sorting, and
RIGHT is the index of the last array element to include in the sorting.
Both LEFT and RIGHT are optional, and the default is to cover the whole
array.

   The comparator function CMP_F should have the following prototype:

     type Comparator = (any,any):int

19.8 CRC Functions
==================

Many file formats use checksums of one sort or another.  Therefore you
may want to write such a checksum or verify a checksum in a constrained
field.  *Note Field Constraints::.

   Some formats use simple additive checksums.  Another common checksum
is the Cyclic Redundancy Checksum (CRC). The standard function 'crc32'
calculates the 32 bit CRC defined by ISO-3309.

     fun crc32 = (byte[] BUF) uint<32>: { ... }

This function returns the 32 bit CRC for the data contained in the array
BUF.

19.9 Dates and Times
====================

Often a format encodes a date and time expressed as the number of
seconds since midnight, January 1st 1970.  You could map these simply as
integers.  However, the standard library provides two types
'POSIX_Time32' and 'POSIX_Time64' which include pretty-printers to
display the date in a human readable format.  The definition is:

     type POSIX_TimeSIZE = struct
     {
       uint<SIZE>  seconds;

       method _print = void:
       { ... }
     }

where SIZE is either 32 or 64.  When pretty printing is enabled, a
mapped value of these types will display similar to

     #<2019-Dec-12 8:54:56>

whereas when pretty printing is not enabled, this example would be
displayed as:

     POSIX_Time32 {seconds=1576140896U}

Note that timestamps of this type do not account for leap seconds and
are agnostic towards timezone.

   Occasionally you might wish to print an unmapped timestamp value in a
human readable format.  To do this, you can use the 'ptime' function,
which is defined as follows:

     fun ptime = (uint<64> seconds) void:
     { ... }

   This pickle also provides a type 'Timespec' that can store Unix times
to nanosecond precision, and has the form:

     type Timespec = struct
     {
       int<64> sec;
       int<64> nsec;
     };

   The function 'gettimeofday' uses the 'get_time' builtin in order to
construct an instance of 'Timespec'.  For example:

     (poke) gettimeofday
     Timespec {
       sec=1604918423L,
       nsec=753492546L
     }
     (poke) ptime (gettimeofday.sec)
     2020-Nov-9 10:40:36

19.10 Offset Functions
======================

19.10.1 alignto
---------------

The 'alignto' function has the prototype:

     fun alignto = (uoff64 offset, uoff64 to) uoff64:
     { ... }

It returns an offset that is the result of aligning the given OFFSET to
the given alignment TO.

20 The Machine-Interface
************************

GNU poke can be executed in a special mode in which it communicates with
a client using a machine-friendly interface.  This makes it possible to
write programs like graphical user interfaces, testing programs and the
like.  This section describes this operation mode and provides a full
description of the interface, for client application writers.

20.1 MI overview
================

A "client application" can communicate with poke through the
machine-interface:

     +--------+       MI     +--------+
     | client |------ O------|  poke  |
     +--------+              +--------+

   There are two ways in which the communication can be performed:
through pipes and through TCP network connections.

   When using the pipe operation mode, the client application runs poke
as a sub process.  Data is obtained from poke reading from its standard
output, and data is fed to poke writing to its standard input:

     +--------+        stdin +--------+
     | client |------------->|  poke  |
     +--------+              +--------+
         ^                       | stdout
         |                       |
         +-----------------------+

   When using the TCP network operation mode the client uses a
bidirectional socket to communicate with a running poke process.  This
has the advantage of allowing having poke and the client application
running on different machines:

     +--------+ socket    (----)__     socket +--------+
     | client |<-------->( network )<-------->|  poke  |
     +--------+           (-------)           +--------+

20.2 Running poke in MI mode
============================

The following poke command-line options are relevant to the
machine-interface.

'--mi'
     Run poke in Machine-Interface mode.
'--mi-socket=PORT'
     Use a network TCP socket to communicate with the client.  PORT is
     the port to use, or zero.  If zero, then poke chooses a port on its
     own.  In both cases, poke prints the port used when it starts.

   If not using network sockets, poke starts talking MI immediately
using the standard input and standard output:

     $ poke --mi
     ^@^@^@G{"poke_mi":0,"type":2,"data":{"type":0,"args":{"version":"0.1-beta"}}}

   When using sockets, poke prints the number of the socket where it is
listening for requests in the standard output:

     $ poke --mi --mi-socket=0
     1234

20.3 MI transport
=================

At the lowest level the communication is performed in terms of "frame
messages".

   The layout of each message is:

     type PMI_FrameMessage =
      struct
      {
         big uint<4> size : size <= 2048;
         byte[size] payload;
      }

Where SIZE is the length of the payload, measured in bytes.  The maximum
length of a frame message payload is two kilobytes.

20.4 MI protocol
================

"Requests" are initiated by the client.  Once a request is sent, it will
trigger a response.  The response is paired with the triggering request
by the request's sequence number.

   "Responses" are initiated by poke, in response to a request received
from the client.

   "Events" are initiated by poke.

20.4.1 MI Requests
------------------

20.4.1.1 Request EXIT
.....................

This request asks poke to exit in an orderly way.  It has no arguments.

20.4.1.2 Request PRINTV
.......................

This request is used to get the printed representation of a given Poke
value.

   Arguments:

VALUE
     The Poke value to get the printed representation of.

20.4.2 MI Responses
-------------------

20.4.2.1 Response EXIT
......................

This is the response to the EXIT request.

   Attributes:

SUCCESS_P
     If 'true', poke will exit.  If 'false', something prevents poke to
     exit.
ERRMSG
     If SUCCESS_P is 'false', this attribute contains a string
     indicating the reason why poke refuses to exit.

20.4.2.2 Response PRINTV
........................

This is the response to the PRINTV request.

   Attributes:

SUCCESS_P
     If 'true', poke was able to get the printed representation of the
     value.  If 'false', an error occurred that prevented poke to handle
     the request.
STRING
     If SUCCESS_P is 'true', this attribute contains the printed
     representation of the Poke value that was passed to the request.
ERRMSG
     If SUCCESS_P is 'false', this attribute contains a string
     indicating the reason why poke could not get the printed
     representation of the value.

20.4.3 MI Events
----------------

20.4.3.1 Event INITIALIZE
.........................

This event is sent by poke when it finishes initializing and is ready to
receive requests.

   Arguments:

MI_VERSION
     An integer with the version of the MI protocol that this poke
     instance speaks.
VERSION
     A string with the printable representation of the version of the
     poke instance.  This is intended to be used for the client to show
     to the users.

21 The Poke Virtual Machine
***************************

21.1 PVM Instructions
=====================

21.1.1 VM instructions
----------------------

21.1.1.1 Instruction canary
...........................

Synopsys:

     canary

   This instruction installs a canary to mark the bottom of the stack,
which is cheched by the 'exit' instruction.  To be most effective this
should be executed before the stack is used for the first time.

   Stack: '( -- )'

21.1.1.2 Instruction exit
.........................

Synopsys:

     exit

   Do some cleanup and finish execution of a PVM program.  This checks
the stack sentinel installed by the 'canary' instruction.

   Stack: '( -- )'

21.1.1.3 Instruction pushend
............................

Synopsys:

     pushend

   Push the current endianness on the stack.  This endianness is part of
the global state of the PVM.

   Stack: '( -- INT )'

21.1.1.4 Instruction popend
...........................

Synopsys:

     popend

   Pop a signed integer from the stack and make it the current
endianness in the PVM. The possible values for endianness are
IOS_ENDIAN_LSB and IOS_ENDIAN_MSB.

   Stack: '( INT -- )'

21.1.1.5 Instruction pushob
...........................

Synopsys:

     pushob

   Push output base.

   This instruction pushes a signed integer value with the output base
that is used when printing PVM values.  Valid values are 2, 8, 10 and
16.

   Stack: '( -- INT )'

21.1.1.6 Instruction popob
..........................

Synopsys:

     popob

   Pop and set output base.

   This instructions pops a signed integer from the stack and uses it to
set the new output base to be used when printing PVM values.  Valid
values are 2, 8, 10 and 16.

   If an invalid obase is specified then this instruction raises
PVM_E_INVAL.

   Stack: '( INT -- )'

   Exceptions: 'PVM_E_INVAL'

21.1.1.7 Instruction pushom
...........................

Synopsys:

     pushom

   Push output mode.

   This instruction pushes a signed integer value with the output mode
that is used when printing PVM values.  Valid values are 0 for flat
mode, and 1 for tree mode.

   Stack: '( -- INT )'

21.1.1.8 Instruction popom
..........................

Synopsys:

     popom

   Pop and set output mode.

   This instructions pops a signed integer from the stack and uses it to
set the new output mode to be used when printing PVM values.  Valid
values are 0 for flat mode and 1 for tree mode.

   If an invalid omode is specified then this instruction raises
PVM_E_INVAL.

   Stack: '( INT -- )'

   Exceptions: 'PVM_E_INVAL'

21.1.1.9 Instruction pushoo
...........................

Synopsys:

     pushoo

   Push output offsets mode.

   This instruction pushes a boolean encoded in a signed integer value
indicating whether to show offsets when printing PVM values.

   Stack: '( -- INT )'

21.1.1.10 Instruction popoo
...........................

Synopsys:

     popoo

   Pop and set output offsets mode.

   This instructions pops a boolean encoded in a signed integer from the
stack and uses it to set the new output offset mode to be used when
printing PVM values.

   Stack: '( INT -- )'

21.1.1.11 Instruction pushoi
............................

Synopsys:

     pushoi

   Push output indentation mode.

   This instructions pushes an integer to the stack with the current
indentation step configured in the VM. The indentation step determines
how many white characters to use in each indentation level when printing
output.

   Stack: '( -- INT )'

21.1.1.12 Instruction popoi
...........................

Synopsys:

     popoi

   Pop and set output indentation step mode.

   This instructions pops an integer from the stack and uses it to set
the current indentation step in the VM. The indentation step determines
how many white characters to use in each indentation level when printing
output.

   Stack: '( INT -- )'

21.1.1.13 Instruction pushod
............................

Synopsys:

     pushod

   Push output depth.

   This instruction pushes a signed integer indicating the depth to use
when printing PVM values.

   Stack: '( -- INT )'

21.1.1.14 Instruction popod
...........................

Synopsys:

     popod

   Pop and set output depth.

   This instructions pops a signed integer indicating the maximum depth
included by the VM when printing values.

   Stack: '( INT -- )'

21.1.1.15 Instruction pushoac
.............................

Synopsys:

     pushoac

   Push output array cutoff.

   This instruction pushes a signed integer indicating the number of
elements that the VM includes in the printed representation of PVM array
values.

   Stack: '( -- INT )'

21.1.1.16 Instruction popoac
............................

Synopsys:

     popoac

   Pop and set output array cutoff.

   This instructions pops a signed integer indicating the number of
elements that the VM includes in the printed representation of PVM array
values.

   Stack: '( INT -- )'

21.1.1.17 Instruction pushopp
.............................

Synopsys:

     pushopp

   Push pretty-print usage.

   This instruction pushes a signed integer indicating whether the VM is
configured to use pretty-printers.

   Stack: '( -- INT )'

21.1.1.18 Instruction popopp
............................

Synopsys:

     popopp

   Pop and set usage of pretty-printers.

   This instructions pops a signed integer indicating whether to use
pretty-printers when printing values and sets it in the VM.

   Stack: '( INT -- )'

21.1.1.19 Instruction pushoc
............................

Synopsys:

     pushoc

   Push the current output color to the stack, encoded as a RGB triplet.

   Stack: '( -- INT INT INT )'

21.1.1.20 Instruction popoc
...........................

Synopsys:

     popoc

   Pop the RGB triplet at the top of the stack and use it to set the new
terminal output color.

   Stack: '( INT INT INT -- )'

21.1.1.21 Instruction pushobc
.............................

Synopsys:

     pushobc

   Push the current output background color to the stack, encoded as a
RGB triplet.

   Stack: '( -- INT INT INT )'

21.1.1.22 Instruction popobc
............................

Synopsys:

     popobc

   Pop the RGB triplet at the top of the stack and use it to set the new
terminal output background color.

   Stack: '( INT INT INT -- )'

21.1.1.23 Instruction sync
..........................

Synopsys:

     sync

   Handle pending signals, and raise exceptions accordingly.  This
instruction should be emitted in strategic places, such as before
backwards jumps and at function prolog, to assure signals are eventually
attended to.

   Stack: '( -- )'

   Exceptions: 'PVM_E_SIGNAL'

21.1.2 IOS related instructions
-------------------------------

21.1.2.1 Instruction open
.........................

Synopsys:

     open

   Open a new IO space.  The handler string and a set of flags are
passed on the stack.  The descriptor of the opened IOS is pushed to the
stack as a signed integer.

   If there is no other IO space opened when this instruction is
executed, then the just opened space becomes the current IO space.

   If it is not possible to open the IO space according to the provided
flags, the PVM_E_IOFLAGS exception is raised.  If there is any other
error performing the operation, PVM_E_IO is raised.

   Stack: '( STR ULONG -- INT )'

   Exceptions: 'PVM_E_IOFLAGS, PVM_E_IO'

21.1.2.2 Instruction close
..........................

Synopsys:

     close

   Close an IO space.  The descriptor of the space to close is provided
on the stack as a signed integer.

   If the specified IO space doesn't exist, this instruction raises
PVM_E_NO_IOS. If the operation fails, it raises PVM_E_IO.

   Stack: '( INT -- )'

   Exceptions: 'PVM_E_NO_IOS, PVM_E_IO'

21.1.2.3 Instruction flush
..........................

Synopsys:

     flush

   Flush an IO space.  The descriptor of the space to flush, and the
bit-offset up to which perform the flushing are provided on the stack.

   If the specified IO space doesn't exist, this instruction raises
PVM_E_IO.

   Stack: '( INT ULONG -- )'

   Exceptions: 'PVM_E_IO'

21.1.2.4 Instruction pushios
............................

Synopsys:

     pushios

   Push the descriptor of the current IO space on the stack, as a signed
integer.  If no IO space is currently opened, raise PVM_E_NO_IOS.

   Stack: '( -- INT )'

   Exceptions: 'PVM_E_NO_IOS'

21.1.2.5 Instruction popios
...........................

Synopsys:

     popios

   Pop an IO space descriptor from the stack and set it as the current
IO space.  If the specified descriptor doesn't identify an IO space,
raise PVM_E_NO_IOS.

   Stack: '( INT -- )'

   Exceptions: 'PVM_E_NO_IOS'

21.1.2.6 Instruction ioflags
............................

Synopsys:

     ioflags

   Push an unsigned 64-bit integer with the flags of the given IO space
on the stack.  The IO space is identified by a descriptor, which is a
signed integer.  If the given IO space doesn't exist, raise
PVM_E_NO_IOS.

   Stack: '( INT -- INT ULONG )'

   Exceptions: 'PVM_E_NO_IOS'

21.1.2.7 Instruction iosize
...........................

Synopsys:

     iosize

   Push the size of the given IO space on the stack, as an offset.  The
IO space is identified by a descriptor, which is a signed integer.  If
the given IO space doesn't exist, raise PVM_E_NO_IOS.

   Stack: '( INT -- INT OFF )'

   Exceptions: 'PVM_E_NO_IOS'

21.1.2.8 Instruction iogetb
...........................

Synopsys:

     iogetb

   Each IO space has a bias associated with it, which by default is 0
bits.  This bias is applied to the offset given to every read/write
operation.

   This instruction pushes the bias associated to the given IO space to
the stack, as an offset.  If the given IO space doesn't exist then the
exception PVM_E_NO_IOS is raised.

