This repo contains custom code as well as a patching utility to add additional functionality to the stock Game and Watch firmware. This is the only custom firmware that allows you to run both the stock firmware as well as retro-go without soldering a higher capacity flash chip!

Features

Works correctly with retro-go in internal flash bank 2.
Press button combination (LEFT + GAME) to launch retro-go from internal flash bank 2.
Ability to store the enitre firmware in internal flash! No external flash required!
- Option to remove the "Mario Song" easter egg.
- Removed the 5 sleeping illustrations.
- LZMA compressed data.
Configurable timeouts.
Run make help to see all configuration options.

Usage

Place your internal_flash_backup.bin and flash_backup.bin in the root of this repo. To extract these from your gnw system, see the game and watch backup repo

Install python dependencies (>=python3.6 required) via:

pip3 install -r requirements.txt

Download STM32 Driver files:

make download_sdk

To just build and flash, you can just run make flash, however it's a bit finicky. You'll probably have better success running the following command (but see the retro-go section for suggested usage).:

make flash_patched_ext
make flash_patched_int

The default programmer interface is stlink, you can chose a different interface via the ADAPTER variable. For example, ADAPTER=rpi.

I recommend pressing the power button at the same time you press enter. Note that the same configuration parameters have to be passed to each make command.

For additional configuration options, run make help.

Retro Go

Since most people are going to be using this with retro-go, want the minimum amount of external storage used, and don't care about the sleeping images or the mario song easter egg, here are the recommend commands. Note that this uses an undocumented 128KB of internal Bank 1 and requires a patched version of openocd installed.

# in this repo
make clean
make PATCH_PARAMS="--internal-only" flash_patched_int

make clean
make -j8 INTFLASH_BANK=2 flash

Advanced usage

Other potentially useful make targets are listed below. Note that external flash only needs to be flashed if the patched external binary is greater than zero bytes.

make clean
make patch  # Generates the patched bin at build/internal_flash_patched.bin, but doesn't flash
make flash_stock_int
make flash_stock_ext
make flash_patch_int
make flash_patch_ext

TODO

Figure out safe place in RAM to store global/static variables. The current configuration described in the linker file is unsafe, but currently we have no global/static variables, so this is low priority.
Custom sprites, icons, and other graphical mods.

Development

Main stages to developing a feature:

Find a place to take control in the stock rom (usually function calls).
Add the stock function and its address to Core/Inc/stock_firmware.h.
Implement your own function, possibly in Core/Src/main.c. There's a good chance your custom function will call the function in (2). You will also probably have to add -Wl,--undefined=my_custom_function to LDFLAGS in the Makefile so that it doesn't get optimized out as unreachable code.
Add a patch definition to patches/patches.py.

Journal

This is my first time ever developing patches for a closed source binary. I documented my journey in hopes that it helps other people. If you have any recommendations, tips, tricks, or anything like that, please leave a github issue and I'll update the documentation!

Acknowledgement

Thanks to the community that made this possible! This repo was built with the help of others. Repos referenced during the development of this project:

game-and-watch-retro-go by kbeckmann
game-and-watch-backup by ghidraninja
game-and-watch-base by ghidraninja
game-and-watch-decrypt by GMMan
game-and-watch-drawing-song-re by jaames

I would also like to thank the stacksmashing discord for all the help (special shoutout to @cyanic)!