8-Bit CPU Emulator in C

Lightweight, modular 8-bit CPU emulator with a multiple-pass assembler written in C/C++ for fun to explore computer architecture, instruction execution, and low-level programming.

click here to view instruction specifications

CPU Emulator & Assembler Demo

fullemuasmdemo.mp4

NOTE: Has only been tested on windows 11

Features

CPU Emulator

• Signed 8-bit arithmetic with overflow detection
• Support for multiple number formats (decimal, hex, binary)
• Supports jmp operations and subroutines JMP, JMP_ZRO, JMP_ABV, JMP_NEG, CALL, RET
• Program loads from external binary file generated by the assembler, (program.bin)
• Has configurable debug output (RAM view, PC tracing, error logging)

Assembler

• Two-pass assembly process (symbol resolution, code generation)
• Label support with forward and backward referencing
• Multiple number formats (decimal, hex, binary)
• Pointer syntax (*rX for register-indirect)
• Syntax validation and robust Error Reporting
• Symbol table generation
• Has configureable debug output (Symbol table view, Binary out)

Assembler Pipeline

flowchart LR
A{{.asm file}} -- raw assembly --> B(CLEANER
removes comments) 
B -- cleaned assembly string --> C[TOKENIZER
lexical analysis]

C -- string of tokens --> D[PASS ONE
symbols & validation]
  
D -- Symbol table --> E[PASS TWO
machine code
generation]
D -- Verified tokens --> E
    
E -- Machine code --> F{{.bin file}}

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How to Run on Your Machine

1. Clone Repo
2. Modify the file program.asm to run your own assembly instructions
3. Build and run CPU Emulator and Assembler

 # build and run with makefile
 make run

 # build and run manually
 g++ -Iassembler/include -o assembler assembler/src/*.cpp
 ./assembler
 gcc -Iemulator/include -o emulator emulator/src/*.c  
 ./emulator.exe

Example ASM Program (Sum of Array)

    ; EXAMPLE PROGRAM: SUMS ARRAY OF NUMNERS (sum = 76)
    ; EXECUTES IN 36 CYCLES
    
    ; initialize array
    LI r15 12; use temp register to load array values
    STR 0xF0 r15
    STR 0xF1 r15
    LI r15 48
    STR 0xF2 r15
    LI r15 4
    STR 0xF3 r15
    LI r15 0; clear r15
    
    LI r0 0; sum
    LI r1 4; no of elements in array 
    LI r2 0xF0; pointer to array start
    
    START:
        LD r3 *r2; load array element with pointer
        ADD r0 r0 r3; add to sum
        ADDI r2 r2 1; increment pointer
        SUBI r1 r1 1; decrement counter
        JMP_ZRO FINISH; when the end of the array is reached, jump to FINISH
        JMP START; jump to START
    
    FINISH:
        STR 0xFE r0; store result in RAM location 0xFE
        HALT; end program

Assembler Output

CPU Emulator Output

Preformance

• Average CPI: 1 cycle per instruction
• Fibonacci(8): 47 cycles, 21 result
• 5! Factorial: 23 cycles, 120 result
• Sum of Array (4 elements): 36 cycles
• Prime Number Checker: (coming soon)
• Bubble Sort: (coming soon)

Roadmaps

CPU Roadmap

• Create ALU that handles signed arithmetic
• Create RAM & ROM and load program
• Create Control Unit
• Can Execute Instructions
• Track Preformance Metrics
• Subroutine support
• State & Stack Operations

Assembler Roadmap

• Load program
• Impliment Tokenizer & Symbol Table
• Impliment First Pass
• Impliment Second Pass
• Generate Binary File

Future Improvements

• Modify Control Unit to decode & process binary instructions
• Transition from 2D RAM to 1D
• Better Handling of edge cases on the CPU emulator
• Run Bubble sorting Algorithm on CPU