   Stack: '( INT - INT OFF )'

   Exceptions: 'PVM_E_NO_IOS'

21.1.2.9 Instruction iosetb
...........................

Synopsys:

     iosetb

   Each IO space has a bias associated with it, which by default is 0
bits.  This bias is applied to the offset given to every read/write
operation.

   This instruction sets the bias associated to the given IO space.  The
bias is specified as an offset.  If the given IO space doesn't exist,
the exception PVM_E_NO_IOS is raised.

   Stack: '( OFF INT -- OFF )'

   Exceptions: 'PVM_E_NO_IOS'

21.1.3 Function management instructions
---------------------------------------

21.1.3.1 Instruction call
.........................

Synopsys:

     call

   Call a closure on the stack, passing the specified arguments.  After
the execution of the closure, control is transferred to the instruction
immediately following the call instruction.

   Stack: '( ARG1 ... ARGN CLOSURE -- RETVAL )'

21.1.3.2 Instruction prolog
...........................

Synopsys:

     prolog

   Prepare the PVM for the execution of a function.  This instruction
shall be the target of every 'call' instruction and shall be the first
instruction in every function body.

   Stack: '( -- )'

21.1.3.3 Instruction return
...........................

Synopsys:

     return

   Return from a function.  A function can have many 'return'
instructions.

   Stack: '( -- )'

21.1.4 Environment instructions
-------------------------------

21.1.4.1 Instruction pushf
..........................

Synopsys:

     pushf N

   Push a new lexical frame.  If the argument N is bigger than zero, it
indicates the number of entries in the frame.  If N is 0, it means we
don't know how many entries will be stored in the frame.

   Stack: '( -- )'

21.1.4.2 Instruction popf
.........................

Synopsys:

     popf N

   Pop N lexical frames.

   Stack: '( -- )'

21.1.4.3 Instruction pushvar
............................

Synopsys:

     pushvar BACK, OVER

   Retrieve the value of a variable from the lexical environment and
push it on the stack.  The lexical address of the variable is specified
as arguments to the instruction.

   Stack: '( -- VAL )'

21.1.4.4 Instruction pushtopvar
...............................

Synopsys:

     pushtopvar OVER

   Retrive the value of a variable from the top-level frame of the
lexical environment and put it on the stack.  The OVER part of the
lexical address of the variable is specified as an argument to the
instruction.

   If the variable is not found then raise E_INVAL.

   Stack: '( -- VAL )'

21.1.4.5 Instruction popvar
...........................

Synopsys:

     popvar BACK, OVER

   Pop a value from the stack and set it as the value of a variable
having the lexical address specified in the arguments, in the current
lexical environment.

   Stack: '( VAL -- )'

21.1.4.6 Instruction regvar
...........................

Synopsys:

     regvar

   Pop a value from the stack and use it as the value for a new variable
in the current lexical environment.

   Stack: '( VAL -- )'

21.1.4.7 Instruction duc
........................

Synopsys:

     duc

   Make a copy of the closure at the top of the stack, and replace it.

   Stack: '( CLS -- CLS )'

21.1.4.8 Instruction pec
........................

Synopsys:

     pec

   Put the current lexical environment to the closure at the top of the
stack.

   Stack: '( CLS -- CLS )'

21.1.5 Printing Instructions
----------------------------

In the following instructions the meaning of the argument BASE is the
following:

   2 - print the number in binary.  8 - print the number in octal.  16 -
print the number in hexadecimal.  Any other value - print the number in
decimal.

21.1.5.1 Instruction indent
...........................

Synopsys:

     indent

   Indent the output in the terminal for LVL levels of indentation,
using STEP white chars in each indentation level.

   LVL is an integer in the under top stack.  STEP is an integer in the
top of the stack.

   Stack: '( INT INT -- )'

21.1.5.2 Instruction printi
...........................

Synopsys:

     printi BITS

   Given a signed integer and a numeration base in the stack, print the
integer to the terminal.

   Stack: '( INT INT -- )'

21.1.5.3 Instruction printiu
............................

Synopsys:

     printiu BITS

   Given an unsigned integer and a numeration base in the stack, print
the integer to the terminal.

   Stack: '( UINT INT -- )'

21.1.5.4 Instruction printl
...........................

Synopsys:

     printl BITS

   Given a long and a numeration base in the stack, print the integer to
the terminal.

   Stack: '( LONG INT -- )'

21.1.5.5 Instruction printlu
............................

Synopsys:

     printlu BITS

   Given an unsigned long and a numeration base in the stack, print the
integer to the terminal.

   Stack: '( ULONG INT -- )'

21.1.5.6 Instruction prints
...........................

Synopsys:

     prints

   Print the string at the top of the stack.

   Stack: '( STR -- )'

21.1.5.7 Instruction beghl
..........................

Synopsys:

     beghl

   Begin an hyperlink, using the URL and ID on the stack.

   Stack: '( STR STR -- )'

21.1.5.8 Instruction endhl
..........................

Synopsys:

     endhl

   End the current hyperlink.

   If no hyperlink is currently being generated, this instruction raises
an exception.

   Stack: '( -- )'

   Exceptions: 'PVM_E_GENERIC'

21.1.5.9 Instruction begsc
..........................

Synopsys:

     begsc

   Begin the styling class whose name is found on the stack.  This class
will be in effect in subsequent print operations until it is explicitly
ended by a 'endsc' instruction.

   Stack: '( STR -- )'

21.1.5.10 Instruction endsc
...........................

Synopsys:

     endsc

   End the styling class whose name is found on the stack.  This class
should have been previously began by a 'begsc' instruction.

   Stack: '( STR -- )'

21.1.6 Format Instructions
--------------------------

In the following instructions the meaning of the argument BASE is the
following:

   2 - format the number in binary.  8 - format the number in octal.  16
- format the number in hexadecimal.  Any other value - format the number
in decimal.

21.1.6.1 Instruction formati
............................

Synopsys:

     formati BITS

   Given a signed integer and a numeration base in the stack, push the
string representation to the stack.

   Stack: '( INT INT -- STR )'

21.1.6.2 Instruction formatiu
.............................

Synopsys:

     formatiu BITS

   Given an unsigned integer and a numeration base in the stack, push
the string representation to the stack.

   Stack: '( UINT INT -- STR )'

21.1.6.3 Instruction formatl
............................

Synopsys:

     formatl BITS

   Given a long and a numeration base in the stack, push the string
representation to the stack.

   Stack: '( LONG INT -- STR )'

21.1.6.4 Instruction formatlu
.............................

Synopsys:

     formatlu BITS

   Given an unsigned long and a numeration base in the stack, push the
string representation to the stack.

   Stack: '( ULONG INT -- STR )'

21.1.7 Main stack manipulation instructions
-------------------------------------------

21.1.7.1 Instruction push
.........................

Synopsys:

     push VAL

   Push the value given as an argument to the main stack.

   Stack: '( -- VAL )'

21.1.7.2 Instruction drop
.........................

Synopsys:

     drop

   Pop the value at the top of the main stack, and discard it.

   Stack: '( VAL -- )'

21.1.7.3 Instruction drop2
..........................

Synopsys:

     drop2

   Pop the two values at the top of the main stack, and discard them.

   Stack: '( VAL VAL -- )'

21.1.7.4 Instruction drop3
..........................

Synopsys:

     drop3

   Pop the three values at the top of the main stack, and discard them.

   Stack: '( VAL VAL VAL -- )'

21.1.7.5 Instruction drop4
..........................

Synopsys:

     drop4

   Pop the four values at the top of the stack, and discard them.

   Stack: '( VAL VAL VAL VAL -- )'

21.1.7.6 Instruction swap
.........................

Synopsys:

     swap

   Exchange the two elements at the top of the stack.

   Stack: '( A B -- B A)'

21.1.7.7 Instruction nip
........................

Synopsys:

     nip

   Discard the element at the under top of the main stack.

   Stack: '( A B -- B )'

21.1.7.8 Instruction nip2
.........................

Synopsys:

     nip2

   Discard the two elements at the under top of the main stack.

   Stack: '( A B C -- C )'

21.1.7.9 Instruction nip3
.........................

Synopsys:

     nip3

   Discard the three elements at the under top of the main stack.

   Stack: '( A B C D -- D )'

21.1.7.10 Instruction dup
.........................

Synopsys:

     dup

   Push a copy of the element at the top of the main stack.

   Stack: '( A -- A A )'

21.1.7.11 Instruction over
..........................

Synopsys:

     over

   Push a copy of the element at the under top of the main stack.

   Stack: '( A B -- A B A )'

21.1.7.12 Instruction rot
.........................

Synopsys:

     rot

   Rotate the three elements at the top of the main stack, clock-wise.

   Stack: '( A B C -- B C A )'

21.1.7.13 Instruction nrot
..........................

Synopsys:

     nrot

   Rotate the three elements at the top of the stack, counter
clock-wise.

   Stack: '( A B C -- C A B )'

21.1.7.14 Instruction tuck
..........................

Synopsys:

     tuck

   Tuck a copy of the element at the top of the stack down two
positions.

   Stack: '( A B -- B A B )'

21.1.7.15 Instruction quake
...........................

Synopsys:

     quake

   Swap the two elements at the under top of the stack.

   Stack: '( A B C - B A C )'

21.1.7.16 Instruction revn
..........................

Synopsys:

     revn N

   Reverse the N elements at the top of the stack.

   Stack: '( VAL... -- VAL... )'

21.1.7.17 Instruction pushhi
............................

Synopsys:

     pushhi VAL

   Push the high 32 bits of the value passed as an argument to the main
stack.  This instruction shall be completed with a 'pushlo'.

   This instruction is a workaround to a limitation of Jitter.

   Stack: '( -- HI32(VAL) )'

21.1.7.18 Instruction pushlo
............................

Synopsys:

     pushlo VAL

   Push the low 32 bits of the value passed as an argument to the main
stack.  This instruction shall be preceded by a 'pushhi'.

   This instruction is a workaround to a limitation of Jitter.

   Stack: '( -- LO32(VAL) )'

21.1.7.19 Instruction push32
............................

Synopsys:

     push32 VAL

   Push the value passed as an argument on the stack.  This assumes that
the internal representation of VAL doesn't require more than 32-bit.

   This instruction is a workaround to a limitation of Jitter.

   Stack: '( -- VAL )'

21.1.8 Registers manipulation instructions
------------------------------------------

21.1.8.1 Instruction pushr
..........................

Synopsys:

     pushr REGNO

   Push the contents of the register REGNO on the stack.

   Stack: '( -- VAL )'

21.1.8.2 Instruction popr
.........................

Synopsys:

     popr REGNO

   Pop the element at the top of the stack and put it in the register
REGNO.

   Stack: '( VAL -- )'

21.1.8.3 Instruction setr
.........................

Synopsys:

     setr REGNO

   Set the element at the top of the stack to the value of the register
REGNO.

   Stack: '( -- )'

21.1.9 Return stack manipulation instructions
---------------------------------------------

21.1.9.1 Instruction saver
..........................

Synopsys:

     saver REGNO

   Push the contents of the register REGNO to the return stack.

   Stack: '( -- )' ReturnStack: ( - VAL )

21.1.9.2 Instruction restorer
.............................

Synopsys:

     restorer REGNO

   Pop the element at the top of the return stack and put it in the
register REGNO.

   Stack: '( -- )' ReturnStack: ( VAL - )

21.1.9.3 Instruction tor
........................

Synopsys:

     tor

   Pop an element from the stack and push it in the return stack.

   Stack: '( VAL -- )' ReturnStack: ( - VAL )

21.1.9.4 Instruction fromr
..........................

Synopsys:

     fromr

   Pop an element from the return stack and push it on the stack.

   Stack: '( -- VAL)' ReturnStack: ( VAL - )

21.1.9.5 Instruction atr
........................

Synopsys:

     atr

   Push a copy of the element at the top of the return stack into the
stack.

   Stack: '( -- VAL )'

21.1.10 Arithmetic instructions
-------------------------------

The following instructions assume that both operands have the same size
in bits.

21.1.10.1 Instruction addi
..........................

Synopsys:

     addi

   Push the result of adding the two integers at the top of the stack.
If the operation would result in overflow, raise PVM_E_OVERFLOW.

   Stack: '( INT INT -- INT INT INT )'

   Exceptions: 'PVM_E_OVERFLOW'

21.1.10.2 Instruction addiu
...........................

Synopsys:

     addiu

   Push the result of adding the two unsigned integers at the top of the
stack.

   Stack: '( UINT UINT -- UINT UINT UINT )' Instruction addl

   Push the result of adding the two longs at the top of the stack.  If
the operation would result in overflow, raise PVM_E_OVERFLOW.

   Stack: '( LONG LONG -- LONG LONG LONG )'

   Exceptions: 'PVM_E_OVERFLOW'

21.1.10.3 Instruction addlu
...........................

Synopsys:

     addlu

   Push the result of adding the two unsigned longs at the top of the
stack.

   Stack: '( ULONG ULONG -- ULONG ULONG ULONG )'

21.1.10.4 Instruction subi
..........................

Synopsys:

     subi

   Push the result of subtracting the two integers at the top of the
stack.

   Stack: '( INT INT -- INT INT INT )'

21.1.10.5 Instruction subiu
...........................

Synopsys:

     subiu

   Push the result of subtracting the two unsigned integers at the top
of the stack.

   Stack: '( UINT UINT -- UINT UINT UINT )'

21.1.10.6 Instruction subl
..........................

Synopsys:

     subl

   Push the result of subtracting the two longs at the top of the stack.

   Stack: '( LONG LONG -- LONG LONG LONG )'

21.1.10.7 Instruction sublu
...........................

Synopsys:

     sublu

   Push the result of subtracting the two unsigned longs at the top of
the stack.

   Stack: '( ULONG ULONG -- ULONG ULONG ULONG )'

21.1.10.8 Instruction muli
..........................

Synopsys:

     muli

   Push the result of multiplying the two integers at the top of the
stack.

   Stack: '( INT INT -- INT INT INT )'

21.1.10.9 Instruction muliu
...........................

Synopsys:

     muliu

   Push the result of multiplying the two unsigned integers at the top
of the stack.

   Stack: '( UINT UINT -- UINT UINT UINT )'

21.1.10.10 Instruction mull
...........................

Synopsys:

     mull

   Push the result of multiplying the two longs at the top of the stack.

   Stack: '( LONG LONG -- LONG LONG LONG )'

21.1.10.11 Instruction mullu
............................

Synopsys:

     mullu

   Push the result of multiplying the two unsigned longs at the top of
the stack.

   Stack: '( ULONG ULONG -- ULONG ULONG ULONG )'

21.1.10.12 Instruction divi
...........................

Synopsys:

     divi

   Push the result of the integer division of the two integers at the
top of the stack.  If the denominator is zero, raise PVM_E_DIV_BY_ZERO.

   Stack: '( INT INT -- INT INT INT )'

   Exceptions: 'PVM_E_DIV_BY_ZERO'

21.1.10.13 Instruction diviu
............................

Synopsys:

     diviu

   Push the result of the integer division of the two unsigned integers
at the top of the stack.  If the denominator is zero, raise
PVM_E_DIV_BY_ZERO.

   Stack: '( UINT UINT -- UINT UINT UINT )'

   Exceptions: 'PVM_E_DIV_BY_ZERO'

21.1.10.14 Instruction divl
...........................

Synopsys:

     divl

   Push the result of the integer division of the two longs at the top
of the stack.  If the denominator is zero, raise PVM_E_DIV_BY_ZERO.

   Stack: '( LONG LONG -- LONG LONG LONG )'

   Exceptions: 'PVM_E_DIV_BY_ZERO'

21.1.10.15 Instruction divlu
............................

Synopsys:

     divlu

   Push the result of the integer division of the two unsigned longs at
the top of the stack.  If the denominator is zero, raise
PVM_E_DIV_BY_ZERO.

