ByteVM

A lightweight stack-based virtual machine with 18 instructions, written in C++17. ByteVM provides a simple environment for understanding how virtual machines work and how instructions are executed.

Features

• Stack-based architecture with 1000-element stack
• 18 essential opcodes covering arithmetic, logic, control flow, and I/O
• Function calls with local memory support
• Bytecode compiler for assembly-like syntax
• Fast opcode lookup using hash maps

Project Structure

bytevm/              - Virtual machine implementation
bytevm-compiler/     - Bytecode compiler
test/                - Example programs

Building

Compile both the VM and compiler using g++:

g++ -std=c++17 -o bytevm-compiler/bin/Debug/bytevm-compiler bytevm-compiler/*.cpp
g++ -std=c++17 -o bytevm/bin/Debug/bytevm bytevm/*.cpp

Usage

Step 1: Write Assembly Code

Create a file with ByteVM assembly instructions (e.g., program.asm):

PUSH 10
PUSH 5
ADD
PRINT
HALT

Step 2: Compile to Bytecode

./bytevm-compiler/bin/Debug/bytevm-compiler program.asm

This generates test/out.bytevm containing the compiled bytecode.

Step 3: Run the Bytecode

./bytevm/bin/Debug/bytevm test/out.bytevm

Instruction Set

Opcode	Value	Description
PUSH	0	Push value onto stack
POP	1	Pop value from stack
ADD	2	Pop two values, push sum
SUB	3	Pop two values, push difference
MUL	4	Pop two values, push product
AND	5	Bitwise AND of top two values
OR	6	Bitwise OR of top two values
NOT	7	Logical NOT of top value
LT	8	Push 1 if second < top, else 0
EQ	9	Push 1 if second == top, else 0
JMP	10	Jump to instruction address
JZ	11	Jump if top of stack is 0
LOAD	12	Load from local memory
STORE	13	Store to local memory
CALL	14	Call function
RET	15	Return from function
PRINT	16	Pop and print top value
HALT	17	Stop execution

Examples

Example 1: Basic Arithmetic

PUSH 15
PUSH 7
SUB
PRINT
HALT

Output: 8

Example 2: Conditional Logic

PUSH 5
PUSH 10
LT
PRINT
HALT

Output: 1 (true, because 5 < 10)

Example 3: Conditional Jump

PUSH 0
JZ 8
PUSH 999
PRINT
HALT
PUSH 42
PRINT
HALT

Output: 42 (jumps to instruction 8 when stack top is 0)

Example 4: Multiple Operations

PUSH 100
PUSH 50
ADD
PRINT
PUSH 10
PUSH 3
MUL
PRINT
PUSH 25
PUSH 5
SUB
PRINT
HALT

Output:

150
30
20

How It Works

The VM operates on a simple fetch-decode-execute cycle:

1. Fetch: Read the opcode at the current instruction pointer
2. Decode: Determine which operation to perform
3. Execute: Perform the operation on the stack
4. Repeat: Move to the next instruction

The compiler tokenizes assembly source code and converts instruction names to their numeric opcodes, producing a space-separated bytecode file that the VM can execute.

Technical Details

• Stack Size: 1000 integers
• Call Stack: 100 frames
• Local Memory: 100 integers per frame
• Opcode Lookup: O(1) using unordered_map
• Language: C++17