GoBoy

GoBoy is a multi-platform Nintendo GameBoy and GameBoy Color emulator written in go. The emulator can run the majority of GB games and some CGB games. There is also colour and sound support. This emulator was primarily built as a development exercise and is still work in progress. Please feel free to contribute if you're interested in GameBoy emulator development.

The program includes debugging functions making it useful for understanding the emulator operation for building one yourself. These functions include printing of opcodes and register values to the console at each step (although will greatly slow down the emulation) and toggling of individual sound channels.

Installation

go get github.com/Humpheh/goboy/cmd/goboy

The program has been tested on MacOS and Windows 10, and is likely to work on Linux. Building on Windows 10 requires MinGW.

GoBoy uses the go library pixel for control binding and graphics rendering, which requires OpenGL. You may need to install some requirements which can be found on the pixels readme.

Usage

goboy zelda.gb

Controls: ← ↑ ↓ → Z X Enter Backspace

The colour palette can be cycled with = (in DMG mode), and the game can be made fullscreen with F.

Other options:

  -dmg
    	set to force dmg mode
  -mute
    	mute sound output

Debug or experimental options:

  -cpuprofile string
    	write cpu profile to file (debugging)
  -disableVsync
    	set to disable vsync (debugging)
  -load string
    	location of save state to load (experimental)
  -stepthrough
    	step through opcodes (debugging)
  -unlocked
    	if to unlock the cpu speed (debugging)

Debugging

There are a few keyboard shortcuts useful for debugging:

Q - force toggle background
W - force toggle sprites
A - print gb background palette data (cgb)
S - print sprite palette data (cgb)
D - print background map to log
E - toggle opcode printing to console (will slow down execution)
7,8,9,0 - toggle sound channels 1 through 4.

Saving

If the loaded rom supports a battery a <rom-name>.sav (e.g. zelda.gb.sav) file will be created next to the loaded rom containing a dump of the RAM from the cartridge. A loop in the program will update this save file every second while the game is running.

Testing

GoBoy currently passes all of the tests in Blargg's cpu_instrs and instr_timing test roms.

These roms are included in the source code along with a test to check the output is as expected (instructions_test.go and timing_test.go). These tests are also run on each commit.

Contributing

Please feel free to open pull requests to this project or play around if you're interested! There are still plenty of small bugs that can easily be found through playing games on the emulator, or take a task from the TODO list below!

Known Bugs

• Sprite Z-drawing
• Sprites near edge of screen not drawing
• Top half of sprite disappearing off top of screen
• Small sprites row glitch
• Many CGB bugs
• BG tile window offset issue - visible on Pokemon Red splash screen - possibly mistimed interrupt?
• Harry Potter and The Chamber of Secrets has odd sprite issues
• Request to set screen to white does not do so

TODO List

• MBC3 banking support
• GameBoy Color support (partial)
• Rewrite APU emulation
• MBC3 clock support
• Complete CGB emulation
• Icon
• White screen when off
• Speed up CPU
• Resizable window
• Some kind of nice UI
• MBC4-7 support (?)
• More colour palettes
• STOP opcode behaviour
• Blargg's test ROMs

Resources

A large variety of resources were used to understand and test the GameBoy hardware. Some of these include:

• http://www.codeslinger.co.uk/pages/projects/gameboy/files/GB.pdf
• https://github.com/retrio/gb-test-roms
• http://www.codeslinger.co.uk/pages/projects/gameboy/beginning.html
• http://bgb.bircd.org/ - invaluable for debugging
• https://github.com/AntonioND/giibiiadvance/tree/master/docs
• https://github.com/trekawek/coffee-gb