   Stack: '( ULONG ULONG -- ULONG ULONG ULONG )'

   Exceptions: 'PVM_E_DIV_BY_ZERO'

21.1.10.16 Instruction modi
...........................

Synopsys:

     modi

   Push the result of the modulus of the two integers at the top of the
stack.  If the denominator is zero, raise PVM_E_DIV_BY_ZERO.

   Stack: '( INT INT -- INT INT INT )'

   Exceptions: 'PVM_E_DIV_BY_ZERO'

21.1.10.17 Instruction modiu
............................

Synopsys:

     modiu

   Push the result of the modulus of the two unsigned integers at the
top of the stack.  If the denominator is zero, raise PVM_E_DIV_BY_ZERO.

   Stack: '( UINT UINT -- UINT UINT UINT )'

   Exceptions: 'PVM_E_DIV_BY_ZERO'

21.1.10.18 Instruction modl
...........................

Synopsys:

     modl

   Push the result of the modulus of the two longs at the top of the
stack.  If the denominator is zero, raise PVM_E_DIV_BY_ZERO.

   Stack: '( LONG LONG -- LONG LONG LONG )'

   Exceptions: 'PVM_E_DIV_BY_ZERO'

21.1.10.19 Instruction modlu
............................

Synopsys:

     modlu

   Push the result of the modulus of the two unsigned longs at the top
of the stack.  If the denominator is zero, raise PVM_E_DIV_BY_ZERO.

   Stack: '( ULONG ULONG -- ULONG ULONG ULONG )'

   Exceptions: 'PVM_E_DIV_BY_ZERO'

21.1.10.20 Instruction negi
...........................

Synopsys:

     negi

   Push the result of the negation of the integer at the top of the
stack.

   Stack: '( INT -- INT INT )'

21.1.10.21 Instruction negiu
............................

Synopsys:

     negiu

   Push the result of the negation of the unsigned integer at the top of
the stack.

   Stack: '( UINT -- UINT UINT UINT )'

21.1.10.22 Instruction negl
...........................

Synopsys:

     negl

   Push the result of the negation of the long at the top of the stack.

   Stack: '( LONG -- LONG LONG )'

21.1.10.23 Instruction neglu
............................

Synopsys:

     neglu

   Push the result of the negation of the unsigned long at the top of
the stack.

   Stack: '( ULONG -- ULONG ULONG )'

21.1.10.24 Instruction powi
...........................

Synopsys:

     powi

   Perform the exponentiation of the integer at the under top of the
stack.  The exponent is the unsigned integer at the top of the stack.
If the exponent is 0, the result is 1.

   Stack: '( INT UINT -- INT UINT INT )'

21.1.10.25 Instruction powiu
............................

Synopsys:

     powiu

   Perform the exponentiation of the unsigned integer at the under top
of the stack.  The exponent is the unsigned integer at the top of the
stack.  If the exponent is 0, the result is 1.

   Stack: '( UINT UINT -- UINT UINT UINT )'

21.1.10.26 Instruction powl
...........................

Synopsys:

     powl

   Perform the exponentiation of the long at the under top of the stack.
The exponent is the unsigned integer at the top of the stack.  If the
exponent is 0, the result is 1.

   Stack: '( LONG UINT -- LONG UINT LONG )'

21.1.10.27 Instruction powlu
............................

Synopsys:

     powlu

   Perform the exponentiation of the unsigned long at the under top of
the stack.  The exponent is the unsigned integer at the top of the
stack.  If the exponent is 0, the result is 1.

   Stack: '( ULONG UINT -- ULONG UINT ULONG )'

21.1.11 Relational instructions
-------------------------------

21.1.11.1 Instruction eqi
.........................

Synopsys:

     eqi

   Push 1 on the stack if the two integers at the top of the stack are
equal.  Otherwise push 0.

   Stack: '( INT INT -- INT INT INT )'

21.1.11.2 Instruction eqiu
..........................

Synopsys:

     eqiu

   Push 1 on the stack if the two unsigned integers at the top of the
stack are equal.  Otherwise push 0.

   Stack: '( UINT UINT -- UINT UINT UINT )'

21.1.11.3 Instruction eql
.........................

Synopsys:

     eql

   Push 1 on the stack if the two longs at the top of the stack are
equal.  Otherwise push 0.

   Stack: '( LONG LONG -- LONG LONG INT )'

21.1.11.4 Instruction eqlu
..........................

Synopsys:

     eqlu

   Push 1 on the stack if the two unsigned longs at the top of the stack
are equal.  Otherwise push 0.

   Stack: '( ULONG ULONG -- ULONG ULONG INT )'

21.1.11.5 Instruction eqs
.........................

Synopsys:

     eqs

   Push 1 on the stack if the two strings at the top of the stack are
equal.  Otherwise push 0.

   Stack: '( STR STR -- STR STR INT )'

21.1.11.6 Instruction nei
.........................

Synopsys:

     nei

   Push 1 on the stack if the two integers at the top of the stack are
not equal.  Otherwise push 0.

   Stack: '( INT INT -- INT INT INT )'

21.1.11.7 Instruction neiu
..........................

Synopsys:

     neiu

   Push 1 on the stack if the two unsigned integers at the top of the
stack are not equal.  Otherwise push 0.

   Stack: '( UINT UINT -- UINT UINT INT )'

21.1.11.8 Instruction nel
.........................

Synopsys:

     nel

   Push 1 on the stack if the two longs at the top of the stack are not
equal.  Otherwise push 0.

   Stack: '( LONG LONG -- LONG LONG INT )'

21.1.11.9 Instruction nelu
..........................

Synopsys:

     nelu

   Push 1 on the stack if the two unsigned longs at the top of the stack
are not equal.  Otherwise push 0.

   Stack: '( ULONG ULONG -- ULONG ULONG INT )'

21.1.11.10 Instruction nes
..........................

Synopsys:

     nes

   Push 1 on the stack if the two strings at the top of the stack are
not equal.  Otherwise push 0.

   Stack: '( STR STR -- STR STR INT )'

21.1.11.11 Instruction nn
.........................

Synopsys:

     nn

   Push 0 on the stack if the value at the top of the stack equals
PVM_NULL. Otherwise push 1.

   Stack: '( VAL -- VAL INT )'

21.1.11.12 Instruction nnn
..........................

Synopsys:

     nnn

   Push 1 on the stack if the value at the top of the stack equals
PVM_NULL. Otherwise push 0.

   Stack: '( VAL -- VAL INT )'

21.1.11.13 Instruction lti
..........................

Synopsys:

     lti

   Push 1 on the stack if the integer at the under top is less that the
integer at the top.  Otherwise push 0.

   Stack: '( INT INT -- INT INT INT )'

21.1.11.14 Instruction ltiu
...........................

Synopsys:

     ltiu

   Push 1 on the stack if the unsigned integer at the under top is less
that the unsigned integer at the top.  Otherwise push 0.

   Stack: '( UINT INT -- UINT UINT INT )'

21.1.11.15 Instruction ltl
..........................

Synopsys:

     ltl

   Push 1 on the stack if the long at the under top is less that the
long at the top.  Otherwise push 0.

   Stack: '( LONG LONG -- LONG LONG INT )'

21.1.11.16 Instruction ltlu
...........................

Synopsys:

     ltlu

   Push 1 on the stack if the unsigned long at the under top is less
that the unsigned long at the top.  Otherwise push 0.

   Stack: '( ULONG ULONG -- ULONG ULONG INT )'

21.1.11.17 Instruction lei
..........................

Synopsys:

     lei

   Push 1 on the stack if the integer at the under top is less or equal
that the integer at the top.  Otherwise push 0.

   Stack: '( INT INT -- INT INT INT )'

21.1.11.18 Instruction leiu
...........................

Synopsys:

     leiu

   Push 1 on the stack if the unsigned integer at the under top is less
or equal that the unsigned integer at the top.  Otherwise push 0.

   Stack: '( UINT UINT -- UINT UINT INT )'

21.1.11.19 Instruction lel
..........................

Synopsys:

     lel

   Push 1 on the stack if the long at the under top is less or equal
that the long at the top.  Otherwise push 0.

   Stack: '( LONG LONG -- LONG LONG INT )'

21.1.11.20 Instruction lelu
...........................

Synopsys:

     lelu

   Push 1 on the stack if the unsigned long at the under top is less or
equal that the unsigned long at the top.  Otherwise push 0.

   Stack: '( ULONG ULONG -- ULONG ULONG INT )'

21.1.11.21 Instruction gti
..........................

Synopsys:

     gti

   Push 1 on the stack if the integer at the under top is greater than
the integer at the top.  Otherwise push 0.

   Stack: '( INT INT -- INT INT INT )'

21.1.11.22 Instruction gtiu
...........................

Synopsys:

     gtiu

   Push 1 on the stack if the unsigned integer at the under top is
greater than the unsigned integer at the top.  Otherwise push 0.

   Stack: '( UINT UINT -- UINT UINT INT )'

21.1.11.23 Instruction gtl
..........................

Synopsys:

     gtl

   Push 1 on the stack if the long at the under top is greater than the
long at the top.  Otherwise push 0.

   Stack: '( LONG LONG -- LONG LONG INT )'

21.1.11.24 Instruction gtlu
...........................

Synopsys:

     gtlu

   Push 1 on the stack if the unsigned long at the under top is greater
than the unsigned long at the top.  Otherwise push 0.

   Stack: '( LONG LONG -- LONG LONG INT )'

21.1.11.25 Instruction gei
..........................

Synopsys:

     gei

   Push 1 on the stack if the integer at the under top is greater or
equal than the integer at the top.  Otherwise push 0.

   Stack: '( INT INT -- INT INT INT )'

21.1.11.26 Instruction geiu
...........................

Synopsys:

     geiu

   Push 1 on the stack if the unsigned integer at the under top is
greater or equal than the unsigned integer at the top.  Otherwise push
0.

   Stack: '( UINT UINT -- UINT UINT INT )'

21.1.11.27 Instruction gel
..........................

Synopsys:

     gel

   Push 1 on the stack if the long at the under top is greater or equal
than the long at the top.  Otherwise push 0.

   Stack: '( LONG LONG -- LONG LONG INT )'

21.1.11.28 Instruction gelu
...........................

Synopsys:

     gelu

   Push 1 on the stack if the unsigned long at the under top is greater
or equal than the unsigned long at the top.  Otherwise push 0.

   Stack: '( ULONG ULONG -- ULONG ULONG INT )'

21.1.11.29 Instruction lts
..........................

Synopsys:

     lts

   Push 1 on the stack if the string at the under top is less than the
string at the top, in lexicographic order.  Otherwise push 0.

   Stack: '( STR STR -- STR STR INT )'

21.1.11.30 Instruction gts
..........................

Synopsys:

     gts

   Push 1 on the stack if the string at the under top is greater than
the string at the top, in lexicographic order.  Otherwise push 0.

   Stack: '( STR STR -- STR STR INT )'

21.1.11.31 Instruction ges
..........................

Synopsys:

     ges

   Push 1 on the stack if the string at the under top is greater or
equal than the string at the top, in lexicographic order.  Otherwise
push 0.

   Stack: '( STR STR -- STR STR INT )'

21.1.11.32 Instruction les
..........................

Synopsys:

     les

   Push 1 on the stack if the string at the under top is less or equal
than the string at the top, in lexicographic order.  Otherwise push 0.

   Stack: '( STR STR -- STR STR INT )'

21.1.11.33 Instruction eqc
..........................

Synopsys:

     eqc

   Push 1 on the stack if the two closures at the top of the stack are
equal.  Otherwise push 0.

   Stack: '( CLS CLS -- CLS CLS INT )'

21.1.11.34 Instruction nec
..........................

Synopsys:

     nec

   Push 1 on the stack if the two closures at the top of the stack are
not equal.  Otherwise push 0.

   Stack: '( CLS CLS -- CLS CLS INT )'

21.1.12 Concatenation instructions
----------------------------------

21.1.12.1 Instruction sconc
...........................

Synopsys:

     sconc

   Push the concatenation of the two strings at the top of the stack.

   Stack: '( STR STR -- STR STR STR )'

21.1.13 Logical instructions
----------------------------

21.1.13.1 Instruction and
.........................

Synopsys:

     and

   Push the logical and of the two elements at the top of the stack.

   Stack: '( INT INT -- INT INT INT )'

21.1.13.2 Instruction or
........................

Synopsys:

     or

   Push the logical or of the two elements at the top of the stack.

   Stack: '( INT INT -- INT INT INT )'

21.1.13.3 Instruction not
.........................

Synopsys:

     not

   Push the logical not of the element at the top of the stack.

   Stack: '( INT -- INT INT )'

21.1.14 Bitwise instructions
----------------------------

21.1.14.1 Instruction bxori
...........................

Synopsys:

     bxori

   Push the bitwise exclusive or of the two integers at the top of the
stack.

   Stack: '( INT INT -- INT INT INT )'

21.1.14.2 Instruction bxoriu
............................

Synopsys:

     bxoriu

   Push the bitwise exclusive or of the two unsigned integers at the top
of the stack.

   Stack: '( UINT UINT -- UINT UINT UINT )'

21.1.14.3 Instruction bxorl
...........................

Synopsys:

     bxorl

   Push the bitwise exclusive or of the two longs at the top of the
stack.

   Stack: '( LONG LONG -- LONG LONG LONG )'

21.1.14.4 Instruction bxorlu
............................

Synopsys:

     bxorlu

   Push the bitwise exclusive or of the two unsigned longs at the top of
the stack.

   Stack: '( ULONG ULONG -- ULONG ULONG ULONG )'

21.1.14.5 Instruction bori
..........................

Synopsys:

     bori

   Push the bitwise or of the two integers at the top of the stack.

   Stack: '( INT INT -- INT INT INT )'

21.1.14.6 Instruction boriu
...........................

Synopsys:

     boriu

   Push the bitwise or of the two unsigned integers at the top of the
stack.

   Stack: '( UINT UINT -- UINT UINT UINT )'

21.1.14.7 Instruction borl
..........................

Synopsys:

     borl

   Push the bitwise or of the two longs at the top of the stack.

   Stack: '( LONG LONG -- LONG LONG LONG )'

21.1.14.8 Instruction borlu
...........................

Synopsys:

     borlu

   Push the bitwise or of the two longs at the top of the stack.

   Stack: '( ULONG ULONG -- ULONG ULONG ULONG )'

21.1.14.9 Instruction bandi
...........................

Synopsys:

     bandi

   Push the bitwise and of the two integers at the top of the stack.

   Stack: '( INT INT -- INT INT INT )'

21.1.14.10 Instruction bandiu
.............................

Synopsys:

     bandiu

   Push the bitwise and of the two unsigned integers at the top of the
stack.

   Stack: '( UINT UINT -- UINT UINT UINT )'

21.1.14.11 Instruction bandl
............................

Synopsys:

     bandl

   Push the bitwise and of the two longs at the top of the stack.

   Stack: '( LONG LONG -- LONG LONG LONG )'

21.1.14.12 Instruction bandlu
.............................

Synopsys:

     bandlu

   Push the bitwise and of the two unsigned longs at the top of the
stack.

   Stack: '( ULONG ULONG -- ULONG ULONG ULONG )'

21.1.14.13 Instruction bnoti
............................

Synopsys:

     bnoti

   Push the bitwise not of the integer at the top of the stack.

   Stack: '( INT -- INT INT INT )'

21.1.14.14 Instruction bnotiu
.............................

Synopsys:

     bnotiu

   Push the bitwise not of the unsigned integer at the top of the stack.

   Stack: '( UINT -- UINT UINT )'

21.1.14.15 Instruction bnotl
............................

Synopsys:

     bnotl

   Push the bitwise not of the long at the top of the stack.

