Coffee GB

Coffee GB is a Gameboy Color emulator written in Java 8. It's meant to be a development exercise. More info can be found in the blog post Why did I spend 1.5 months creating a Gameboy emulator?

Building

The emulator can be build with Maven:

mvn clean package

The jar file will be available in the ./target directory.

Usage

Usage:
java -jar coffee-gb.jar [OPTIONS] ROM_FILE

Available options:
  -d  --force-dmg                Emulate classic GB (DMG) for universal ROMs
  -c  --force-cgb                Emulate color GB (CGB) for all ROMs
  -b  --use-bootstrap            Start with the GB bootstrap
  -db --disable-battery-saves    Disable battery saves
      --debug                    Enable debug console
      --headless                 Start in the headless mode

Play with ←, ↑, ↓, →, Z, X, Enter, Backspace.

Features

• Cycle-exact Gameboy CPU emulation. Each opcode is split into a few micro-operations (load value from memory, store it to register, etc.) and each micro-operation is run in a separate CPU cycle.
• Quite compatible (all the Blargg's tests are passed, although some game still doesn't work)
• GPU
• Joypad
• Timer
• Sound
• MBC1-5 support
• Battery saves
• Support for zipped ROMs
• ROM-based compatibility tests run from Maven

Running Blargg's tests

The Blargg's test ROMs are used for testing the compatibility. Tests can be launched from Maven using appropriate profile:

mvn clean test -Ptest-blargg
mvn clean test -Ptest-blargg-individual # for running "single" tests providing more diagnostic info

They are also part of the Travis-based CI.

The tests output (normally displayed on the Gameboy screen) is redirected to the stdout:

cpu_instrs

01:ok  02:ok  03:ok  04:ok  05:ok  06:ok  07:ok  08:ok  09:ok  10:ok  11:ok

Passed all tests

Coffee GB passes all the tests:

• cgb_sound
• cpu_instrs
• dmg_sound-2
• halt_bug
• instr_timing
• interrupt_time
• mem_timing-2
• oam_bug-2

Mooneye tests

The Mooneye GB emulator comes with a great set of test ROMs. They can be used to test the Coffee GB as well. Use -Ptest-mooneye profile:

mvn clean test -Ptest-mooneye

Screenshots

Key bindings

The default key bindings can be changed with the ~/.coffeegb.properties file. The file has following format:

btn_up=VK_UP
btn_down=VK_DOWN
btn_left=VK_LEFT
btn_right=VK_RIGHT
btn_a=VK_Z
btn_b=VK_X
btn_start=VK_ENTER
btn_select=VK_BACK_SPACE

The key list can be found in the KeyEvent JavaDoc.

Resources

• GameBoy CPU manual
• The Ultimate GameBoy talk
• Gameboy opcodes
• Nitty Gritty Gameboy cycle timing
• Video Timing
• BGB emulator --- good for testing / debugging, works fine with Wine
• The Cycle-Accurate Game Boy Docs
• Test ROMs - included in the src/test/resources/roms
• Pandocs
• Mooneye GB - an accurate emulator written in Rust, contains great ROM-based acceptance tests