This is a work-in-progress full port of Sonic the Hedgehog for the Mega Drive (revision 1) to C.
What's ported:
- All "core" game code (graphics management, collision handling (object/level and object/object), level-specific code)
- All game objects, including the entire Sonic object
- Sound driver, integrating code from CKDEV's reimplementation of the sound driver (used with permission)
What's not ported:
- Horizontal blank handling (tile drawing will be corrupted when interrupting demo playback and while Labyrinth Zone's water surface is onscreen)
A REV01 ROM of the original game is required, for reading data from it. Place it in the same folder as the executable and name it sonic1.bin.
I accomplished this by writing a program that generates "nearly-correct" C code from all the code it can find in the original ROM, assisted by a symbol map from a disassembly and a few heuristics. Then, I go through all the generated code and manually fix any issues from the generator that would be too complex to fix automatically (dynamic jumps, complex control flow, code that wasn't picked up by the converter). I compile the resulting C with TCC (though other compilers will probably work), and load it through a separate program written in Rust.
The program acts as a specialized "frontend" for speedshoes, a minimal Mega Drive emulation framework I wrote for this project. Speedshoes is capable of running the original ROM, albeit with some slight runtime patching to accomodate for extra features. Notably, it can detect whenever 68k subroutines are called, and is able to intercept those calls with calls into C "scripts". For the sake of accuracy with the game, the C scripts hold their own copy of game state, and after every hook, the state of the C side is compared with the emulator. Additionally, Speedshoes supports checking accuracy at the level of individual instructions, allowing one to construct a perfect reimplementation of a Mega Drive game down to the CPU register, without interpreting a single byte of code in ROM.
I intend for this port to be a basis from which myself and others can create derivative works of the game, retaining faithfulness to the original gameplay while not having to work in the original's 68000 assembly language.
To build the game DLL:
- Grab a distribution of TCC and put it at the repo root, such that the path to TCC is
<repo root>/tcc/tcc.exe - Enter the
sonic1cfolder and runbuild_game.cmd. (If you want faster code without any accuracy checking, runbuild_game_release.cmd). - You'll end up with
sonic1.dllandsonic1.def(Windows)/libsonic1.so(*nix) in the repo root.
To build Speedshoes and the game runner:
- Install Rust. If you already have Rust installed, make sure you're using the 2021 edition.
- Enter the repo root and
cargo buildfor a debug build, orcargo build --releasefor a release build. - Enter the
sonic1folder andcargo runfo a debug build, orcargo run --releasefor a release build. Release builds expect the game DLL to be in the same folder as it, so copy the DLL over as well.