   Stack: '( LONG -- LONG LONG )'

21.1.14.16 Instruction bnotlu
.............................

Synopsys:

     bnotlu

   Push the bitwise not of the unsigned long at the top of the stack.

   Stack: '( ULONG -- ULONG ULONG )'

21.1.15 Shift instructions
--------------------------

21.1.15.1 Instruction bsli
..........................

Synopsys:

     bsli

   Left-shift the integer at the under top of the stack the number of
bits indicated by the unsigned int at the top of the stack.

   If the bit count is equal or bigger than the size of the left
operand, then raise PVM_E_OUT_OF_BOUNDS.

   Stack: '( INT UINT -- INT UINT INT )'

   Exceptions: 'PVM_E_OUT_OF_BOUNDS'

21.1.15.2 Instruction bsliu
...........................

Synopsys:

     bsliu

   Left-shift the unsigned integer at the under top of the stack the
number of bits indicated by the unsigned int at the top of the stack.

   If the bit count is equal or bigger than the size of the left
operand, then raise PVM_E_OUT_OF_BOUNDS.

   Stack: '( UINT UINT -- UINT UINT UINT )'

   Exceptions: 'PVM_E_OUT_OF_BOUNDS'

21.1.15.3 Instruction bsll
..........................

Synopsys:

     bsll

   Left-shift the long at the under top of the stack the number of bits
indicated by the unsigned int at the top of the stack.

   If the bit count is equal or bigger than the size of the left
operand, then raise PVM_E_OUT_OF_BOUNDS.

   Stack: '( LONG UINT -- LONG UINT LONG )'

   Exceptions: 'PVM_E_OUT_OF_BOUNDS'

21.1.15.4 Instruction bsllu
...........................

Synopsys:

     bsllu

   Left-shift the unsigned long at the under top of the stack the number
of bits indicated by the unsigned int at the top of the stack.

   If the bit count is equal or bigger than the size of the left
operand, then raise PVM_E_OUT_OF_BOUNDS.

   Stack: '( ULONG UINT -- ULONG UINT ULONG )'

   Exceptions: 'PVM_E_OUT_OF_BOUNDS'

21.1.15.5 Instruction bsri
..........................

Synopsys:

     bsri

   Right-shift the integer at the under top of the stack the number of
tis indicated by the unsigned int at the top of the stack.

   Stack: '( INT UINT -- INT UINT INT )'

21.1.15.6 Instruction bsriu
...........................

Synopsys:

     bsriu

   Right-shift the unsigned integer at the under top of the stack the
number of tis indicated by the unsigned int at the top of the stack.

   Stack: '( UINT UINT -- UINT UINT UINT )'

21.1.15.7 Instruction bsrl
..........................

Synopsys:

     bsrl

   Right-shift the long at the under top of the stack the number of tis
indicated by the unsigned int at the top of the stack.

   Stack: '( LONG UINT -- LONG UINT LONG )'

21.1.15.8 Instruction bsrlu
...........................

Synopsys:

     bsrlu

   Right-shift the unsigned long at the under top of the stack the
number of tis indicated by the unsigned int at the top of the stack.

   Stack: '( ULONG UINT -- ULONG UINT ULONG )'

21.1.16 Compare-and-swap instructions
-------------------------------------

21.1.16.1 Instruction swapgti
.............................

Synopsys:

     swapgti

   Swap the two integers at the top of the stack if the element at the
under-top is greater than the element at the top.

   Stack: '( INT INT -- INT INT )'

21.1.16.2 Instruction swapgtiu
..............................

Synopsys:

     swapgtiu

   Swap the two unsigned integers at the top of the stack if the element
at the under-top is greater than the element at the top.

   Stack: '( UINT UINT -- UINT UINT )'

21.1.16.3 Instruction swapgtl
.............................

Synopsys:

     swapgtl

   Swap the two longs at the top of the stack if the element at the
under-top is greater than the element at the top.

   Stack: '( LONG LONG -- LONG LONG )'

21.1.16.4 Instruction swapgtlu
..............................

Synopsys:

     swapgtlu

   Swap the two unsigned longs at the top of the stack if the element at
the under-top is greater than the element at the top.

   Stack: '( ULONG ULONG -- ULONG ULONG )'

21.1.17 Branch instructions
---------------------------

21.1.17.1 Instruction ba
........................

Synopsys:

     ba LABEL

   Branch unconditionally to the given LABEL.

   Stack: '( -- )'

21.1.17.2 Instruction bn
........................

Synopsys:

     bn LABEL

   Branch to the given LABEL if the value at the top of the stack is
PVM_NULL.

   Stack: '( VAL -- VAL )'

21.1.17.3 Instruction bnn
.........................

Synopsys:

     bnn LABEL

   Branch to the given LABEL if the value at the top of the stack is not
PVM_NULL.

   Stack: '( VAL -- VAL )'

21.1.17.4 Instruction bzi
.........................

Synopsys:

     bzi LABEL

   Branch to the given LABEL if the integer at the top of the stack is
zero.

   Stack: '( INT -- INT )'

21.1.17.5 Instruction bziu
..........................

Synopsys:

     bziu LABEL

   Branch to the given LABEL if the unsigned integer at the top of the
stack is zero.

   Stack: '( UINT -- UINT )'

21.1.17.6 Instruction bzl
.........................

Synopsys:

     bzl LABEL

   Branch to the given LABEL if the long at the top of the stack is
zero.

   Stack: '( LONG -- LONG )'

21.1.17.7 Instruction bzlu
..........................

Synopsys:

     bzlu LABEL

   Branch to the given LABEL if the unsigned long at the top of the
stack is zero.

   Stack: '( ULONG -- ULONG )'

21.1.17.8 Instruction bnzi
..........................

Synopsys:

     bnzi LABEL

   Branch to the given LABEL if the integer at the top of the stack is
nonzero.

   Stack: '( INT -- INT )'

21.1.17.9 Instruction bnziu
...........................

Synopsys:

     bnziu LABEL

   Branch to the given LABEL if the unsigned integer at the top of the
stack is nonzero.

   Stack: '( UINT -- UINT )'

21.1.17.10 Instruction bnzl
...........................

Synopsys:

     bnzl LABEL

   Branch to the given LABEL if the long at the top of the stack is
nonzero.

   Stack: '( LONG -- LONG )'

21.1.17.11 Instruction bnzlu
............................

Synopsys:

     bnzlu LABEL

   Branch to the given LABEL if the unsigned long at the top of the
stack is nonzero.

   Stack: '( ULONG -- ULONG )'

21.1.18 Conversion instructions
-------------------------------

21.1.18.1 Instruction ctos
..........................

Synopsys:

     ctos

   Convert the character encoded as an unsigned integer at the top of
the stack to a string that contains just that character.

   Stack: '( UINT -- UINT STR )'

21.1.18.2 Instruction itoi
..........................

Synopsys:

     itoi NBITS

   Convert the integer at the top of the stack to an integer featuring
NBITS bits.

   NBITS can be any number from 1 to 32.

   Stack: '( INT -- INT INT )'

21.1.18.3 Instruction itoiu
...........................

Synopsys:

     itoiu NBITS

   Convert the integer at the top of the stack to an unsigned integer
featuring NBITS bits.

   NBITS can be any number from 1 to 32.

   Stack: '( INT -- INT UINT )'

21.1.18.4 Instruction itol
..........................

Synopsys:

     itol NBITS

   Convert the integer at the top of the stack to a long featuring NBITS
bits.

   NBITS can be any number from 1 to 64.

   Stack: '( INT -- INT LONG )'

21.1.18.5 Instruction itolu
...........................

Synopsys:

     itolu NBITS

   Convert the integer at the top of the stack to an unsigned long
featuring NBITS bits.

   NBITS can be any number from 1 to 64.

   Stack: '( INT -- INT ULONG )'

21.1.18.6 Instruction iutoi
...........................

Synopsys:

     iutoi NBITS

   Convert the unsigned integer at the top of the stack to an integer
featuring NBITS bits.

   NBITS can be any number from 1 to 32.

   Stack: '( UINT -- UINT INT )'

21.1.18.7 Instruction iutoiu
............................

Synopsys:

     iutoiu NBITS

   Convert the unsigned integer at the top of the stack to an unsigned
integer featuring NBITS bits.

   NBITS can be any number from 1 to 32.

   Stack: '( UINT -- UINT UINT )'

21.1.18.8 Instruction iutol
...........................

Synopsys:

     iutol NBITS

   Convert the unsigned integer at the top of the stack to a long
featuring NBITS bits.

   NBITS can be any number from 1 to 64.

   Stack: '( UINT -- UINT LONG )'

21.1.18.9 Instruction iutolu
............................

Synopsys:

     iutolu NBITS

   Convert the unsigned integer at the top of the stack to an unsigned
long featuring NBITS bits.

   NBITS can be any number from 1 to 64.

   Stack: '( UINT -- UINT ULONG )'

21.1.18.10 Instruction ltoi
...........................

Synopsys:

     ltoi NBITS

   Convert the long at the top of the stack to an integer featuring
NBITS bits.

   NBITS can be any number from 1 to 32.

   Stack: '( LONG -- LONG INT )'

21.1.18.11 Instruction ltoiu
............................

Synopsys:

     ltoiu NBITS

   Convert the long at the top of the stack to an unsigned integer
featuring NBITS bits.

   NBITS can be any number from 1 to 32.

   Stack: '( LONG -- LONG UINT )'

21.1.18.12 Instruction ltol
...........................

Synopsys:

     ltol NBITS

   Convert the long at the top of the stack to a long featuring NBITS
bits.

   NBITS can be any number from 1 to 64.

   Stack: '( LONG -- LONG LONG )'

21.1.18.13 Instruction ltolu
............................

Synopsys:

     ltolu NBITS

   Convert the long at the top of the stack to an unsigned long
featuring NBITS bits.

   NBITS can be any number from 1 to 64.

   Stack: '( LONG -- LONG ULONG )'

21.1.18.14 Instruction lutoi
............................

Synopsys:

     lutoi NBITS

   Convert the unsigned long at the top of the stack to an integer
featuring NBITS bits.

   NBITS can be any number from 1 to 32.

   Stack: '( ULONG -- ULONG INT )'

21.1.18.15 Instruction lutoiu
.............................

Synopsys:

     lutoiu NBITS

   Convert the unsigned long at the top of the stack to an unsigned
integer featuring NBITS bits.

   NBITS can be any number from 1 to 32.

   Stack: '( ULONG -- ULONG UINT )'

21.1.18.16 Instruction lutol
............................

Synopsys:

     lutol NBITS

   Convert the unsigned long at the top of the stack to a long featuring
NBITS bits.

   Stack: '( ULONG -- ULONG LONG )'

21.1.18.17 Instruction lutolu
.............................

Synopsys:

     lutolu NBITS

   Convert the unsigned long at the top of the stack to an unsigned long
featuring NBITS bits.

   Stack: '( ULONG -- ULONG ULONG )'

21.1.19 String instructions
---------------------------

21.1.19.1 Instruction strref
............................

Synopsys:

     strref

   Given a string and an unsigned long at the top of the stack, push an
unsigned integer with the code of the character that occupies that
position in the string, on the stack.

   The index is zero-based.  If it is less than 0 or exceeds the length
of the string, then PVM_E_OUT_OF_BOUNDS is raised.

   Stack: '( STR ULONG -- STR ULONG UINT )'

   Exceptions: 'PVM_E_OUT_OF_BOUNDS'

21.1.19.2 Instruction strset
............................

Synopsys:

     strset

   Given a string STR, an index FROM and a string NEWSTR, copy the
content of NEWSTR to STR at index FROM.

   Index is zero-based.

   If FROM >= the size of the string, or if FROM+len(NEWSTR) > the size
of the string, raise the PVM_E_OUT_OF_BOUNDS exception.

   Stack: '( STR ULONG NEWSTR -- STR )'

   Exceptions: 'PVM_E_OUT_OF_BOUNDS'

21.1.19.3 Instruction substr
............................

Synopsys:

     substr

   Given a string and two indices FROM and TO conforming a semi-open
interval [FROM,TO), push the substring enclosed by that interval.

   Both indexes are zero-based.

   If FROM >= the size of the string, or if TO > the size of the string,
or if FROM >= TO, raise the PVM_E_OUT_OF_BOUNDS exception.

   Stack: '( STR ULONG(from) ULONG(to) -- STR ULONG(from) ULONG(to) STR
)'

   Exceptions: 'PVM_E_OUT_OF_BOUNDS'

21.1.19.4 Instruction muls
..........................

Synopsys:

     muls

   Given a string and an unsigned long on the stack, push a new string
value whose value is the concatenation of the argument string applied to
itself as many times as the unsigned long.  If the second argument to
muls is 0 then the result of the operation is the empty string.

   Stack: '( STR ULONG -- STR ULONG STR )'

21.1.19.5 Instruction sprops
............................

Synopsys:

     sprops

   Given a string STR, set the styling class of the substring with
length LEN from index IDX to CLASS.

   Stack: '( STR IDX LEN CLASS -- STR )'

21.1.19.6 Instruction sproph
............................

Synopsys:

     sproph

   Given a string STR, set the hyperlink property (which characterized
by a URL and an ID) of the substring with length LEN from index IDX.

   Stack: '( STR IDX LEN URL ID -- STR )'

21.1.19.7 Instruction spropc
............................

Synopsys:

     spropc

   Given a string on the stack, push the copy of the string with all
properties cleared.

   Stack: '( STR -- STR )'

21.1.20 Array instructions
--------------------------

21.1.20.1 Instruction mka
.........................

Synopsys:

     mka

   Make a new empty array value.

   TYP is the type of the new array.

   NELEM is a hint on how many elements to use to initialize the array
value.  This is to avoid allocating memory that will never be used.  Use
0UL when the number of elements in the array are not known in advance;
this will make the PVM to choose a reasonable default.

   Stack: '( TYP ULONG(nelem) -- ARR )'

21.1.20.2 Instruction ains
..........................

Synopsys:

     ains

   Insert a new element VAL, at the end of the array ARR, making it
grow.

   If IDX is less than the current size of the array, the value is
stored in the referred argument.

   If IDX is equal or bigger than the current size of the array, the
same element is replicated in the previous elements.

   Examples:

   a = [a1, a2, a3]

   [a1, a2, a3] 2 VAL ains -> INVAL exception [a1, a2, a3] 3 VAL ains ->
[a1, a2, a3, VAL] [a1, a2, a3] 5 VAL ains -> [a1, a2, a3, VAL, VAL, VAL]

   Stack: '( ARR IDX VAL -- ARR )'

   Exceptions: 'PVM_E_INVAL'

21.1.20.3 Instruction arem
..........................

Synopsys:

     arem

   Remove an element from an array at the specified index, making it
schrink.

   If IDX doesn't correspond to an element in the array, raise
PVM_E_OUT_OF_BOUNDS. This always happens if the array is empty.

   Stack: '( ARR IDX -- ARR )' Exception: PVM_E_OUT_OF_BOUNDS

21.1.20.4 Instruction aset
..........................

Synopsys:

     aset

   Set the value with index ULONG in the array ARR to have the value
VAL.

   If the specified index exceeds the capability of the array, then
PVM_E_OUT_OF_BOUNDS is raised.  If the array is bounded by size and the
new value makes the total size of the array to change, then PVM_E_CONV
is raised.

   Stack: '( ARR ULONG VAL -- ARR )'

   Exceptions: 'PVM_E_CONV, PVM_E_OUT_OF_BOUNDS'

21.1.20.5 Instruction aref
..........................

Synopsys:

     aref

   Given an array ARR and an index ULONG, push the element of the array
occupying that position on the stack.

   If the provided index is out of bounds, then raise
PVM_E_OUT_OF_BOUNDS.

