Asm6502

Asm6502 is a lightweight C# library for the 6502/6510 that combines a fluent, strongly typed assembler/disassembler with a cycle-accurate CPU emulator (pluggable 64 KiB memory bus). Use it to generate binaries, disassemble existing code, and run or step programs with precise timing.

✨ Features

• Assembler/disassembler with full support for all core 6502 instructions and 6510 instructions (6502 + undocumented opcodes)
• New: cycle-accurate 6502/6510 CPU emulator (use Mos6510Cpu for full opcode coverage; Mos6502Cpu for documented opcodes)
  • Accurate cycle timing and passes known 6502 timing from Thomas Harte's 2,560,000 tests for the 6502
  • Pluggable 64 KiB memory bus via IMos6502CpuMemoryBus
• Unique strongly typed and fluent assembler API
• Support producing debug information (C# file and line numbers) for each instruction
• Easily disassemble instructions and operand.
• High performance / zero allocation library for disassembling / assembling instructions.
• Compatible with net8.0+ and NativeAOT.
• Integrated assembler API documentation via API XML comments.

📖 User Guide

For more details on how to use Asm6502, please visit the user guide.

🧠 6502 CPU Emulator

Asm6502 includes a cycle-accurate CPU emulator with two variants:

• Mos6502Cpu: documented 6502 instruction set
• Mos6510Cpu: full instruction set including all undocumented opcodes (recommended)

Both reuse the same decode tables as the assembler/disassembler and expose a simple, pluggable 64 KiB memory bus.

Quick start:

using Asm6502;

// Minimal 64 KiB RAM bus
public sealed class RamBus : IMos6502CpuMemoryBus
{
    private readonly byte[] _ram;
    public RamBus(byte[] ram) => _ram = ram;
    public byte Read(ushort address) => _ram[address];
    public void Write(ushort address, byte value) => _ram[address] = value;
}

// Create memory and a tiny program at $C000: LDA #$01; ADC #$01;
var mem = new byte[65536];
mem[0xC000] = 0xA9;
mem[0xC001] = 0x01; // LDA #$01
mem[0xC002] = 0x69;
mem[0xC003] = 0x01; // ADC #$01

// Set Reset vector to $C000
mem[0xFFFC] = 0x00;
mem[0xFFFD] = 0xC0;

var cpu = new Mos6510Cpu(new RamBus(mem));
cpu.Reset(); // fetch reset vector and begin executing
cpu.Steps(2); // Run 2 instructions (LDA and ADC)
var a = cpu.A; // 2
// cpu.PC is at 0xC004 (next instruction)

Notes:

• RMW instructions perform read + two writes; your bus should tolerate that pattern.
• Use cpu.Step()/cpu.Steps(n) for instruction stepping, cpu.Cycle()/cpu.Cycles(n) for cycle stepping.
• cpu.Nmi(), cpu.Irq(), and cpu.Reset() helpers are provided; RaiseNmi/Irq/Reset schedule the interrupt on the next cycle.
• InstructionCycles reports cycles for the last completed instruction; TimestampCounter is a monotonic cycle counter.
• The implementation targets accurate cycle timing and passes known 6502 timing test suites.

🧪 6502 Assembler/Disassembler Example

Suppose the following 6502 assembly code:

       .org $c000     ; Start address
       
       ; Initialization
start: LDX #$00       ; X = 0, index into buffer
       LDY #$10       ; Y = 16, number of bytes to process
       
loop:  LDA $0200,X    ; Load byte at $0200 + X
       CMP #$FF       ; Check if byte is already 0xFF
       BEQ skip       ; If so, skip incrementing
       
       CLC            ; Clear carry before addition
       ADC #$01       ; Add 1
       STA $0200,X    ; Store result back to memory
       
skip:  INX            ; X = X + 1
       DEY            ; Y = Y - 1
       BNE loop       ; Loop until Y == 0
       
       ; Call subroutine to flash border color
       JSR flash_border
       
       ; Infinite loop
end:   JMP end

; ------------------------------
; Subroutine: flash_border
; Cycles border color between 0–7
; (Useful on C64, otherwise dummy)
; ------------------------------
flash_border:
        LDX #$00

flash_loop:
        STX $D020     ; C64 border color register
        INX
        CPX #$08
        BNE flash_loop

        RTS

And the equivalent in C# using Asm6502 library:

using var asm = new Mos6510Assembler();
asm.Org(0xc000);

// Initialization
asm.Label(out var start)
    .LDX_Imm(0x00)     // X = 0, index into buffer
    .LDY_Imm(0x10);    // Y = 16, number of bytes to process

asm.Label(out var loop)
    .LDA(0x0200, X)    // Load byte at $0200 + X
    .CMP_Imm(0xFF)     // Check if byte is already 0xFF
    .BEQ(out var skip) // If so, skip incrementing (forward label)

    .CLC()             // Clear carry before addition
    .ADC_Imm(0x01)     // Add 1
    .STA(0x0200, X);   // Store result back to memory

asm.Label(skip)        // X = X + 1
    .INX()
    .DEY()             // Y = Y - 1
    .BNE(loop)         // Loop until Y == 0

    // Call subroutine to flash border color
    .JSR(out var flash_border); // Declare a forward label

// Infinite loop
asm.Label(out var end)
    .JMP(end);

// ------------------------------
// Subroutine: FLASH_BORDER
// Cycles border color between 0–7
// (Useful on C64, otherwise dummy)
// -----------------------------
asm.Label(flash_border)
    .LDX_Imm(0x00);

asm.Label(out var flash_loop)
    .STX(0xD020)       // C64 border color register
    .INX()
    .CPX_Imm(0x08)
    .BNE(flash_loop)
    .RTS()

    .End();            // Resolve labels

// Get the assembled buffer
var buffer = asm.Buffer;

Disassembling the same code can be done using the Mos6502Disassembler class:

var dis = new Mos6510Disassembler(new Mos6502DisassemblerOptions()
{
    PrintLabelBeforeFirstInstruction = false,
    PrintAddress = true,
    PrintAssemblyBytes = true,
});

var asmText = dis.Disassemble(asm.Buffer);
Console.WriteLine(asmText);

Will generate the following disassembled code:

C000  A2 00      LDX #$00
C002  A0 10      LDY #$10

LL_02:
C004  BD 00 02   LDA $0200,X
C007  C9 FF      CMP #$FF
C009  F0 06      BEQ LL_01

C00B  18         CLC
C00C  69 01      ADC #$01
C00E  9D 00 02   STA $0200,X

LL_01:
C011  E8         INX
C012  88         DEY
C013  D0 EF      BNE LL_02

C015  20 1B C0   JSR LL_03

LL_04:
C018  4C 18 C0   JMP LL_04

LL_03:
C01B  A2 00      LDX #$00

LL_05:
C01D  8E 20 D0   STX $D020
C020  E8         INX
C021  E0 08      CPX #$08
C023  D0 F8      BNE LL_05

C025  60         RTS

🪪 License

This software is released under the BSD-2-Clause license.

🌟 Credits

Thanks to Norbert Landsteiner for providing the 6502 Instruction Set.

Thanks to Jacob Paul for providing a C++ version cycle accurate version of the 6502 CPU. See Mos6502Cpu.cs for more details about the improvements made in Asm6502.

🤗 Author

Alexandre Mutel aka xoofx.