MadNES (A crappy NES emulator)

This is a long term side project I was inspired to start after one of the assignments I did for CMPT 295 (Introduction to Computing Systems) at SFU. In that project we implemented a few functions to complete a disassembler/simulator for a subset of RISC-V assembly. I thought it was a pretty fun project, and it got me thinking: could I adapt/expand this concept to make an emulator for a game console? Well this is my attempt at exactly that. I'm going to attempt to implement an NES emulator from scratch in C.

Goals of this project

This is a very ambitious project, so only time will tell how far I actually get, but these are my goals for this project in roughly the order I plan to achieve them in:

• Create a disassembler for 6502 assembly
• Create C implementations for all 56 instructions to emulate a normal 6502 program (i.e. no graphics/sound/etc.)
• Write unit tests for all the instructions (probably should've done that while implementing them 😅)
• Implement a parser for the iNES ROM format
• Begin researching NES graphics and maybe try to get some simple homebrew applications running
• Same thing for audio
• Figure out how to emulate controller input (probably just from the keyboard)
• Play the OG Super Mario Bros. on a homemade emulator! (probably after a LOT of debugging)

I feel fairly confident in my ability to complete the first 2 goals, so I'm aiming for those for now and will reassess once I feel satisfied with them. Playing a retail NES game feels very daunting right now, but if I can make it work that would be pretty awesome! However, if I don't get all the way there, I'll still see this project as a win as it's going to teach me a lot about low-level computing, which I think is valuable regardless of what field I actually end up programming in.

Non-goals of this project

This is purely an experiment to see if I can do it. I have no plans whatsoever to make this user-friendly or create releases or anything. There are plenty of excellent NES emulators out there already that will work 1000000x better than this project ever will, and that's ok.

Update: Due to a lack of understanding of how the NES actually works, I assumed that I could get away with just implementing the 6502 instructions and that would be all I need. After a bit of preliminary research though, I have found out that there's more to it than just the 6502, which I probably should've guessed. Because of this, I will have to implement a parser for the iNES ROM format, which will make this a lot more "user-friendly" than I originally planned 😅

I also don't plan on making this cross-platform at all. This is a purely "it works on my machine" project.

Usage

If you decide you want to try using this project, here are the steps to build/run it. The only prerequisites should be clang and make.

1. Clone the repository (obviously)
2. cd /path/to/repo/ && make
3. The binary will be outputted to ./build/bin/nes.

Just run this file with the appropriate flag and a path to a file that is formatted like the one above. For example, to disassemble the file I made to test the LDA instruction, run this command:

./build/bin/nes -d ./input/d_tests/lda.input

and you should get the output

LDA  #$01
LDA  $02
LDA  $03,X
LDA  $0405
LDA  $0607,X
LDA  $0809,Y
LDA  ($0a,X)
LDA  ($0b),Y

Similarly, you can run any of the test input files like this:

./build/bin/nes -r ./input/snake.input

and you'll get and output like this:

-------- Debug Output --------
     A=$1f  X=$ff  Y=$00
      SP=$fb  PC=$0735
          NV-BDIZC
          00110011
------------------------------

This output shows the contents of the A, X, and Y registers, the Stack Pointer (SP), the Program Counter (PC), and the processor status flags, allowing you to verify that the program at least did something.