   Stack: '( ARR ULONG -- ARR ULONG VAL )'

   Exceptions: 'PVM_E_OUT_OF_BOUNDS'

21.1.20.6 Instruction arefo
...........................

Synopsys:

     arefo

   Given an array ARR and an index ULONG, push the offset of the element
occupying that position in the array.

   If the provided index is out of bounds, then raise
PVM_E_OUT_OF_BOUNDS.

   Stack: '( ARR ULONG -- ARR ULONG OFF )'

   Exceptions: 'PVM_E_OUT_OF_BOUNDS'

21.1.20.7 Instruction asettb
............................

Synopsys:

     asettb

   Given an array ARR and a closure BOUND, set the later as the array's
bounder function.  This is a function that, once executed with no
arguments, returns the size of the array.

   Stack: '( ARR BOUND -- ARR )'

21.1.21 Struct instructions
---------------------------

21.1.21.1 Instruction mksct
...........................

Synopsys:

     mksct

   Given an offset, a list of fields, a list of methods and a struct
type, create a struct value and push it on the stack.

   Each field is specified as a triplet [OFF STR VAL] where OFF is the
offset of field, STR the name of the field or PVM_NULL if the field is
anonymous, and VAL is a value.

   Each method is specified as a tuple [STR VAL] where STR is the name
of the method and VAL is the closure value corresponding to the method.

   Stack: '( OFF [OFF STR VAL]... [STR VAL]... ULONG ULONG TYP -- SCT )'

21.1.21.2 Instruction sset
..........................

Synopsys:

     sset

   Given a struct, a field name and a value, replace the value of the
referred struct field with the given value.  If the struct does not have
a field with the given name, then raise PVM_E_ELEM.

   Stack: '( SCT STR VAL -- SCT )'

   Exceptions: 'PVM_E_ELEM'

21.1.21.3 Instruction sseti
...........................

Synopsys:

     sseti

   Given a struct, a field index and a value, replace the value of the
referred struct field with the given value.  If the given index does not
refer to a struct field, then raise PVM_E_ELEM.

   Stack: '(SCT IDX VAL -- SCT)'

   Exceptions: 'PVM_E_ELEM'

21.1.21.4 Instruction sref
..........................

Synopsys:

     sref

   Given a struct and a field name, push the value contained in the
referred struct field on the stack.  If the struct does not have a field
with the given name, or if the field is absent from the struct value
then raise PVM_E_ELEM.

   Stack: '( SCT STR -- SCT STR VAL )'

   Exceptions: 'PVM_E_ELEM'

21.1.21.5 Instruction srefo
...........................

Synopsys:

     srefo

   Given a struct and a field name, push the bit-offset of the referred
field on the stack.  If the struct does not have a field with the given
name, or if the field is absent from the struct value then raise
PVM_E_ELEM.

   Stack: '( SCT STR -- SCT STR BOFF )'

   Exceptions: 'PVM_E_ELEM'

21.1.21.6 Instruction srefmnt
.............................

Synopsys:

     srefmnt

   Given a struct and a method name, push the closure value
corresponding to that method on the stack.  If the struct does not have
a method with the given name then push PVM_NULL.

   Stack: '( SCT STR - SCT STR CLS )'

21.1.21.7 Instruction srefnt
............................

Synopsys:

     srefnt

   Given a struct and a field name, push the value contained in the
struct field on the stack.  If the struct does not have a field with the
given name, or if the field is absent from the struct value then push
PVM_NULL.

   Stack: '( SCT STR -- SCT STR VAL )'

21.1.21.8 Instruction srefi
...........................

Synopsys:

     srefi

   Given a struct and an index, push the value of the field occupying
the position specified by the index in the given struct.  If the struct
doesn't have that many fields, raise PVM_E_OUT_OF_BOUNDS.

   Stack: '( SCT ULONG -- SCT ULONG VAL )'

   Exceptions: 'PVM_E_OUT_OF_BOUNDS'

21.1.21.9 Instruction srefia
............................

Synopsys:

     srefia

   Given a struct and an index, push 1 if the field occupying the
position specified by the index in the given struct is absent.  Push 0
otherwise.  If the struct doesn't have that many fields, raise
PVM_E_OUT_OF_BOUNDS.

   Stack: '( SCT ULONG -- SCT ULONG INT )'

   Exceptions: 'PVM_E_OUT_OF_BOUNDS'

21.1.21.10 Instruction srefio
.............................

Synopsys:

     srefio

   Given a struct and an index, push the offset of the field occupying
the position specified by the index in the given struct.  If the struct
doesn't have that many fields, raise PVM_E_OUT_OF_BOUNDS.

   Stack: '( SCT ULONG -- SCT ULONG BOFF )'

   Exceptions: 'PVM_E_OUT_OF_BOUNDS'

21.1.21.11 Instruction smodi
............................

Synopsys:

     smodi

   Given a struct and an index, push the modified flags of the field
occupying the position specified by the index in the given struct.  If
the struct doesn't have that many fields, raise PVM_E_OUT_OF_BOUNDS.

   Stack: '( SCT ULONG -- SCT ULONG BOOL )'

   Exceptions: 'PVM_E_OUT_OF_BOUNDS'

21.1.22 Offset Instructions
---------------------------

21.1.22.1 Instruction mko
.........................

Synopsys:

     mko

   Given an integral magnitude VAL and an unit expressed in an ULONG,
make an offset value and push it on the stack.

   Stack: '( VAL ULONG -- OFF )'

21.1.22.2 Instruction ogetm
...........................

Synopsys:

     ogetm

   Given an offset OFF, push its magnitude on the stack.

   Stack: '( OFF -- OFF VAL )'

21.1.22.3 Instruction osetm
...........................

Synopsys:

     osetm

   Given an offset OFF and an integral value VAL, make it the offset's
magnitude.

   Stack: '( OFF VAL -- OFF )'

21.1.22.4 Instruction ogetu
...........................

Synopsys:

     ogetu

   Given an offset OFF, push its unit on the stack.

   Stack: '( OFF -- OFF ULONG )'

21.1.22.5 Instruction ogetbt
............................

Synopsys:

     ogetbt

   Given an offset OFF, push its base type on the stack.

   Stack: '( OFF -- OFF TYP )'

21.1.23 Instructions to handle mapped values
--------------------------------------------

21.1.23.1 Instruction mm
........................

Synopsys:

     mm

   Given a value, push 1 on the stack if the value is mapped.  Push 0
otherwise.

   Stack: '( VAL -- VAL INT )'

21.1.23.2 Instruction map
.........................

Synopsys:

     map

   Given a value, mark it as as mapped.  If the value can't be mapped
then PVM_E_INVAL is raised.

   Stack: '( VAL -- VAL )'

   Exceptions: 'PVM_E_INVAL'

21.1.23.3 Instruction unmap
...........................

Synopsys:

     unmap

   Given a value, mark it as as not mapped.  If the value can't be
mapped then this is a no-operation.

   Stack: '( VAL -- VAL )'

21.1.23.4 Instruction reloc
...........................

Synopsys:

     reloc

   Given a value, a IO space expressed in an ulong, and a bit-offset
expressed in an ulong, relocate the value to the given bit-offset at the
given IO space.

   If the given value is not map-able then raise PVM_E_INVAL.

   Stack: '( VAL ULONG ULONG -- VAL ULONG ULONG )'

   Exceptions: 'PVM_E_INVAL'

21.1.23.5 Instruction ureloc
............................

Synopsys:

     ureloc

   Given a value, undo the last reloc performed on the value.

   If the given value is not map-able then raise PVM_E_INVAL.

   Stack: '( VAL -- VAL )'

   Exceptions: 'PVM_E_INVAL'

21.1.23.6 Instruction mgets
...........................

Synopsys:

     mgets

   Given a value, push a boolean indicating whether the value is strict.
If the given value is not map-able then push false, i.e.  0.

   Stack: '( VAL -- VAL INT )'

21.1.23.7 Instruction msets
...........................

Synopsys:

     msets

   Given a value and a boolean, set the strictness of the value to the
given boolean.  If the value is not map-able this is a no-operation.

   Stack: '( VAL INT -- VAL )'

21.1.23.8 Instruction mgeto
...........................

Synopsys:

     mgeto

   Given a map-able value, push its bit-offset on the stack as an
unsigned long.  If the given value is not map-able then push PVM_NULL.

   Stack: '( VAL -- VAL ULONG )'

21.1.23.9 Instruction mseto
...........................

Synopsys:

     mseto

   Given a map-able value an a bit-offset, set its offset to the value.
If the given value is not map-able, then the offset is ignored.

   Stack: '( VAL ULONG -- VAL )'

21.1.23.10 Instruction mgetios
..............................

Synopsys:

     mgetios

   Given a map-able value, push its associated IO space on the stack.
If the given value is not map-able, then push PVM_NULL.

   Stack: '( VAL -- VAL INT )'

21.1.23.11 Instruction msetios
..............................

Synopsys:

     msetios

   Given a map-able value and an IOS descriptor, set it as its
associated IO space.  If the IOS descriptor is PVM_NULL then it uses the
current IO space.  If the given value is not map-able then the IO space
is ignored.

   Stack: '( VAL INT -- VAL )'

21.1.23.12 Instruction mgetm
............................

Synopsys:

     mgetm

   Given a map-able value, push its mapper closure on the stack.  If the
given value is not map-able, then push PVM_NULL.

   Stack: '( VAL -- VAL CLS )'

21.1.23.13 Instruction msetm
............................

Synopsys:

     msetm

   Given a map-able value and a closure, set it as its mapper.  If the
given value is not map-able then the closure is ignored.

   Stack: '( VAL CLS -- VAL )'

21.1.23.14 Instruction mgetw
............................

Synopsys:

     mgetw

   Given a map-able value, push its writer closure on the stack.  If the
given value is not map-able, then push PVM_NULL.

   Stack: '( VAL -- VAL CLS )'

21.1.23.15 Instruction msetw
............................

Synopsys:

     msetw

   Given a map-able value and a closure, set it as its writer.  If the
given value is not map-able then the closure is ignored.

   Stack: '( VAL CLS -- VAL )'

21.1.23.16 Instruction mgetsel
..............................

Synopsys:

     mgetsel

   Given a map-able value in the TOS, push the number of elements to
which its mapping is bounded to.  If the value is not mapped, or if it
is not bounded by number of elements, push PVM_NULL.

   Note that only array values can have mappings bounded by number of
elements.

   Stack: '( VAL -- VAL ULONG )'

21.1.23.17 Instruction msetsel
..............................

Synopsys:

     msetsel

   Given a map-able value and an unsigned long, set it as the mapping
bound by number of elements.  If the value is not map-able the unsigned
long is ignored.

   Note that only array values can have mappings bounded by number of
elements.

   Stack: '( VAL ULONG -- VAL )'

21.1.23.18 Instruction mgetsiz
..............................

Synopsys:

     mgetsiz

   Given a map-able value in the TOS, push its mapping size-bound as a
bit-offset.  If the value is not map-able, or if it is not bounded by
size, push PVM_NULL.

   Note that only array values can have mappings bounded by size.

   Stack: '( VAL -- VAL ULONG )'

21.1.23.19 Instruction msetsiz
..............................

Synopsys:

     msetsiz

   Given a map-able value and a bit-offset, set it as the mapping
size-bound.  If the value is not map-able, the bit-offset is ignored.

   Note that only array values can have mappings bounded by size.

   Stack: '( VAL ULONG -- VAL )'

21.1.24 Type related instructions
---------------------------------

21.1.24.1 Instruction isa
.........................

Synopsys:

     isa

   Given a value and a type, push 1 on the stack if the value is of the
given type.  Push 0 otherwise.

   Stack: '( VAL TYPE -- TYPE VAL INT )'

21.1.24.2 Instruction typof
...........................

Synopsys:

     typof

   Given a value that is not itself a type, push its type on the stack.
Given a value that is a type, push the value on the stack.

   Stack: '( VAL -- VAL TYPE )'

21.1.24.3 Instruction tyisc
...........................

Synopsys:

     tyisc

   Given a value, push 1 on the stack if it is a closure.  Push 0
otherwise.

   Stack: '( VAL -- VAL INT )'

21.1.24.4 Instruction tyissct
.............................

Synopsys:

     tyissct

   Given a value, push 1 on the stack if it is a struct.  Push 0
otherwise.

   Stack: '( VAL -- VAL INT )'

21.1.24.5 Instruction mktyv
...........................

Synopsys:

     mktyv

   Build a "void" type and push it on the stack.

   Stack: '( -- TYPE )'

21.1.24.6 Instruction mktyany
.............................

Synopsys:

     mktyany

   Build an "any" type and push it on the stack.

   Stack: '( -- TYPE )'

21.1.24.7 Instruction mktyi
...........................

Synopsys:

     mktyi

   Given an unsigned long denoting a bit width, and an unsigned int
denoting signedness (0 is unsigned, 1 is signed), build a an integral
type with these features and push it on the stack.

   Stack: '( ULONG UINT -- TYPE )'

21.1.24.8 Instruction mktys
...........................

Synopsys:

     mktys

   Push a string type on the stack.

   Stack: '( -- TYPE )'

21.1.24.9 Instruction mktyo
...........................

Synopsys:

     mktyo

   Given a base integral type and an integer denoting an offset unit
(multiple of the base unit) construct an offset type having these
features, and push it on the stack.

   Stack: '( TYPE INT -- TYPE )' define F(res, a, b) \ undef F

21.1.24.10 Instruction mktya
............................

Synopsys:

     mktya

   Given an elements type and an unsigned long denoting a length, build
an array type having these features and push it on the stack.  If the
type array is unbounded then length is PVM_NULL.

   Stack: '( TYPE (ULONG|NULL) -- TYPE )'

21.1.24.11 Instruction tyagett
..............................

Synopsys:

     tyagett

   Given an array type, push the type of its elements on the stack.

   Stack: '( TYPE -- TYPE TYPE )'

21.1.24.12 Instruction tyagetb
..............................

Synopsys:

     tyagetb

   Given an array type, push its bound on the stack.

   Stack: '( TYPE -- TYPE (ULONG|NULL) )'

21.1.24.13 Instruction mktyc
............................

Synopsys:

     mktyc

   Given a list of argument types, a return type and a number of
arguments, build a closure type and push it on the stack.

   Stack: '( TYPE... TYPE ULONG -- TYPE )'

21.1.24.14 Instruction mktysct
..............................

Synopsys:

     mktysct

   Given a list of field descriptors, a number of fields and a struct
type name, build a struct type and push it on the stack.

   Each field descriptor has the form [STRING TYPE] and contains the
name of the field and its type.

   Stack: '( [STRING TYPE]... ULONG STR -- TYPE )'

21.1.24.15 Instruction tysctn
.............................

Synopsys:

     tysctn

   Given a struct type, push its name to the stack.  If the struct type
is not named push PVM_NULL.

   Stack: '( SCT -- SCT STR )'

21.1.25 IO instructions
-----------------------

21.1.25.1 Instruction write
...........................

Synopsys:

     write

   If the value at the TOS is mapped, then write it to its associated IO
space.  Otherwise, this is a no-op.

   Stack: '( VAL -- VAL )'

   Exceptions: 'PVM_E_IOS_FULL, PVM_E_CONSTRAINT_ERROR'

21.1.25.2 Instruction peeki
...........................

Synopsys:

     peeki NENC,ENDIAN,BITS

   Given an IOS descriptor and a bit-offset, peek an integer value of
width BITS bits.  The negative encoding and endianness to be used are
specified in the instruction arguments.

   Stack: '( INT ULONG -- INT )'

21.1.25.3 Instruction peekiu
............................

Synopsys:

     peekiu ENDIAN,BITS

   Given an IOS descriptor and a bit-offset, peek an unsigned integer
value of width BITS bits.  The endianness to be used is specified in the
instruction arguments.

