Famicom Disk System Preload Patcher Example

A standalone example Famicom Disk System bootloader. It boots from its own tiny, license-free disk, waits for you to insert an unmodified The Legend of Zelda disk, loads the original game into RAM, patches it in memory at runtime, and launches the game, so Link starts with the Magical Sword. The original game disk is never written to or modified.

The patch is deliberately trivial (one tiny byte-diff). The point of this repo is the bootloader mechanism: seize control of the FDS before its copyright check, load a different disk yourself, splice changes into the loaded code, and hand off to the game, all from a distributable disk that contains no copyrighted content.

How it works

The boot disk has no game, no graphics, and no copyright/license ("KYODAKU") file. The boot sequence:

1. Seize control before the copyright check. It boots with the standard homebrew "false reset", as demonstrated in Brad Smith's FDS license-bypass example.
2. Prompt + wait for the disk swap. The bootloader shows "PLEASE LOAD NEXT DISK", then resets the disk drive and finishes the BIOS init by hand (the false reset skipped it), then waits, polling the drive status ($4032), for you to remove the boot disk and insert your Zelda disk.
3. Load Zelda ourselves. We call the BIOS LoadFiles subroutine (it loads named files and returns to us) to read Zelda side A into RAM ($6000–$AF4F).
4. Patch in RAM. We copy a small byte-diff over the freshly-loaded code (this is the magic-sword change), then install Zelda's real interrupt vectors and JMP into Zelda's own reset. The game runs as if it had always had the patch, but the disk was never altered.

The bootloader itself lives in the 1200-byte gap at $AF50–$B3FF that a Zelda side-A load never touches; the splash screen + font live at $7000 (free during the boot-disk load, harmlessly overwritten when Zelda loads).

Build

./setup.sh                 # one-time: build the asm6f assembler
make                       # -> build/boot.fds

setup.sh is macOS-only and self-contained: it builds the asm6f assembler locally.

• build/boot.fds: the shippable bootloader disk. Contains no copyrighted content. Write it to a disk via FDSStick, boot it, and swap in your own Zelda disk when prompted.

Testing

./setup.sh                 # one-time: build asm6f + the custom FCEUX fork
make test                  # run the test suite

The end-to-end test runs in a custom FCEUX fork whose Lua disk-switching support drives the disk swap. On macOS, ./setup.sh installs the build dependencies via Homebrew (see Brewfile), clones the fork into ./fceux, and builds it; the test suite picks it up automatically (override with FCEUX=/path/to/fceux if you built it elsewhere).

You must also provide your own original.fds (an unmodified The Legend of Zelda disk image) in the repo root. Without it the end-to-end test cannot run.