byteVM

A toy stack-based bytecode VM written in C. Built because I wanted to understand how virtual machines work from the inside.

How it works

The assembler works in two passes:

1. Scans labels and calculates addresses
2. Emits bytecode

The VM executes bytecode using a stack, local variables, and call support.

Instruction set

Op	Name	Description
0	halt	Stop execution
1	push N	Push N onto stack
2	pop	Pop from stack
3	dup	Duplicate top of stack
4	add	Addition
5	sub	Subtraction
6	mul	Multiplication
7	div	Division
8	mod	Modulo
9	eq	Equality
10	ne	Not equal
11	lt	Less than
12	gt	Greater than
13	jmp	Unconditional jump
14	jz	Jump if zero
15	jnz	Jump if not zero
16	call	Call subroutine
17	ret	Return from subroutine
18	load N	Load local variable N
19	store N	Store in local variable N
20	print	Print integer
21	prc	Print character

Build

make

Usage

./vm program.basm

Example

push 1
push 5
call fact
print
push 10
prc
halt

:fact
store 0
load 0
push 1
gt
jz base
load 0
load 0
push 1
sub
call fact
mul
ret

:base
push 1
ret

References

• Crafting Interpreters (Robert Nystrom) — the book that got me into this