   Stack: '( INT ULONG -- INT )'

21.1.25.4 Instruction peekl
...........................

Synopsys:

     peekl NENC,ENDIAN,BITS

   Given an IOS descriptor and a bit-offset, peek a long value of width
BITS bits.  The negative encoding and endianness to be used are
specified in the instruction arguments.

   Stack: '( INT ULONG -- LONG )'

21.1.25.5 Instruction peeklu
............................

Synopsys:

     peeklu ENDIAN,BITS

   Given an IOS descriptor and a bit-offset, peek an unsigned long value
of width BITS bits.  The endianness to be used is specified in the
instruction arguments.

   Stack: '( INT ULONG -- ULONG )'

21.1.25.6 Instruction peekdi
............................

Synopsys:

     peekdi BITS

   Given an IOS descriptor and a bit-offset, peek an integer value of
width BITS bits.  Use the default endianness and negative encoding.

   Stack: '( INT ULONG -- INT )'

21.1.25.7 Instruction peekdiu
.............................

Synopsys:

     peekdiu BITS

   Given an IOS descriptor and a bit-offset, peek an unsigned integer
value of width BITS bits.  Use the default endianness.

   Stack: '( INT ULONG -- UINT )'

21.1.25.8 Instruction peekdl
............................

Synopsys:

     peekdl BITS

   Given an IOS descriptor and a bit-offset, peek a long value of width
BITS bits.  Use the default endianness and negative encoding.

   Stack: '( INT ULONG -- LONG )'

21.1.25.9 Instruction peekdlu
.............................

Synopsys:

     peekdlu BITS

   Given an IOS descriptor and a bit-offset, peek an unsigned long value
of width BITS bits.  Use the default endianness.

   Stack: '( INT ULONG -- ULONG )'

21.1.25.10 Instruction pokei
............................

Synopsys:

     pokei NENC,ENDIAN,BITS

   Given an IOS descriptor, a bit-offset and an integer value of BITS
bits, poke it.  Use the negative encoding and endianness specified in
the instruction arguments.

   Stack: '( INT ULONG INT -- )'

21.1.25.11 Instruction pokeiu
.............................

Synopsys:

     pokeiu ENDIAN,BITS

   Given an IOS descriptor, a bit-offset and an unsigned integer value
of BITS bits, poke it.  Use the endianness specified in the instruction
arguments.

   Stack: '( INT ULONG INT -- )'

21.1.25.12 Instruction pokel
............................

Synopsys:

     pokel NENC,ENDIAN,BITS

   Given an IOS descriptor, a bit-offset and a long value of BITS bits,
poke it.  Use the negative encoding and endianness specified in the
instruction arguments.

   Stack: '( INT ULONG LONG -- )'

21.1.25.13 Instruction pokelu
.............................

Synopsys:

     pokelu ENDIAN,BITS

   Given an IOS descriptor, a bit-offset and an unsigned long value of
BITS bits, poke it.  Use the endianness specified in the instruction
arguments.

   Stack: '( INT ULONG ULONG -- )'

21.1.25.14 Instruction pokedi
.............................

Synopsys:

     pokedi BITS

   Given an IOS descriptor, a bit-offset and an integer of BITS bits,
poke it.  Use the default negative encoding and endianness.

   Stack: '( INT ULONG INT -- )'

21.1.25.15 Instruction pokediu
..............................

Synopsys:

     pokediu BITS

   Given an IOS descriptor, a bit-offset and an unsigned integer of BITS
bits, poke it.  Use the default endianness.

   Stack: '( INT ULONG UINT -- )'

21.1.25.16 Instruction pokedl
.............................

Synopsys:

     pokedl BITS

   Given an IOS descriptor, a bit-offset and a long of BITS bits, poke
it.  Use the default negative encoding and endianness.

   Stack: '( INT ULONG LONG -- )'

21.1.25.17 Instruction pokedlu
..............................

Synopsys:

     pokedlu BITS

   Given an IOS descriptor, a bit-offset and an unsigned long of BITS
bits, poke it.  Use the default endianness.

   Stack: '( INT ULONG ULONG -- )'

21.1.25.18 Instruction peeks
............................

Synopsys:

     peeks

   Given an IOS descriptor and a bit-offset, peek a string.

   Stack: '( INT ULONG -- STR )'

21.1.25.19 Instruction pokes
............................

Synopsys:

     pokes

   Given an IOS descriptor, a bit-offset and a string, poke it.

   Stack: '( INT ULONG STR -- )'

21.1.26 Exceptions handling instructions
----------------------------------------

21.1.26.1 Instruction pushe
...........................

Synopsys:

     pushe LABEL

   Given an Exception struct in the stack, push a handler for it on the
exceptions stack.  An exception code 0 means any exception.

   Stack: '( INT -- )'

   Exception Stack: '( -- EXCEPTION_HANDLER )'

21.1.26.2 Instruction pope
..........................

Synopsys:

     pope

   Pop an exception handler from the exceptions stack.

   Stack: '( -- )'

   Exception Stack: '( EXCEPTION_HANDLER -- )'

21.1.26.3 Instruction raise
...........................

Synopsys:

     raise

   Raise the given exception.

   Stack: '( EXCEPTION -- )'

   Exception Stack: '( -- )'

21.1.26.4 Instruction popexite
..............................

Synopsys:

     popexite

   Pops the exception on the stack and sets it in the VM.

   Stack: '( EXCEPTION -- )'

   Exception Stack: '( -- )'

21.1.27 Debugging Instructions
------------------------------

21.1.27.1 Instruction strace
............................

Synopsys:

     strace DEPTH

   Print a debugging trace with the elements of the top of the stack.
The number of elements to print is specified in DEPTH. A depth of zero
means to print the whole stack.

   Stack: '( -- )'

21.1.27.2 Instruction disas
...........................

Synopsys:

     disas

   Print out the disassembling of the program executed by the closure in
the top of the stack.

   Stack: '( CLS -- CLS )'

21.1.27.3 Instruction note
..........................

Synopsys:

     note VALUE

   This instruction is intended to be used to insert annotations that
help to understand disassemblies.  Most of the times VALUE is a string.

   Semantically, this instruction does nothing.

   Stack: '( -- )'

21.1.28 System Interaction Instructions
---------------------------------------

21.1.28.1 Instruction getenv
............................

Synopsys:

     getenv

   This instruction gets the name of an environment variable on the
stack and pushes the value of the corresponding environment variable.
If no variable with the given name is defined on the environment, then
push PVM_NULL.

   Stack: '( STR -- STR STR )'

21.1.29 Miscellaneous Instructions
----------------------------------

21.1.29.1 Instruction nop
.........................

Synopsys:

     nop

   Do nothing.

   Stack: '( -- )'

21.1.29.2 Instruction rand
..........................

Synopsys:

     rand

   Push a pseudo-random integer to the stack.

   If the argument is 0U then it is ignored.  Otherwise it is used to
set the seed for a new sequence of pseudo-random numbers.

   Stack: '( UINT -- INT )'

21.1.29.3 Instruction time
..........................

Synopsys:

     time

   Push the current system time to the stack in the form of an array of
two long elements containing the number of seconds and nanoseconds since
the epoch.

   Stack: '( -- ARR )'

21.1.29.4 Instruction sleep
...........................

Synopsys:

     sleep

   Sleep for a given number of seconds and nanoseconds.

   If the provided number of nanoseconds are not in the range 0 to
999999999 or the number of provided seconds is negative, raise
PVM_E_INVAL.

   If there is any other error performing the operation then raise
PVM_E_GENERIC.

   Stack: '( LONG LONG -- LONG LONG )'

   Exceptions: 'PVM_E_INVAL, PVM_E_GENERIC'

21.1.29.5 Instruction siz
.........................

Synopsys:

     siz

   Given a value, push its size as a bit-offset.

   Stack: '( VAL -- VAL ULONG )'

21.1.29.6 Instruction sel
.........................

Synopsys:

     sel

   Given a value, push its length as an unsigned long.

   The length of an array is the number of values contained in it.  The
lenght of a struct is the number of fields contained in it.  The lenght
of a string is the number of characters contained in it.  The length of
any other value is 1.

   Stack: '( VAL -- VAL ULONG )'

Appendix A Table of ASCII Codes
*******************************

Oct     Dec     Hex     Char
---------------------------------------------------------------------------
000     0       00      NUL '\0' (null character)
001     1       01      SOH (start of heading)
002     2       02      STX (start of text)
003     3       03      ETX (end of text)
004     4       04      EOT (end of transmission)
005     5       05      ENQ (enquiry)
006     6       06      ACK (acknowledge)
007     7       07      BEL '\a' (bell)
010     8       08      BS '\b' (backspace)
011     9       09      HT '\t' (horizontal tab)
012     10      0A      LF '\n' (new line)
013     11      0B      VT '\v' (vertical tab)
014     12      0C      FF '\f' (form feed)
015     13      0D      CR '\r' (carriage ret)
016     14      0E      SO (shift out)
017     15      0F      SI (shift in)
020     16      10      DLE (data link escape)
021     17      11      DC1 (device control 1)
022     18      12      DC2 (device control 2)
023     19      13      DC3 (device control 3)
024     20      14      DC4 (device control 4)
025     21      15      NAK (negative ack.)
026     22      16      SYN (synchronous idle)
027     23      17      ETB (end of trans.  blk)
030     24      18      CAN (cancel)
031     25      19      EM (end of medium)
032     26      1A      SUB (substitute)
033     27      1B      ESC (escape)
034     28      1C      FS (file separator)
035     29      1D      GS (group separator)
036     30      1E      RS (record separator)
037     31      1F      US (unit separator)
040     32      20      SPACE
041     33      21      !
042     34      22      "
043     35      23      #
044     36      24      $
045     37      25      %
046     38      26      &
047     39      27      '
050     40      28      (
051     41      29      )
052     42      2A      *
053     43      2B      +
054     44      2C      ,
055     45      2D      -
056     46      2E      .
057     47      2F      /
060     48      30      0
061     49      31      1
062     50      32      2
063     51      33      3
064     52      34      4
065     53      35      5
066     54      36      6
067     55      37      7
070     56      38      8
071     57      39      9
072     58      3A      :
073     59      3B      ;
074     60      3C      <
075     61      3D      =
076     62      3E      >
077     63      3F      ?
100     64      40      @
101     65      41      A
102     66      42      B
103     67      43      C
104     68      44      D
105     69      45      E
106     70      46      F
107     71      47      G
110     72      48      H
111     73      49      I
112     74      4A      J
113     75      4B      K
114     76      4C      L
115     77      4D      M
116     78      4E      N
117     79      4F      O
120     80      50      P
121     81      51      Q
122     82      52      R
123     83      53      S
124     84      54      T
125     85      55      U
126     86      56      V
127     87      57      W
130     88      58      X
131     89      59      Y
132     90      5A      Z
133     91      5B      [
134     92      5C      \ '\\'
135     93      5D      ]
136     94      5E      ^
137     95      5F      _
140     96      60      '
141     97      61      a
142     98      62      b
143     99      63      c
144     100     64      d
145     101     65      e
146     102     66      f
147     103     67      g
150     104     68      h
151     105     69      i
152     106     6A      j
153     107     6B      k
154     108     6C      l
155     109     6D      m
156     110     6E      n
157     111     6F      o
160     112     70      p
161     113     71      q
162     114     72      r
163     115     73      s
164     116     74      t
165     117     75      u
166     118     76      v
167     119     77      w
170     120     78      x
171     121     79      y
172     122     7A      z
173     123     7B      {
174     124     7C      |
175     125     7D      }
176     126     7E      ~
177     127     7F      DEL

Appendix B GNU Free Documentation License
*****************************************

                     Version 1.3, 3 November 2008

     Copyright (C) 2000, 2001, 2002, 2007, 2008, 2021 Free Software
     Foundation, Inc.
     <http://fsf.org/>

     Everyone is permitted to copy and distribute verbatim copies
     of this license document, but changing it is not allowed.

  0. PREAMBLE

     The purpose of this License is to make a manual, textbook, or other
     functional and useful document "free" in the sense of freedom: to
     assure everyone the effective freedom to copy and redistribute it,
     with or without modifying it, either commercially or
     noncommercially.  Secondarily, this License preserves for the
     author and publisher a way to get credit for their work, while not
     being considered responsible for modifications made by others.

     This License is a kind of "copyleft", which means that derivative
     works of the document must themselves be free in the same sense.
     It complements the GNU General Public License, which is a copyleft
     license designed for free software.

     We have designed this License in order to use it for manuals for
     free software, because free software needs free documentation: a
     free program should come with manuals providing the same freedoms
     that the software does.  But this License is not limited to
     software manuals; it can be used for any textual work, regardless
     of subject matter or whether it is published as a printed book.  We
     recommend this License principally for works whose purpose is
     instruction or reference.

  1. APPLICABILITY AND DEFINITIONS

     This License applies to any manual or other work, in any medium,
     that contains a notice placed by the copyright holder saying it can
     be distributed under the terms of this License.  Such a notice
     grants a world-wide, royalty-free license, unlimited in duration,
     to use that work under the conditions stated herein.  The
     "Document", below, refers to any such manual or work.  Any member
     of the public is a licensee, and is addressed as "you".  You accept
     the license if you copy, modify or distribute the work in a way
     requiring permission under copyright law.

     A "Modified Version" of the Document means any work containing the
     Document or a portion of it, either copied verbatim, or with
     modifications and/or translated into another language.

     A "Secondary Section" is a named appendix or a front-matter section
     of the Document that deals exclusively with the relationship of the
     publishers or authors of the Document to the Document's overall
     subject (or to related matters) and contains nothing that could
     fall directly within that overall subject.  (Thus, if the Document
     is in part a textbook of mathematics, a Secondary Section may not
     explain any mathematics.)  The relationship could be a matter of
     historical connection with the subject or with related matters, or
     of legal, commercial, philosophical, ethical or political position
     regarding them.

     The "Invariant Sections" are certain Secondary Sections whose
     titles are designated, as being those of Invariant Sections, in the
     notice that says that the Document is released under this License.
     If a section does not fit the above definition of Secondary then it
     is not allowed to be designated as Invariant.  The Document may
     contain zero Invariant Sections.  If the Document does not identify
     any Invariant Sections then there are none.

     The "Cover Texts" are certain short passages of text that are
     listed, as Front-Cover Texts or Back-Cover Texts, in the notice
     that says that the Document is released under this License.  A
     Front-Cover Text may be at most 5 words, and a Back-Cover Text may
     be at most 25 words.

     A "Transparent" copy of the Document means a machine-readable copy,
     represented in a format whose specification is available to the
     general public, that is suitable for revising the document
     straightforwardly with generic text editors or (for images composed
     of pixels) generic paint programs or (for drawings) some widely
     available drawing editor, and that is suitable for input to text
     formatters or for automatic translation to a variety of formats
     suitable for input to text formatters.  A copy made in an otherwise
     Transparent file format whose markup, or absence of markup, has
     been arranged to thwart or discourage subsequent modification by
     readers is not Transparent.  An image format is not Transparent if
     used for any substantial amount of text.  A copy that is not
     "Transparent" is called "Opaque".

     Examples of suitable formats for Transparent copies include plain
     ASCII without markup, Texinfo input format, LaTeX input format,
     SGML or XML using a publicly available DTD, and standard-conforming
     simple HTML, PostScript or PDF designed for human modification.
     Examples of transparent image formats include PNG, XCF and JPG.
     Opaque formats include proprietary formats that can be read and
     edited only by proprietary word processors, SGML or XML for which
     the DTD and/or processing tools are not generally available, and
     the machine-generated HTML, PostScript or PDF produced by some word
     processors for output purposes only.

