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. Find all posts about Play Kid here.

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.
• Multiple color palettes.
• Save screenshot of current frame buffer.
• FPS counter.
• Respects 160:144 aspect ratio by letter-boxing.
• Debug panel:
  • Step instruction.
  • Step scanline.
  • Pause/continue current execution.
  • Displays internal state of CPU, PPU, and Joypad.
  • Full program disassembly, with breakpoints.
• Save RAM to .sav files to emulate the battery-backed SRAM. Those are saved every minute.
• Working games/roms:
  • Passes dmg-acid2
  • Tetris
  • Pokémon
  • Super Mario Land
  • Super Mario Land 2: 6 Golden Coins
  • Wario Land (Super Mario Land 3)
  • Wario Land II
  • Bugs Bunny Crazy Castle
  • The Amazing Spider-Man
  • Dr. Mario
  • Probably many, many more
• For Linux, macOS, and Windows.

Downloads

You can grab packages for Linux, macOS, and Windows here:

• Downloads
• Mirror (GitHub)

Build

Build the project with cargo build.

Run

The usual Rust stuff.

  cargo run

You can also pass in a ROM file with cargo run -- your-rom.gb.

Make the binary with:

  cargo build --release

Operation

If you don’t pass in any ROM file as an argument, you need to select using the top menu bar, File▶Open ROM….

Here are the Joypad keyboard mappings:

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

The keyboard is clumsy for playing Game Boy games, so you can use any game controller. Controllers are detected when hot-plugged.

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

You can also use the provided UI.

Debug panel

You can open the debug panel any time by pressing d, by clicking on Machine▶Debug panel…, or activate it at launch with the -d/--debug flag. The debug panel shows up in a translucent window. It provides a view of the internal state of the emulator, with:

• Current address, instruction, operands, and opcode, to the top.
• Internal state of CPU, PPU, and JOYP, to the left.
• Disassembly of the program, to the right.
• Breakpoints.

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 at the bottom. To create a breakpoint, either click on the address in the disassembly panel, or enter it (in $abcd format) into the text field and click +. Remove a breakpoint by clicking the × in the breakpoints list. Clear all current breakpoints with Clear all.

CLI args

There are some CLI arguments that you can use:

Play Kid 0.4.0
Toni Sagristà - tonisagrista.com

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

SDL2 version

Play Kid started as an SDL2 application, but it was moved to a pure Rust tech stack using pixels, winit, egui, and rodio. This makes it much easier to build for different targets (including WASM!). Additionally, the SDL2 version contains a minimalist homegrown UI library that I’m particularly proud about, but it can’t hold a candle to egui. It looks like this:

The SDL2 version is forever tagged playkid-sdl2 (playkid-sdl2@codeberg, playkid-sdl2@github).

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 CPU instructions: https://meganesu.github.io/generate-gb-opcodes/