Play Kid is yet another Game Boy emulator, written in Rust. But hey, it is MY Game Boy emulator, and I’m proud of it. I wrote some words about its development here.

• Download
• Repository (Codeberg)
• Mirror (GitHub)

Here are the main features of Play Kid:

• All CPU instructions implemented.
• Full memory map implemented.
• Modes: ROM, MBC1, MBC2, MBC3.
• Audio is implemented, with 4 channels, envelopes, sweep, and stereo.
• Supports game controllers via SDL.
• Multiple color palettes.
• Save screenshot of current frame buffer.
• Respects 160:144 aspect ratio by letter-boxing.
• Debug mode:
  • Step instruction.
  • Step scanline.
  • Pause/continue current execution.
  • FPS counter.
  • Displays internal state.
  • Breakpoints.
  • Uses own minimal UI library with horizontal/vertical layouts, labels, buttons, and text fields.
• Save RAM to .sav files to emulate the battery-backed SRAM.
• Automatically adapts to multi-DPI setups by scaling the UI.
• Tested and working games/roms:
  • Passes dmg-acid2
  • Tetris
  • Pokémon Red
  • Super Mario Land
  • Super Mario Land 2: 6 Golden Coins
  • Super Mario Land 3: Wario Land
  • Wario Land II
  • Bugs Bunny Crazy Castle
  • The Amazing Spider-Man
  • Kirby’s Dream Land
  • Dr. Mario
  • Probably many more
• For Linux, macOS, and Windows.

Run

The usual Rust stuff. Check out the repository and run with cargo.

git clone https://codeberg.org/langurmonkey/rts-engine.git
cargo run -- [ROM_FILE]

Make the binary with:

cargo build --release

Operation

Here are the keyboard mappings:

• enter - Start button
• space - Select button
• a - A button
• b - B button

You can also use any game controller. The SDL2 usual mappings apply.

Additionally, there are some more actions available:

• p - change the palette colors
• w - trigger the SRAM save operation to .sav file.
• f - toggle FPS monitor
• s - save a screenshot, with name screenshot_[time].jpg
• d - enter debug mode
• Esc - exit the emulator

Debug mode

You can enter the debug mode any time by pressing d, or activate it at launch with the -d/--debug flag.

You can use the provided UI controls to work with debug mode. You can also use the keyboard. These are the key bindings:

• F6 - step a single instruction
• F7 - step a scanline
• F9 - continue execution until breakpoint (if paused), or pause execution (if running)
• r - reset the CPU
• d - exit debug mode and go back to normal full-speed emulation
• Esc - exit the emulator

You can also use breakpoints. A list with the current breakpoint addresses is provided in yellow. To create a breakpoint, enter the desired address (in $abcd format) into the text field and click Add BR. Remove a breakpoint with Remove BR. Clear all current breakpoints with Clear all.

CLI args

There are some CLI arguments that you can use:

Play Kid 0.1.0

Minimalist Game Boy emulator for the cool kids.

Usage: playkid [OPTIONS] <INPUT>

Arguments:
  <INPUT>  Path to the input ROM file to load

Options:
  -s, --scale <SCALE>  Initial window scale. It can also be resized manually [default: 4]
  -d, --debug          Activate debug mode. Use `d` to stop program at any point
  -f, --fps            Show FPS counter. Use `f` to toggle on and off
      --skipcheck      Skip global checksum, header checksum, and logo sequence check
  -h, --help           Print help
  -V, --version        Print version

Useful links

• Pandocs: https://gbdev.io/pandocs/
• Complete tech reference: https://gekkio.fi/files/gb-docs/gbctr.pdf
• Game Boy CPU manual: http://marc.rawer.de/Gameboy/Docs/GBCPUman.pdf
• Game Boy instruction set: https://www.pastraiser.com/cpu/gameboy/gameboy_opcodes.html