An experimental Kotlin Multiplatform, Compose Multiplatform, GameBoy Emulator that targets Android, iOS, Desktop and JS/WASM.
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Note
KocoBoy can run many games, demos and tests but it is not a M-Cycle or micro-ops accurate emulator.
Accuracy and syncronization between the various memory mapped devices relies on hardcoded fixed values and varies from 4 to 24 CPU cycles depending on the executed opcode or hardware interrupt.
Game Boy catalog compatibility support is focused on the most popular cartridges types (MBCs 1,2,3 and 5).
The full list goes as follows:
| Supported Cartridge Types | Unsupported Cartridge Types |
|---|---|
| 00h ROM ONLY | 08h ROM+RAM |
| 01h MBC1 | 09h ROM+RAM+BATTERY |
| 02h MBC1+RAM | 0Bh MMM01 |
| 03h MBC1+RAM+BATTERY | 0Ch MMM01+RAM |
| 05h MBC2 | 0Dh MMM01+RAM+BATTERY |
| 06h MBC2+BATTERY | 1Ch MBC5+RUMBLE |
| 0Fh MBC3+TIMER+BATTERY | 1Dh MBC5+RUMBLE+RAM |
| 10h MBC3+TIMER+RAM+BATTERY | 1Eh MBC5+RUMBLE+RAM+BATTERY |
| 11h MBC3 | 20h MBC6 |
| 12h MBC3+RAM | 22h MBC7+SENSOR+RUMBLE+RAM+BATTERY |
| 13h MBC3+RAM+BATTERY | FCh POCKET CAMERA |
| 19h MBC5 | FDh BANDAI TAMA5 |
| 1Ah MBC5+RAM | FEh HuC3 |
| 1Bh MBC5+RAM+BATTERY | FFh HuC1+RAM+BATTERY |
Note
SRAM save files are not supported at the moment so your progress on games will be lost on exit.
Run the App, Click the cog / settings icon to see the sidebar.
On the sidebar you can Load a rom, Power On/Off or select a theme.
Click again the cog / settings icon to collapese the sidebar.
On platforms with touch input you can actually use the UI as an input.
On platforms with keyboard input is mapped as:
- D-Pad UP: W
- D-Pad Left: A
- D-Pad Down: S
- D-Pad Right: D
- A: K
- B: L
- Start: P
- Select: O
The project is splitted in 2 modules:
core: It contains just commonMain Kotlin code. It represents the full emulator core with CPU, PPU, Bus and other components.
composeApp: A Compose Multiplatform gradle module it is mainly a commonMain centric module with a couple of per platform functions, for image and audio handling.
This is nowhere a full list of issues just some of them from a high view perspective:
- iOS lacks sound output (yet)
- Main loop and audio sync have quite room for improvement.
- There are little differences on TextAutoSize, shadows and UI in different platforms (Compose Multiplatform 1.8.0-alpha2 issues).
- Can i use this emulator to play?
Yes you can, but you shouldn't. There are a lot of other more capable emulators out there.
This is just a personal project to play with Kotlin Multiplatform / Compose Multiplatform
- Why are you using Compose Multiplatform 1.8.0 alphas.
This is an experimental project, also TextAutoSize is only available there.
- Why are you using Compose Multiplatform 1.8.0-alpha2 instead of alpha3?
Even thought they fixed things like shadow inconsistencies per platform, the AndroidX dependencies are bumped to Compose 1.8.0-beta1 and that breaks TextAutoSize in animated custom layouts.
- Why are the previews of composables on androidMain instead of commonMain?
Because @Preview annotations on Android Studio only works there.
- Why parts of the core code use unsigned while others use signed masking?
Never been a fan of Kotlin approach to unsigned. When you add limited infix bitwise operators to the mix you end on quite messy code.
But then I learned that on KMP the inline classes actually get lowered to native primitives and even on the JVM/Android/R8 can produce better bytecode. (thanks Kotlin Explorer!)
By that time the CPU was already finished, so yeah... I should rewrite the masked part to unsigned even if some parts are experimental and the developer experience is not the best.
- How is performance overall for an emulator?
At it's current iteration it can trigger around 2800vps, so around 46x the real device running on a phone while running uncapped. (tested on a Pixel 6).
- Why sometimes it hiccups on audio or it goes below 60 when capped?
Aparently I'm kinda bad at writting main loops (also delay may be not the best tool there as it behaves differently per platform...)
- Why did you insert random thing here on that way? I can totally do it better!
Make a pull request!
- But... I have questions!
Ask on the Emudev Discord Server