`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.

bstien/Swift6502

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