Swift6502 – a simple 6502 emulator and disassembler

This repo contains a simple, but working, software implementation of a generic system based on the 6502 processor containing a CPU, bus and RAM.

It is capable of running programs written for this architecture. No GUI or CLI capabilities have been added yet.

A working disassembler also exists.

Usage

Emulator

The emulator has no output yet, so all that's currently possible is to run some code and make assertions in a test scenario. Clock frequency is not emulated, so unless code with an infinite loop is given it'll finish quite fast.

// Provide a program as a byte array.
let program: [UInt8] = [0x20, 0x09, 0x00, 0x20, 0x0C, 0x00, 0x20, 0x12, 0x00, 0xA2, 0x00, 0x60, 0xE8, 0xE0, 0x05, 0xD0, 0xFB, 0x60, 0x00]

// Create RAM (of length 0xFFFF + 1), with the program placed at the beginning.
let ram = Array.createRam(withProgram: program)

// Create the CPU.
let cpu = CPU.create(ram: ram)

// Run the program.
cpu.run()

Disassembler

The disassembler is capable of disassembling a program back into human readable code.

// Provide a program as a byte array.
let program: [UInt8] = [0x20, 0x09, 0x00, 0x20, 0x0C, 0x00, 0x20, 0x12, 0x00, 0xA2, 0x00, 0x60, 0xE8, 0xE0, 0x05, 0xD0, 0xFB, 0x60, 0x00]

// Create the disassembler with the given program.
let disassembler = Disassembler(program: program)

// Disassemble the program. The return is a `[String]` containing human readable code.
let code = disassembler.disassemble()

Motivation

I've had a lot of fun watching videos on YouTube of people either writing or disassembling games for the NES, which uses the 6502, and I wanted to see if I could write a software emulator that could run code assembled for that processor.

The main goal of this repo is to just have fun and learn something new I don't work with on a daily basis.

Ideas / todo

• Make Bus own CPU – not the other way around.
• Make types public, if they need to.
• Don't break program flow when encountering opcode 0x00, aka BRK.
• Add support for decimal mode.
• Implement the interrupt vector.
• Programs don't start at address 0x0000. Fix this somehow.
• Emulate clock frequency.
• Improve disassembler output.
• Add labels for disassembly output for i.e. JSR.
• Create a CLI tool for the disassembler.
• Create a visual tool/app that can/has:
  • Load binaries from file.
  • Step debugging support.
  • Visualize registers and status flags.
  • Live disassemble while stepping through.

References

What	Description
Masswerk	Probably the most useful resource I've used. Contains description of all instructions and addressing modes, as well as descriptions on when/how status flags are set.
Instruction set	Have answered some questions I couldn't find on Masswerk's site.
Overflow explanation	Setting the overflow flag properly was probably the thing I struggled the most with. This site gave some highly appreciated insights.
Online assembler	This page has a nice step debugger and visualization of registers and flags.
Online disassembler	Used to compare the output of my disassembler against a result from someone who actually knows what they're doing.