     The "Title Page" means, for a printed book, the title page itself,
     plus such following pages as are needed to hold, legibly, the
     material this License requires to appear in the title page.  For
     works in formats which do not have any title page as such, "Title
     Page" means the text near the most prominent appearance of the
     work's title, preceding the beginning of the body of the text.

     The "publisher" means any person or entity that distributes copies
     of the Document to the public.

     A section "Entitled XYZ" means a named subunit of the Document
     whose title either is precisely XYZ or contains XYZ in parentheses
     following text that translates XYZ in another language.  (Here XYZ
     stands for a specific section name mentioned below, such as
     "Acknowledgements", "Dedications", "Endorsements", or "History".)
     To "Preserve the Title" of such a section when you modify the
     Document means that it remains a section "Entitled XYZ" according
     to this definition.

     The Document may include Warranty Disclaimers next to the notice
     which states that this License applies to the Document.  These
     Warranty Disclaimers are considered to be included by reference in
     this License, but only as regards disclaiming warranties: any other
     implication that these Warranty Disclaimers may have is void and
     has no effect on the meaning of this License.

  2. VERBATIM COPYING

     You may copy and distribute the Document in any medium, either
     commercially or noncommercially, provided that this License, the
     copyright notices, and the license notice saying this License
     applies to the Document are reproduced in all copies, and that you
     add no other conditions whatsoever to those of this License.  You
     may not use technical measures to obstruct or control the reading
     or further copying of the copies you make or distribute.  However,
     you may accept compensation in exchange for copies.  If you
     distribute a large enough number of copies you must also follow the
     conditions in section 3.

     You may also lend copies, under the same conditions stated above,
     and you may publicly display copies.

  3. COPYING IN QUANTITY

     If you publish printed copies (or copies in media that commonly
     have printed covers) of the Document, numbering more than 100, and
     the Document's license notice requires Cover Texts, you must
     enclose the copies in covers that carry, clearly and legibly, all
     these Cover Texts: Front-Cover Texts on the front cover, and
     Back-Cover Texts on the back cover.  Both covers must also clearly
     and legibly identify you as the publisher of these copies.  The
     front cover must present the full title with all words of the title
     equally prominent and visible.  You may add other material on the
     covers in addition.  Copying with changes limited to the covers, as
     long as they preserve the title of the Document and satisfy these
     conditions, can be treated as verbatim copying in other respects.

     If the required texts for either cover are too voluminous to fit
     legibly, you should put the first ones listed (as many as fit
     reasonably) on the actual cover, and continue the rest onto
     adjacent pages.

     If you publish or distribute Opaque copies of the Document
     numbering more than 100, you must either include a machine-readable
     Transparent copy along with each Opaque copy, or state in or with
     each Opaque copy a computer-network location from which the general
     network-using public has access to download using public-standard
     network protocols a complete Transparent copy of the Document, free
     of added material.  If you use the latter option, you must take
     reasonably prudent steps, when you begin distribution of Opaque
     copies in quantity, to ensure that this Transparent copy will
     remain thus accessible at the stated location until at least one
     year after the last time you distribute an Opaque copy (directly or
     through your agents or retailers) of that edition to the public.

     It is requested, but not required, that you contact the authors of
     the Document well before redistributing any large number of copies,
     to give them a chance to provide you with an updated version of the
     Document.

  4. MODIFICATIONS

     You may copy and distribute a Modified Version of the Document
     under the conditions of sections 2 and 3 above, provided that you
     release the Modified Version under precisely this License, with the
     Modified Version filling the role of the Document, thus licensing
     distribution and modification of the Modified Version to whoever
     possesses a copy of it.  In addition, you must do these things in
     the Modified Version:

       A. Use in the Title Page (and on the covers, if any) a title
          distinct from that of the Document, and from those of previous
          versions (which should, if there were any, be listed in the
          History section of the Document).  You may use the same title
          as a previous version if the original publisher of that
          version gives permission.

       B. List on the Title Page, as authors, one or more persons or
          entities responsible for authorship of the modifications in
          the Modified Version, together with at least five of the
          principal authors of the Document (all of its principal
          authors, if it has fewer than five), unless they release you
          from this requirement.

       C. State on the Title page the name of the publisher of the
          Modified Version, as the publisher.

       D. Preserve all the copyright notices of the Document.

       E. Add an appropriate copyright notice for your modifications
          adjacent to the other copyright notices.

       F. Include, immediately after the copyright notices, a license
          notice giving the public permission to use the Modified
          Version under the terms of this License, in the form shown in
          the Addendum below.

       G. Preserve in that license notice the full lists of Invariant
          Sections and required Cover Texts given in the Document's
          license notice.

       H. Include an unaltered copy of this License.

       I. Preserve the section Entitled "History", Preserve its Title,
          and add to it an item stating at least the title, year, new
          authors, and publisher of the Modified Version as given on the
          Title Page.  If there is no section Entitled "History" in the
          Document, create one stating the title, year, authors, and
          publisher of the Document as given on its Title Page, then add
          an item describing the Modified Version as stated in the
          previous sentence.

       J. Preserve the network location, if any, given in the Document
          for public access to a Transparent copy of the Document, and
          likewise the network locations given in the Document for
          previous versions it was based on.  These may be placed in the
          "History" section.  You may omit a network location for a work
          that was published at least four years before the Document
          itself, or if the original publisher of the version it refers
          to gives permission.

       K. For any section Entitled "Acknowledgements" or "Dedications",
          Preserve the Title of the section, and preserve in the section
          all the substance and tone of each of the contributor
          acknowledgements and/or dedications given therein.

       L. Preserve all the Invariant Sections of the Document, unaltered
          in their text and in their titles.  Section numbers or the
          equivalent are not considered part of the section titles.

       M. Delete any section Entitled "Endorsements".  Such a section
          may not be included in the Modified Version.

       N. Do not retitle any existing section to be Entitled
          "Endorsements" or to conflict in title with any Invariant
          Section.

       O. Preserve any Warranty Disclaimers.

     If the Modified Version includes new front-matter sections or
     appendices that qualify as Secondary Sections and contain no
     material copied from the Document, you may at your option designate
     some or all of these sections as invariant.  To do this, add their
     titles to the list of Invariant Sections in the Modified Version's
     license notice.  These titles must be distinct from any other
     section titles.

     You may add a section Entitled "Endorsements", provided it contains
     nothing but endorsements of your Modified Version by various
     parties--for example, statements of peer review or that the text
     has been approved by an organization as the authoritative
     definition of a standard.

     You may add a passage of up to five words as a Front-Cover Text,
     and a passage of up to 25 words as a Back-Cover Text, to the end of
     the list of Cover Texts in the Modified Version.  Only one passage
     of Front-Cover Text and one of Back-Cover Text may be added by (or
     through arrangements made by) any one entity.  If the Document
     already includes a cover text for the same cover, previously added
     by you or by arrangement made by the same entity you are acting on
     behalf of, you may not add another; but you may replace the old
     one, on explicit permission from the previous publisher that added
     the old one.

     The author(s) and publisher(s) of the Document do not by this
     License give permission to use their names for publicity for or to
     assert or imply endorsement of any Modified Version.

  5. COMBINING DOCUMENTS

     You may combine the Document with other documents released under
     this License, under the terms defined in section 4 above for
     modified versions, provided that you include in the combination all
     of the Invariant Sections of all of the original documents,
     unmodified, and list them all as Invariant Sections of your
     combined work in its license notice, and that you preserve all
     their Warranty Disclaimers.

     The combined work need only contain one copy of this License, and
     multiple identical Invariant Sections may be replaced with a single
     copy.  If there are multiple Invariant Sections with the same name
     but different contents, make the title of each such section unique
     by adding at the end of it, in parentheses, the name of the
     original author or publisher of that section if known, or else a
     unique number.  Make the same adjustment to the section titles in
     the list of Invariant Sections in the license notice of the
     combined work.

     In the combination, you must combine any sections Entitled
     "History" in the various original documents, forming one section
     Entitled "History"; likewise combine any sections Entitled
     "Acknowledgements", and any sections Entitled "Dedications".  You
     must delete all sections Entitled "Endorsements."

  6. COLLECTIONS OF DOCUMENTS

     You may make a collection consisting of the Document and other
     documents released under this License, and replace the individual
     copies of this License in the various documents with a single copy
     that is included in the collection, provided that you follow the
     rules of this License for verbatim copying of each of the documents
     in all other respects.

     You may extract a single document from such a collection, and
     distribute it individually under this License, provided you insert
     a copy of this License into the extracted document, and follow this
     License in all other respects regarding verbatim copying of that
     document.

  7. AGGREGATION WITH INDEPENDENT WORKS

     A compilation of the Document or its derivatives with other
     separate and independent documents or works, in or on a volume of a
     storage or distribution medium, is called an "aggregate" if the
     copyright resulting from the compilation is not used to limit the
     legal rights of the compilation's users beyond what the individual
     works permit.  When the Document is included in an aggregate, this
     License does not apply to the other works in the aggregate which
     are not themselves derivative works of the Document.

     If the Cover Text requirement of section 3 is applicable to these
     copies of the Document, then if the Document is less than one half
     of the entire aggregate, the Document's Cover Texts may be placed
     on covers that bracket the Document within the aggregate, or the
     electronic equivalent of covers if the Document is in electronic
     form.  Otherwise they must appear on printed covers that bracket
     the whole aggregate.

  8. TRANSLATION

     Translation is considered a kind of modification, so you may
     distribute translations of the Document under the terms of section
     4.  Replacing Invariant Sections with translations requires special
     permission from their copyright holders, but you may include
     translations of some or all Invariant Sections in addition to the
     original versions of these Invariant Sections.  You may include a
     translation of this License, and all the license notices in the
     Document, and any Warranty Disclaimers, provided that you also
     include the original English version of this License and the
     original versions of those notices and disclaimers.  In case of a
     disagreement between the translation and the original version of
     this License or a notice or disclaimer, the original version will
     prevail.

     If a section in the Document is Entitled "Acknowledgements",
     "Dedications", or "History", the requirement (section 4) to
     Preserve its Title (section 1) will typically require changing the
     actual title.

  9. TERMINATION

     You may not copy, modify, sublicense, or distribute the Document
     except as expressly provided under this License.  Any attempt
     otherwise to copy, modify, sublicense, or distribute it is void,
     and will automatically terminate your rights under this License.

     However, if you cease all violation of this License, then your
     license from a particular copyright holder is reinstated (a)
     provisionally, unless and until the copyright holder explicitly and
     finally terminates your license, and (b) permanently, if the
     copyright holder fails to notify you of the violation by some
     reasonable means prior to 60 days after the cessation.

     Moreover, your license from a particular copyright holder is
     reinstated permanently if the copyright holder notifies you of the
     violation by some reasonable means, this is the first time you have
     received notice of violation of this License (for any work) from
     that copyright holder, and you cure the violation prior to 30 days
     after your receipt of the notice.

     Termination of your rights under this section does not terminate
     the licenses of parties who have received copies or rights from you
     under this License.  If your rights have been terminated and not
     permanently reinstated, receipt of a copy of some or all of the
     same material does not give you any rights to use it.

  10. FUTURE REVISIONS OF THIS LICENSE

     The Free Software Foundation may publish new, revised versions of
     the GNU Free Documentation License from time to time.  Such new
     versions will be similar in spirit to the present version, but may
     differ in detail to address new problems or concerns.  See
     <http://www.gnu.org/copyleft/>.

     Each version of the License is given a distinguishing version
     number.  If the Document specifies that a particular numbered
     version of this License "or any later version" applies to it, you
     have the option of following the terms and conditions either of
     that specified version or of any later version that has been
     published (not as a draft) by the Free Software Foundation.  If the
     Document does not specify a version number of this License, you may
     choose any version ever published (not as a draft) by the Free
     Software Foundation.  If the Document specifies that a proxy can
     decide which future versions of this License can be used, that
     proxy's public statement of acceptance of a version permanently
     authorizes you to choose that version for the Document.

  11. RELICENSING

     "Massive Multiauthor Collaboration Site" (or "MMC Site") means any
     World Wide Web server that publishes copyrightable works and also
     provides prominent facilities for anybody to edit those works.  A
     public wiki that anybody can edit is an example of such a server.
     A "Massive Multiauthor Collaboration" (or "MMC") contained in the
     site means any set of copyrightable works thus published on the MMC
     site.

     "CC-BY-SA" means the Creative Commons Attribution-Share Alike 3.0
     license published by Creative Commons Corporation, a not-for-profit
     corporation with a principal place of business in San Francisco,
     California, as well as future copyleft versions of that license
     published by that same organization.

     "Incorporate" means to publish or republish a Document, in whole or
     in part, as part of another Document.

     An MMC is "eligible for relicensing" if it is licensed under this
     License, and if all works that were first published under this
     License somewhere other than this MMC, and subsequently
     incorporated in whole or in part into the MMC, (1) had no cover
     texts or invariant sections, and (2) were thus incorporated prior
     to November 1, 2008.

     The operator of an MMC Site may republish an MMC contained in the
     site under CC-BY-SA on the same site at any time before August 1,
     2009, provided the MMC is eligible for relicensing.

ADDENDUM: How to use this License for your documents
====================================================

To use this License in a document you have written, include a copy of
the License in the document and put the following copyright and license
notices just after the title page:

       Copyright (C)  YEAR  YOUR NAME.
       Permission is granted to copy, distribute and/or modify this document
       under the terms of the GNU Free Documentation License, Version 1.3
       or any later version published by the Free Software Foundation;
       with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
       Texts.  A copy of the license is included in the section entitled ``GNU
       Free Documentation License''.

   If you have Invariant Sections, Front-Cover Texts and Back-Cover
Texts, replace the "with...Texts."  line with this:

         with the Invariant Sections being LIST THEIR TITLES, with
         the Front-Cover Texts being LIST, and with the Back-Cover Texts
         being LIST.

   If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the
situation.

   If your document contains nontrivial examples of program code, we
recommend releasing these examples in parallel under your choice of free
software license, such as the GNU General Public License, to permit
their use in free software.

