8-Bit CPU Emulator

This project is an educational 8-bit CPU emulator written in Rust, it's build to understand core computer architecture concepts such as instruction sets, flags, control flow, stacks and system calls.

This project is not a production CPU and not accurate. It is designed purely for learning and experimentation.

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Features

• Custom 8-bit ISA (control-flow and arithmetic operations)
• 4 general-purpose registers
• 64 KB (0x0000-0xFFFF) byte-addressable memory
• Two-pass assembler with label support
• Stack-based function calls
• Stack is software-managed and grows downward

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Registers

Register	Size	Description
A	8-bit	General-purpose register
B	8-bit	General-purpose register
C	8-bit	General-purpose register
D	8-bit	General-purpose register
PC	16-bit	Program Counter
SP	16-bit	Stack pointer

Flags Register

The CPU maintains a small flags register.

Flag	Description
Z	Zero flag - set if last result was 0
C	Carry/Borrow flag

• Control-flow instructions do not modify flags.

Supported Instructions

Instruction	Syntax
MOV	mov dest, src OR mov reg, imm
ADD	add r1, r2 OR add reg, imm
SUB	sub r1, r2 OR sub reg, imm
JMP	jmp addr OR jmp addr
JZ	jz addr OR jz addr
JNZ	jnz addr OR jnz addr
CMP	cmp r1, r2 OR cmp reg, imp
MUL	mul r1, r2
DIV	div r1, r2
CALL	call <label>
SYS	sys <syscall_no>
HLT (Halt)	hlt

Syscalls

Imm	Meaning
0	Exit Program
1	Print register A as integer
2	Print register A as ASCII char

• Syscalls also do not modify flags

Example assembly program

mov b, 3
mov a, 1

loop:
    sub b, a
    jnz loop

sys 1   ; print register A
sys 0   ; exit

Usage

cargo run -- --f <examples/filename.asc>

Non-goals

1. No pipelining
2. No interrupts
3. No virtual memory
4. No privilege levels
5. No hardware I/O

Goals

• Assembler
  • Lexer/Tokenizer
  • Add label support (supporting JMP/JZ/JNZ)
• Add instructions
  • CMP
  • MUL
  • DIV
  • CALL
  • RET
  • SYS
• Better error-handling
• TUI debugger