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Cowboy

Howdy! Cowboy is a Game Boy emulator built in Rust. It was built as an educational exercise to learn both Rust and about emulation however it is pretty much feature complete and runs most games which target the original Game Boy. In the future I may extend it to support Game Boy Color. Cowboy also comes with a pretty nifty debug tooling for games running in the emulator. The tooling was created primarily to support the development of Cowboy however it is useful for understanding Game Boy games work in general since it includes a full decompiler, breakpoints, as well as full memory, register and interrupt inspection.

Screenshot 2024-10-01 at 12 52 30

Running

You will need rust and cargo installed. By default the emulator will run the super mario land rom from the roms/ directory.

$ cargo run

You can also specify a ROM by passing the path as an argument:

$ cargo run roms/tetris.gb

References

Creating this emulator was a very educational experience for me. I'd like to highlight the following resources which were very helpful for me when building this and might be helpful for you if you're thinking of building an emulator as well.

  • gbdev.io Documentation. This documentation is very comprehensive.
  • gbdev.io Opcode table. Table of all opcode instructions and includes as JSON file which is useful for testing.
  • gbdev.io Opcode reference. Comprehensive opcode documentation
  • TLMBoy: Exploring the Game Boy's Boot. The first point of call for building a gameboy emulator should be emulating the boot rom. This blog post is a very thorough disection of the gameboy boot ROM and was invaluable for getting the first version of Cowboy running.
  • Game Boy: Complete Technical Reference Probably one of the highest quality technical references I have worked with. The level of precision and attention to detail is unparalleled especially when debugging edge cases for specific CPU instructions or timing details.
  • The Ultimate Game Boy Talk (33c3) An excellent talk from the chaos computer club conference. This is very useful when you are getting started and want to get a feel for the system.
  • Tetris disassembly. This tetris disassembly has been commented. It was useful when my emulator got stuck because I could quickly understand the intention of the code it was struggling with.
  • Super mario land disassembly. Likewise this SML disassembly was useful as it was the second game I approached after tetris.