#### 8.2.3 Additional VM functions ¶

When writing non-trivial Scheme extensions using the MIX/Scheme library, you will probably need to evaluate the contents of the virtual machine components (registers, memory cells and so on). For instance, you may need to store the contents of the `A` register in a variable. The Scheme functions described so far are of no help: you can print the contents of `A` using `(mix-preg 'A)`, but you cannot define a variable containing the contents of `A`. To address this kind of problems, the MIX/Scheme library provides the following additional functions:

Function: mixvm-status ¶
Function: mix-vm-status ¶

  Return the current status of the virtual machine, as a number (`mixvm-status`) or as a symbol (`mix-vm-status`). Possible return values are:

  `(mixvm-status)` | `(mix-vm-status)` |
  0 | MIX_ERROR | Loading or execution error
  1 | MIX_BREAK | Breakpoint encountered
  2 | MIX_COND_BREAK | Conditional breakpoint
  3 | MIX_HALTED | Execution terminated
  4 | MIX_RUNNING | Execution stopped after `next`
  5 | MIX_LOADED | Program successfully loaded
  6 | MIX_EMPTY | No program loaded

Function: mix-vm-error? ¶
Function: mix-vm-break? ¶
Function: mix-vm-cond-break? ¶
Function: mix-vm-halted? ¶
Function: mix-vm-running? ¶
Function: mix-vm-loaded? ¶
Function: mix-vm-empty? ¶

  Predicates asking whether the current virtual machine status is `MIX_ERROR`, `MIX_BREAK`, etc.

Function: mix-reg register ¶
Function: mix-set-reg! register value ¶

  `mix-reg` evaluates to a number which is the contents of the specified register. `mix-set-reg` sets the contents of the given register to value. The register can be specified either as a string (`"A"`, `"X"`, etc.) or as a symbol (`'A`, `'X`, etc.). For instance,

      guile> (mix-reg 'A)
      2341
      guile> (mix-set-reg! "A" 2000)
      ok
      guile> (define reg-a (mix-reg 'A))
      guile> (display reg-a)
      2000
      guile>

Function: mix-cell cell_no ¶
Function: mix-set-cell! cell_no value ¶

  Evaluate and set the contents of the memory cell number cell_no. Both cell_no and value are Scheme numbers.

Function: mix-loc ¶

  Evaluates to the value of the location counter (i.e., the address of the next instruction to be executed).

Function: mix-over ¶
Function: mix-set-over! #t|#f ¶

  `mix-over` evaluates to `#t` if the overflow toggle is set, and to `#f` otherwise. The value of the overflow toggle can be modified using `mix-set-over!`.

Function: mix-cmp ¶
Function: mix-set-cmp! ’L|’E|’G ¶

  Evaluate and set the comparison flag. Possible values are the scheme symbols `L` (lesser), `E` (equal) and `G` (greater).

Function: mix-up-time ¶

  Evaluates to the current virtual machine uptime.

Function: mix-lap-time ¶

  Evaluates to the current virtual machine lapsed time, i.e., the time elapsed since the last `run` or `next` command.

Function: mix-prog-time ¶

  Evaluates to the total time spent executing the currently loaded program.

Function: mix-prog-name ¶

  Evaluates to a string containing the basename (without any leading path) of the currently loaded MIX program.

Function: mix-prog-path ¶

  Evaluates to a string containing the full path to the currently loaded MIX program.

Function: mix-src-path ¶

  Evaluates to a string containing the full path to the source file of the currently loaded MIX program.

Function: mix-src-line [lineno] ¶
Function: mix-src-line-no ¶

  `mix-src-line-no` evaluates to the current source file number during the execution of a program. `mix-src-line` evaluates to a string containing the source file line number lineno; when invoked without argument, it evaluates to `(mix-src-line (mix-src-line-no))`.

Function: mix-ddir ¶

  Evaluates to a string containing the full path of the current device directory.