Mooneye GB

Mooneye GB is a Game Boy research project and emulator written in Rust.

The main goals of this project are accuracy and documentation. Some existing emulators are very accurate (Gambatte, BGB >= 1.5) but are not documented very clearly, so they are not that good references for emulator developers. I want this project to document as clearly as possible why certain behaviour is emulated in a certain way. This also means writing a lot of test ROMs to figure out corner cases and precise behaviour on real hardware.

For documentation about known behaviour, see Game Boy: Complete Technical Reference

Binary test ROMs are available here in a zip package and also as individual .gb files. They are automatically built and deployed whenever there's new changes in the master branch.

Non-goals:

• CGB (Game Boy Color) support. It would be nice, but I want to make the normal Game Boy support extremely robust first.
• A debugger
• A good user interface. Building native UIs with Rust is a bit painful at the moment.

Warning:

• Project is WIP
• Doesn't work properly without a boot ROM
• The emulator is lagging behind hardware research. I don't want to spend time making short-lived and probably incorrect fixes to the emulator if I'm not sure about the hardware behaviour.

Hardware testing

There's tons of documentation and tons of emulators in the internet, but in the end I only trust real hardware. I follow a fairly "scientific" process when developing emulation for a feature:

1. Think of different ways how it might behave on real hardware
2. Make a hypothesis based on the most probable behaviour
3. Write a test ROM for such behaviour
4. Run the test ROM on real hardware. If the test ROM made an invalid hypothesis, go back to 1.
5. Replicate the behaviour in the emulator

All test ROMs are manually run with these devices:

Additional devices

I also have access to more devices with different mainboard revisions, but I think the CPU revision is all that matters if we study the behaviour and not analog characteristics (e.g. audio filtering). Even if audio sounded different between two units with the same CPU revision but different mainboard revisions, I'd expect the difference to be caused by individual device variation or different revisions of support chips (e.g. RAM/AMP/REG).

The main "test fleet" is already very big, so I will only use these devices if there's evidence of behaviour that depends on mainboard revision or individual units.

I'm still looking for the following mainboards, but these are probably not required for reverse engineering:

• SGB-R-01
• SGB-N-01
• SGB-N-10
• C/AGS-CPU-20
• DOL-GBS-01

For now, the focus is on DMG/MGB/SGB/SGB2 emulation, so not all tests pass on CGB/AGB/AGS or emulators emulating those devices.

Performance

Always compile in release mode if you care about performance!

On a i7-3770K desktop machine I can usually run ROMs with 2000 - 4000% speed. Without optimizations the speed drops to 150 - 200%, which is still fine for development purposes.

Raspberry Pi with X11 desktop works but is too slow because there is no OpenGL acceleration.

The emulator is runnable on Android, but cross-compiling and packaging is a huge pain and touch controls would have to be implemented, so I'm not supporting Android at the moment.

Running the emulator

Requirements:

• Rust 1.26
• SDL2 development libraries for your platform must be installed

GUI

1. cargo run --release
2. Follow the instructions

Command-line

1. Acquire a Game Boy bootrom, and put it to $HOME/.local/share/mooneye-gb/bootroms/dmg_boot.bin
2. cargo build --release
3. cargo run --release -- PATH_TO_GAMEBOY_ROM

On Windows, also download an SDL2 package containing SDL2.dll, and put it to target/debug and target/release.

Game Boy keys

Other keys

Test suite

Blargg's tests

Notes:

• cpu_instrs fails on MGB/SGB2 hardware and emulators emulating them correctly. The ROM incorrectly detects the device as CGB, and attempts to perform a CPU speed change which causes a freeze (STOP instruction with joypad disabled)
• dmg_sound-2 test #10 can fail randomly on real hardware and seems to depend on non-deterministic behaviour.
• oam_bug-2 fails on all CGB, AGB, and AGS devices
• cgb_sound-2 test #03 fails on CPU CGB, CPU CGB A, and CPU CGB B

Mooneye GB acceptance tests

Bits (unusable bits in memory and registers)

Instructions

Interrupt handling

OAM DMA

PPU

Serial

Timer

Mooneye GB emulator-only tests

MBC1

MBC2

MBC5

Mooneye GB manual tests

Mooneye GB misc tests

Bits

PPU

Test naming

Some tests are expected to pass only a single model:

• dmg = Game Boy
• mgb = Game Boy Pocket
• sgb = Super Game Boy
• sgb2 = Super Game Boy 2
• cgb = Game Boy Color
• agb = Game Boy Advance
• ags = Game Boy Advance SP

In addition to model differences, CPU revisions can affect the behaviour. Revision 0 refers always to the initial version of a CPU (e.g. CPU CGB). AGB and AGS use the same CPU models. The following CPU models have several revisions:

• DMG: 0, A, B, C
• CGB: 0, A, B, C, D, E
• AGB: 0, A, A E, B, B E. Revision E also exists, but only in Game Boy Micro (OXY) so it is out of this project's scope. However, A E and B E are most likely actually just E revision in A or B-compatible package.

In general, hardware can be divided to a couple of groups based on their behaviour. Some tests are expected to pass on a single or multiple groups:

• G = dmg+mgb
• S = sgb+sgb2
• C = cgb+agb+ags
• A = agb+ags

For example, a test with GS in the name is expected to pass on dmg+mgb + sgb+sgb2.

License and copyright

Mooneye GB is licensed under GPLv3+. Copyright (C) 2014-2020 Joonas Javanainen [email protected]

The test framework and hardware tests under tests/ are licensed under MIT. Copyright (C) 2014-2020 Joonas Javanainen [email protected]