Concept Index
*************

* Menu:

* .close:                                close command.     (line  7563)
* .doc:                                  doc command.       (line  7575)
* .editor:                               editor command.    (line  7601)
* .exit:                                 exit command.      (line  7846)
* .file:                                 file command.      (line  7458)
* .info:                                 info command.      (line  7613)
* .ios:                                  ios command.       (line  7553)
* .load:                                 Pickles.           (line  4131)
* .load <1>:                             load command.      (line  7431)
* .mem:                                  mem command.       (line  7489)
* .nbd:                                  nbd command.       (line  7504)
* .pokerc:                               Invoking poke.     (line   954)
* .pokerc <1>:                           Dumping File Contents.
                                                            (line  1579)
* .pokerc <2>:                           pokerc.            (line  7023)
* .proc:                                 proc command.      (line  7531)
* .quit:                                 quit command.      (line  7856)
* .set:                                  set command.       (line  7659)
* .source:                               source command.    (line  7447)
* .sub:                                  sub command.       (line  7542)
* .vm:                                   vm command.        (line  7805)
* ::                                     Conditional Expression.
                                                            (line 10393)
* ?:                                     Conditional Expression.
                                                            (line 10393)
* @:                                     The Map Operator.  (line 11062)
* ^L:                                    Vertical separator.
                                                            (line 11862)
* __FILE__:                              __LINE__ and __FILE__.
                                                            (line 12099)
* __LINE__:                              __LINE__ and __FILE__.
                                                            (line 12099)
* addition:                              Offset Operations. (line  8580)
* alignment:                             Padding and Alignment.
                                                            (line  4917)
* AND:                                   Bitwise Operators. (line  8347)
* any, the any type:                     type.              (line 10281)
* any, the any type <1>:                 The any Type.      (line 10286)
* arguments:                             Commands and Dot-Commands.
                                                            (line  1103)
* arguments <1>:                         Optional Arguments.
                                                            (line 10540)
* arithmetic:                            Arithmetic Operators.
                                                            (line  8316)
* array functions:                       Array Functions.   (line 12292)
* arrays:                                Arrays.            (line  8847)
* arrays, trimming:                      Modifying SBM Images.
                                                            (line  3912)
* ASCII:                                 Character Sets.    (line  2790)
* assert:                                assert.            (line 11637)
* assignment:                            Assignments.       (line 10324)
* atoi:                                  atoi.              (line 12261)
* attributes, array attributes:          Array Attributes.  (line  9229)
* attributes, function attributes:       Function Attributes.
                                                            (line 10676)
* attributes, integer attributes:        Integer Attributes.
                                                            (line  8390)
* attributes, offset:                    Offset Attributes. (line  8664)
* attributes, string attributes:         String Attributes. (line  8803)
* attributes, struct attributes:         Struct Attributes. (line 10219)
* base, argument in atoi:                atoi.              (line 12268)
* big endian:                            Big and Little Endians.
                                                            (line  1936)
* binary:                                Integer Literals.  (line  8123)
* binary <1>:                            printf.            (line 11772)
* binary files:                          Binary Files.      (line  1316)
* bitsize:                               Integer Literals.  (line  8144)
* bitwise operators:                     Bitwise Operators. (line  8343)
* boolean operators:                     Boolean Operators. (line  8375)
* boolean values:                        Booleans.          (line  8210)
* break:                                 while.             (line 10418)
* break <1>:                             for-in.            (line 10469)
* break <2>:                             try-until.         (line 11568)
* calling, function calls:               Calling Functions. (line 10571)
* casting, integration, integrate, arrays: Casting Arrays.  (line  9019)
* casts:                                 Negative Integers. (line  2140)
* casts <1>:                             Casting Integers.  (line  8257)
* casts <2>:                             Casting Offsets.   (line  8556)
* catos:                                 catos.             (line 12226)
* character:                             Character Sets.    (line  2784)
* character set:                         Character Sets.    (line  2784)
* character, locating in a string:       strchr.            (line 12333)
* characters:                            Characters.        (line  8181)
* checksum:                              CRC Functions.     (line 12366)
* close:                                 close.             (line 10971)
* closures:                              Array Types.       (line  8916)
* coercions:                             Poking a SBM Image.
                                                            (line  3281)
* command files:                         Command Files.     (line  1133)
* commands:                              Commands and Dot-Commands.
                                                            (line  1086)
* commands, customizing:                 Dumping File Contents.
                                                            (line  1561)
* commands, passing arguments:           Dumping File Contents.
                                                            (line  1522)
* comments:                              Comments.          (line 11843)
* comparing, arrays:                     Array Comparison.  (line  9109)
* comparing, structs:                    Struct Comparison. (line  9378)
* comparison:                            Relational Operators.
                                                            (line  8300)
* complement:                            Bitwise Operators. (line  8370)
* compound statements:                   Compound Statements.
                                                            (line 10353)
* concatenation, bitwise:                Bitwise Operators. (line  8350)
* concatenation, strings:                String Concatenation.
                                                            (line  8760)
* conditional expressions:               Conditional Expression.
                                                            (line 10393)
* conditional statements:                if-else.           (line 10381)
* constraints:                           Field Constraints. (line  9471)
* constructing, arrays:                  Array Constructors.
                                                            (line  9074)
* continue:                              while.             (line 10428)
* continue <1>:                          for-in.            (line 10472)
* conversion functions:                  Conversion Functions.
                                                            (line 12220)
* conversions:                           Poking a SBM Image.
                                                            (line  3281)
* converting, arrays to strings:         catos.             (line 12226)
* converting, strings to arrays:         stoca.             (line 12248)
* converting, strings to integers:       atoi.              (line 12261)
* copy:                                  copy.              (line  7983)
* CRC:                                   CRC Functions.     (line 12366)
* date:                                  Dates and Times.   (line 12382)
* decimal:                               Integer Literals.  (line  8110)
* decimal <1>:                           printf.            (line 11766)
* declarations, function declarations:   Function Declarations.
                                                            (line 10496)
* deflate:                               Offsets and Sizes. (line  2557)
* digits separator:                      Integer Literals.  (line  8134)
* disassembler:                          .vm disassemble.   (line  7812)
* division:                              Offset Operations. (line  8615)
* doc:                                   doc command.       (line  7575)
* dot-commands:                          Commands and Dot-Commands.
                                                            (line  1059)
* dump:                                  dump.              (line  7864)
* dump <1>:                              Calling Functions. (line 10600)
* eBPF:                                  Endian built-ins.  (line 10748)
* editor:                                editor command.    (line  7601)
* ELF:                                   Offset Units.      (line  8510)
* ELF <1>:                               Field Constraints. (line  9481)
* ELF <2>:                               Endian built-ins.  (line 10821)
* end of file:                           Array maps bounded by number of elements.
                                                            (line 11197)
* end of file <1>:                       Exceptions.        (line 11468)
* end of file <2>:                       try-until.         (line 11557)
* endianness:                            Big and Little Endians.
                                                            (line  1936)
* endianness <1>:                        Endianness.        (line 10684)
* endianness <2>:                        set endian.        (line 10693)
* errors:                                Evaluation.        (line  1040)
* escape sequence:                       From Bytes to Characters.
                                                            (line  2905)
* escape sequence <1>:                   Characters.        (line  8185)
* exceptions:                            String Indexing.   (line  8749)
* exceptions <1>:                        Array Indexing.    (line  9133)
* exceptions <2>:                        Exception Handling.
                                                            (line 11423)
* exclusive OR:                          Bitwise Operators. (line  8348)
* executing command files:               source command.    (line  7447)
* exit:                                  exit command.      (line  7846)
* expressions:                           Evaluation.        (line  1022)
* extract:                               extract.           (line  8042)
* fields, anonymous:                     Struct Types.      (line  9287)
* file name:                             __LINE__ and __FILE__.
                                                            (line 12107)
* flags:                                 Commands and Dot-Commands.
                                                            (line  1071)
* flow control:                          Loops.             (line 10404)
* flush:                                 flush.             (line 10988)
* for:                                   for.               (line 10441)
* for-in:                                for-in.            (line 10452)
* form feed:                             Vertical separator.
                                                            (line 11862)
* formatted output:                      printf.            (line 11746)
* fun:                                   Function Declarations.
                                                            (line 10496)
* function types:                        Function Types.    (line 10616)
* functions:                             Functions.         (line 10493)
* gettimeofday:                          Dates and Times.   (line 12382)
* get_ios:                               get_ios.           (line 11005)
* gibibits:                              Standard Units.    (line 12213)
* gibibytes:                             Standard Units.    (line 12215)
* gigabits:                              Standard Units.    (line 12198)
* gigabytes:                             Standard Units.    (line 12200)
* global settings:                       set command.       (line  7659)
* hexadecimal:                           Integer Literals.  (line  8117)
* hexadecimal <1>:                       printf.            (line 11768)
* history, session history:              The REPL.          (line   997)
* inclusive OR:                          Bitwise Operators. (line  8349)
* indexing, into arrays:                 Array Indexing.    (line  9122)
* indexing, into strings:                String Indexing.   (line  8738)
* initialization file:                   Dumping File Contents.
                                                            (line  1579)
* integer literals:                      Integer Literals.  (line  8108)
* integer literals <1>:                  Integer Literals.  (line  8144)
* integers:                              Integers.          (line  8096)
* integral structs:                      Integral Structs.  (line  9721)
* integral types:                        Standard Integral Types.
                                                            (line 12131)
* invoking:                              Invoking poke.     (line   897)
* IO devices:                            Unaligned Integers.
                                                            (line  2404)
* IO space:                              file command.      (line  7458)
* IO space <1>:                          mem command.       (line  7489)
* IO space <2>:                          nbd command.       (line  7504)
* IO space <3>:                          close command.     (line  7563)
* IO space <4>:                          info command.      (line  7624)
* IO space <5>:                          IO Spaces.         (line 10872)
* IO space <6>:                          open.              (line 10881)
* IO spaces:                             Unaligned Integers.
                                                            (line  2404)
* ioflags:                               ioflags.           (line 11044)
* iosize:                                iosize.            (line 11033)
* isa operator:                          The isa Operator.  (line 10309)
* kibibits:                              Standard Units.    (line 12205)
* kibibytes:                             Standard Units.    (line 12207)
* kilobits:                              Offset Types.      (line  8546)
* kilobits <1>:                          Offset Operations. (line  8626)
* kilobits <2>:                          Standard Units.    (line 12190)
* kilobytes:                             Offset Literals.   (line  8430)
* kilobytes <1>:                         Standard Units.    (line 12192)
* Latin-1:                               Character Sets.    (line  2790)
* license, GNU Free Documentation License: GNU Free Documentation License.
                                                            (line 16814)
* line number:                           __LINE__ and __FILE__.
                                                            (line 12104)
* little endian:                         Big and Little Endians.
                                                            (line  1936)
* load:                                  Modules.           (line 11869)
* load path:                             Load Path.         (line  7057)
* loading files:                         Pickles.           (line  4131)
* location:                              __LINE__ and __FILE__.
                                                            (line 12099)
* loops:                                 Loops.             (line 10404)
* ltos:                                  ltos.              (line 12280)
* ltrim:                                 ltrim.             (line 12313)
* magnitude:                             Offset Literals.   (line  8430)
* magnitude <1>:                         Offset Units.      (line  8530)
* magnitude <2>:                         Casting Offsets.   (line  8565)
* mapped values:                         Modifying SBM Images.
                                                            (line  3667)
* mapping:                               Mapping.           (line 10865)
* mapping <1>:                           The Map Operator.  (line 11055)
* matrices:                              Array Literals.    (line  8875)
* mebibits:                              Standard Units.    (line 12209)
* mebibytes:                             Standard Units.    (line 12211)
* megabits:                              Standard Units.    (line 12194)
* megabytes:                             Standard Units.    (line 12196)
* minus:                                 Arithmetic Operators.
                                                            (line  8335)
* modules:                               Modules.           (line 11869)
* modulus:                               Offset Operations. (line  8650)
* MP3:                                   MP3.               (line  7195)
* multiplication:                        Offset Operations. (line  8598)
* negation:                              Boolean Operators. (line  8385)
* negative encoding:                     Negative Integers. (line  2074)
* octal:                                 Integer Literals.  (line  8113)
* octal <1>:                             printf.            (line 11770)
* offset:                                Offsets.           (line  8416)
* OFFSET:                                The OFFSET variable.
                                                            (line  9694)
* offset algebra:                        Offset Operations. (line  8575)
* offset types:                          Standard Offset Types.
                                                            (line 12166)
* one complement:                        Negative Integers. (line  2074)
* opening files:                         file command.      (line  7458)
* opening files <1>:                     open.              (line 10881)
* opening memory buffers:                mem command.       (line  7489)
* opening NBD buffers:                   nbd command.       (line  7504)
* openproc:                              openproc.          (line 10959)
* opensub:                               opensub.           (line 10934)
* OR:                                    Bitwise Operators. (line  8349)
* output:                                print.             (line 11732)
* padding:                               Padding and Alignment.
                                                            (line  4917)
* pickle:                                Nomenclature.      (line   867)
* pinned structs:                        Pinned Structs.    (line  9671)
* poke:                                  Nomenclature.      (line   867)
* Poke:                                  Nomenclature.      (line   867)
* polymorphism:                          The any Type.      (line 10286)
* POSIX_Time32:                          Dates and Times.   (line 12382)
* print:                                 print.             (line 11732)
* printf:                                printf.            (line 11746)
* profiler:                              .vm profile.       (line  7834)
* ptime:                                 Dates and Times.   (line 12382)
* ptime <1>:                             Dates and Times.   (line 12410)
* qsort:                                 qsort.             (line 12347)
* quick sort:                            qsort.             (line 12347)
* quit:                                  quit command.      (line  7856)
* quitting:                              exit command.      (line  7846)
* quitting <1>:                          quit command.      (line  7856)
* rabbit herd:                           Offsets and Sizes. (line  2628)
* raise:                                 raise.             (line 11574)
* readline:                              The REPL.          (line   992)
* REPL:                                  The REPL.          (line   981)
* reverse:                               reverse.           (line 12298)
* rtrim:                                 rtrim.             (line 12323)
* save:                                  save.              (line  8008)
* scrabble:                              scrabble.          (line  8056)
* scripts:                               Scripts.           (line  1171)
* separator, digits separator:           Integer Literals.  (line  8134)
* set_ios:                               set_ios.           (line 11018)
* shebang, #!:                           Scripts.           (line  1171)
* shifting:                              Bitwise Operators. (line  8346)
* side effects:                          Expression Statements.
                                                            (line 10478)
* sign promotion:                        Arithmetic Operators.
                                                            (line  8323)
* signedness:                            Integer Literals.  (line  8162)
* signedness <1>:                        Casting Integers.  (line  8276)
* signedness <2>:                        Arithmetic Operators.
                                                            (line  8323)
* signedness <3>:                        Integer Attributes.
                                                            (line  8404)
* size of variables:                     Integer Literals.  (line  8144)
* sorting:                               Sorting Functions. (line 12344)
* statements:                            Evaluation.        (line  1029)
* statements, compound statements:       Compound Statements.
                                                            (line 10353)
* stoca:                                 stoca.             (line 12248)
* strchr:                                strchr.            (line 12333)
* string functions:                      String Functions.  (line 12307)
* strings:                               Strings.           (line  8694)
* strings, formatting:                   String Formatting. (line  8828)
* struct fields:                         Optional Fields.   (line 10066)
* structs:                               Structs.           (line  9265)
* styled output:                         Invoking poke.     (line   930)
* styled output <1>:                     Styling.           (line  7084)
* styled output <2>:                     printf.            (line 11811)
* subtraction:                           Offset Operations. (line  8580)
* tags, file ID tags:                    file command.      (line  7463)
* tags, file ID tags <1>:                mem command.       (line  7494)
* tags, file ID tags <2>:                nbd command.       (line  7509)
* tags, file ID tags <3>:                ios command.       (line  7553)
* tags, format tags:                     printf.            (line 11755)
* ternary conditional operator:          Conditional Expression.
                                                            (line 10393)
* text files:                            Binary Files.      (line  1316)
* time:                                  Dates and Times.   (line 12382)
* Timespec:                              Dates and Times.   (line 12382)
* tracing:                               __LINE__ and __FILE__.
                                                            (line 12099)
* truncation:                            Poking a SBM Image.
                                                            (line  3281)
* try-catch:                             try-catch.         (line 11517)
* try-until:                             try-until.         (line 11547)
* two complement:                        Negative Integers. (line  2074)
* type:                                  Defining Types.    (line  4001)
* type <1>:                              type.              (line 10260)
* types:                                 Types.             (line 10257)
* types, function types:                 Function Types.    (line 10616)
* types, integral types:                 Standard Integral Types.
                                                            (line 12131)
* types, offset types:                   Standard Offset Types.
                                                            (line 12166)
* unbounded arrays:                      Array Types.       (line  8908)
* Unicode:                               Character Sets.    (line  2794)
* unions:                                Unions.            (line  9989)
* united values:                         Offsets.           (line  8416)
* units:                                 Standard Units.    (line 12178)
* unmap:                                 Unmapping.         (line 11406)
* utf-8:                                 Character Sets.    (line  2808)
* variables:                             info command.      (line  7638)
* variadic functions:                    Variadic Functions.
                                                            (line 10560)
* virtual machine:                       vm command.        (line  7805)
* while:                                 while.             (line 10410)
* whitespace, trimming:                  ltrim.             (line 12313)
* whitespace, trimming <1>:              rtrim.             (line 12323)

