EmeraldRecomp

EmeraldRecomp — Pokémon Emerald, Recompiled

This recompilation is a byproduct of developing gbarecomp — the games are the proving ground, the framework is the goal. These are in-development previews, not finished ports — expect rough edges, and depth will keep landing over months, not days. My time for any one title is limited, so I ask for your patience. Contributions are welcome — testing, issues, and PRs to the game or framework all help and will accelerate this game's polish. More on the why at: Recomp + AI: 5 Months Later »

Static recompilation of Pokémon Emerald (Game Boy Advance) to native PC, built on the gbarecomp framework.

Its Gen3 siblings live in FireRedLeafGreenRecomp (FireRed + LeafGreen) and RubySapphireRecomp (Ruby + Sapphire).

Status — playable bring-up (v0.0.1), and self-improving

This is a static-recompilation base + runner, not a finished port. Emerald boots through the BIOS intro to the title screen and into gameplay. It is early — not every code path is statically recompiled yet, and content has not been exhaustively tested. (Emerald's RTC and its larger battle/contest engine are the notable deltas from the other Gen3 games.)

It gets better the more you play. Any code path the static recompiler hasn't covered runs through a built-in interpreter the first time it's hit, then is JIT-compiled to native (in-process, no toolchain needed) and remembered on disk — so the next launch runs it natively from the start. Interpreted once, native ever after; coverage grows toward fully-native as the game is played. See How it self-improves.

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Screenshots

Pokémon Emerald — title screen	Pokémon Emerald — a wild encounter

Native recompiled builds (no emulator), captured running the original ROM.

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What "static recompilation" means here

The ROM's ARM7TDMI machine code is statically translated to native C — every function the game runs becomes a real generated C function. Unlike most recomp projects, the GBA BIOS is recompiled and executed too (not HLE'd or stubbed), so the boot sequence and interrupt/SWI handlers run as real recompiled code. The rest of the console — the PPU (graphics), APU + M4A sound engine, DMA, timers, the cartridge flash save chip + RTC, and hardware I/O — is modeled by the gbarecomp runtime.

Only symbol metadata (function names, addresses, sizes) from the pret/pokeemerald decompilation enters this repo — never its C source, build output, or toolchain. The ROM is never redistributed; you supply your own legally-dumped copy.

ROM

Target	Game	ROM (USA)	SHA-1	Debug port
EmeraldRecomp	Pokémon Emerald	USA	f3ae088181bf583e55daf962a92bb46f4f1d07b7	19892

The runtime refuses to launch on an unrecognized ROM — the SHA-1 must match.

Quick start

1. Download the latest EmeraldRecomp-windows-x64 zip from Releases and extract it (or build from source — see below).
2. Run EmeraldRecomp.
3. Supply your own legally-obtained Pokémon Emerald (USA) ROM when prompted. The path is cached next to the exe for future launches.
4. Play. Early on you may briefly see the interpreter warm up new code paths; once warmed (and cached), they run native.

Multiplayer: Link Cable and Wireless Adapter

EmeraldRecomp supports two-player Emerald ↔ Emerald netplay with a virtual Link Cable or Wireless Adapter. Each peer simulates both GBAs and the local link hardware; recomp-net exchanges controller inputs using delay-sync or rollback. LAN / Direct IP and Internet lobbies use the same simulation.

1. Use matching builds and the supported Emerald USA ROM on both machines.
2. Open Netplay, create or join a lobby, and use a separate trainer save for each player.
3. The host selects Lobby Settings → Connection type: Link Cable (default) or Wireless Adapter. All players inherit the host's choice.
4. Start the session, then use the corresponding upstairs Pokémon Center desk. Wireless Adapter enables the Union Room; the original game's progression and party requirements still apply.

Cable multiplayer has been play-tested. Wireless currently has validated Union Room entry, mutual discovery and contact, deterministic replay, and input netplay under simulated latency/jitter. Full wireless trades/battles and all RFU recovery behavior have not yet been qualified. See the engine's wireless validation and limits.

v0.1.1 binaries include experimental Wireless Adapter Union Room support; v0.1.0 contains cable support only. Merging source does not replace old release downloads. FireRed/LeafGreen/Ruby/Sapphire cross-version linking, larger lobbies and Single-Pak multiboot are future work.

Controls

GBA button	Keyboard
D-Pad	Arrow keys
A	Z
B	X
Start	Enter
Select	Backspace

Save states: Shift+F1–F9 save to a slot, F1–F9 load it.

How it self-improves

gbarecomp's coverage is honest: a path that wasn't statically recompiled is bridged through the interpreter the first time, loudly, then healed:

• First hit: the interpreter runs the missed function (correct, just not native) and the runtime records it.
• Heal: the function is JIT-compiled to native in-process via a toolchain-less backend (sljit) — no compiler required on your machine.
• Persist: the healed path is written to a per-ROM cache (recomp_cache/<rom-sha1>/), so the next launch re-JITs it up front and it runs native from the start.

The result is a game that converges toward fully-native execution the more it's played, and stays improved across launches. A handful of instruction patterns the JIT can't lower yet stay on the interpreter (precision over recall); those are emitter gaps that close over time. Self-improvement is on by default; set GBARECOMP_SELFHEAL_RECOMPILE=0 for a pure-interpreter run.

Building from source

An opt-in overworld widescreen experiment adds Fit to window, 16:9, 21:9 and 32:9 choices through the Mods catalog. This version expands scenery and live NPC visibility. Starting with v0.0.6, Fit also fills portrait windows, overworld menus anchor to the viewport edges, and door animations retain expanded scenery. Distant object spawning and field effects retain the game's original limits; unsupported scenes use the original centered view. Mod 0.2.0 is bundled with v0.0.6, disabled by default.

Prerequisites (Windows): MSYS2 with the mingw64 toolchain (gcc/g++), CMake 3.16+, Ninja, and SDL2 (mingw64 package). Builds are invoked from PowerShell with the mingw64 toolchain on PATH.

1. Clone this repo next to gbarecomp (the game repo builds against the sibling engine checkout on main):

git clone https://github.com/mstan/gbarecomp.git
git clone https://github.com/mstan/EmeraldRecomp.git
cd EmeraldRecomp

2. Supply your ROM at variants/emerald/roms/emerald_usa.gba (SHA-1 above). ROMs are gitignored and never committed.

3. Recompile + build. The committed variants/emerald/symbols/*.toml are the importer output, so you can regenerate the C and build directly:

# from PowerShell, mingw64 on PATH
gba_recompile --rom variants/emerald/roms/emerald_usa.gba \
              --config variants/emerald/symbols/emerald_usa.toml \
              --out variants/emerald/generated
cmake -S . -B build -G Ninja
cmake --build build --target EmeraldRecomp

(gba_recompile is built from the gbarecomp checkout; see that repo's README.) The recompiled translation unit is large — expect a multi-minute compile.

License

PolyForm Noncommercial 1.0.0 — see LICENSE. Third-party components retain their own licenses.

Legal

This project contains no copyrighted ROM data, no Nintendo BIOS, and no decomp source — only original recompiler/runtime code and symbol metadata. You must supply your own legally-dumped ROM (and BIOS, where the runtime requires one). Pokémon and Emerald are trademarks of Nintendo / Game Freak / The Pokémon Company; this project is an unaffiliated, non-commercial preservation and research effort.

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