# Thread Details

Thread objects in vvp are created by .thread statements in the input source file.

A thread object includes a program counter and private bit registers. The program counter is used to step the processor through the code space as it executes instructions. The bit registers each hold Verilog-style 4-value bits and are for use by the arithmetic operators as they operate.

The program counter normally increments by one instruction after the instruction is fetched. If the instruction is a branching instruction, then the execution of the instruction sets a new value for the pc.

Instructions that use the bit registers have as an operand a <bit> value. There is usually space in the instruction for 2 <bit> operands. Instructions that work on vectors pull the vector values from the bit registers starting with the LSB and up.

The bit addresses 0, 1, 2 and 3 are special constant bits 0, 1, x and z, and are used as read-only immediate values. If the instruction takes a single bit operand, then the appropriate value is simply read out. If the instruction expects a vector, then a vector of the expected width is created by replicating the constant value.

Bits 4, 5, 6 and 7 are read/write bits but are reserved by many instructions for special purposes. Comparison operators, for example, use these as comparison flag bits.

The remaining 64K-8 possible <bit> values are read-write bit registers that can be accessed singly or as vectors. This obviously implies that a bit address is 16 bits.

Threads also contain 16 numeric registers. These registers can hold a real value or a 64bit integer, and can be used in certain cases where numeric values are needed. The thread instruction set includes %ix/* instructions to manipulate these registers. The instructions that use these registers document which register is used, and what the numeric value is used for. Registers 0-3 are often given fixed meanings to instructions that need an integer value.

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     * Copyright (c) 2001-2024 Stephen Williams (steve@icarus.com)
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