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Armored Core Decompilation

A matching decompilation of Armored Core (FromSoftware, 1997) for the Sony PlayStation.

The goal is to recover readable C that, when compiled with a period-correct toolchain, produces files byte-identical to the original executable and code overlays, then a moddable full-source tree.

Item	Value
Platform	PlayStation (PSX / PS1)
Disc (USA)	SCUS-94182
Main executable	SCUS_941.82, SHA-1 ee884a2cb3ce30bc13255142fecd5a2f02ba04ad (169984 bytes), loaded at 0x80011000
Program overlays	GG/COM/FDAT.T entries 201-204, loaded at 0x8004ADA0
Mission overlays	58 more code entries in FDAT.T (0x801C4B40 / 0x801C8460 / 0x801B80C8)
Compiler	PsyQ 4.0: CC1PSX GCC 2.7.2 (SN32 3.7.0002) + ASPSX 2.56
License (project code)	CC0 1.0

You must own the game. This repository contains no ROMs, disc images, disassembly or copyrighted assets. rom/ dumps/ asm/ linkers/ assets/ build/ are made from your own disc and are gitignored. Obtain a legal dump of your own disc.

Status

Bring-up is done. The executable, the four program overlays and the 58 mission overlays (57 unique; one pair is byte-identical and built once) are split, every unit relinks byte-identical from the split (C files with INCLUDE_ASM stubs plus data), the C compile pipeline is live, and the compiler is pinned. Matching has started.

Progress by component

Component	Functions	Matched	Progress
Main executable (SCUS_941.82, game code)	147	60	▰▰▰▰▰▰▰▰▱▱▱▱▱▱▱▱▱▱▱▱ 40.82%
Program overlays (FDAT.T)	2871	825	▰▰▰▰▰▰▱▱▱▱▱▱▱▱▱▱▱▱▱▱ 28.74%
└ FDAT_201	1091	529	▰▰▰▰▰▰▰▰▰▰▱▱▱▱▱▱▱▱▱▱ 48.49%
└ FDAT_202	619	106	▰▰▰▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱ 17.12%
└ FDAT_203	579	95	▰▰▰▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱ 16.41%
└ FDAT_204	582	95	▰▰▰▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱ 16.32%
Mission overlays (FDAT.T, 58 entries, 57 unique)	2768	60	▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱ 2.17%
PsyQ 3.7 libraries (configs/USA/psyq_funcs.txt, kept as asm, not counted)	528
Total	5786	945	▰▰▰▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱ 16.33%

Counts come from objdiff (tools/progress_report.py, published to decomp.dev by .github/workflows/progress.yml). The empty jr ra stubs splat already wrote as C count as matched. PsyQ library code (528 functions, identified by signature, listed in configs/USA/psyq_funcs.txt) stays as asm and is excluded from the counts.

Findings so far

Where the code lives. The boot exe is small (164 KB of code and data). Almost all game code is in GG/COM/FDAT.T, a FromSoftware .T archive (u16 count, then count + 1 sector offsets). Of its 205 entries, 62 contain code:

FDAT.T entries	Load address	Content
201	0x8004ADA0	program, 0x84800 bytes, 1091 functions (memory card, key config, ranking strings)
202	0x8004ADA0	program, 0x46000 bytes, 619 functions (mission engine: "ENERGY", "MISSION TIMER")
203	0x8004ADA0	program, 0x41800 bytes, 579 functions (variant of 202)
204	0x8004ADA0	program, 0x41800 bytes, 582 functions (variant of 202)
0-98, 120, 140 (even)	0x801C4B40	per-mission code overlay, followed by its data entry
160-168 (even)	0x801C8460	small per-mission code overlays (base from FDAT_204's loader)
180-186 (even)	0x801B80C8	small per-mission code overlays (base from FDAT_203's loader)

0x8004ADA0 is exactly the end of the exe's .bss (crt0 clears 0x80039CB8-0x8004ADA0), so the four programs share one slot and replace each other. Load addresses were found from the jal targets.

Compiler. Retail code almost never fills the epilogue delay slot (exe 363 unfilled vs 12 filled, overlays similar), which rules out GCC 2.8.x. A differential compile of 11 retail functions against nine compiler builds leaves exactly one: PsyQ 4.0's CC1PSX (GCC 2.7.2 SN32 3.7.0002), identical to decompals' gcc-2.7.2-psx. GCC 2.6.x matches 10 of 11 and fails a function with a switch and a divide by a constant; PsyQ 4.1's cygnus 2.7.2 and GCC 2.8.1 fail several. The assembler is ASPSX 2.56: lw d,sym(reg) expands through $at, and divides carry no divide-by-zero checks. Flags: -O2 -G0.

tools/compiler_probe/run.sh reproduces the comparison.

Layout

Path	What
src/ / include/	decompiled C and headers
configs/	splat configs + symbol maps
tools/	PsyQ cc1 builds, maspsx, m2c, decomp-permuter, asm-differ, mkpsxiso, build scripts

Quick start

Supply your own Armored Core (SCUS-94182) disc image. If it is ECM-compressed, decode it first (tools/unecm.c, any C compiler):

gcc -O2 -o unecm tools/unecm.c
./unecm "Armored Core [NTSC-U] [SCUS-94182].img.ecm" "rom/Armored Core [NTSC-U] [SCUS-94182].img"
tools/windows/mkpsxiso/dumpsxiso.exe -pt -x dumps/disc -s dumps/ac.xml "rom/Armored Core [NTSC-U] [SCUS-94182].img"

python3 -m venv venv                                # venv\Scripts on Windows
pip install -r requirements.txt
python3 tools/acdisc.py                             # unpack GG/COM/FDAT.T to dumps/fdat/
python3 -m splat split configs/USA/SCUS_941.82.yaml # regenerate asm/ and linkers/
python3 -m splat split configs/USA/fdat201.yaml     # same for fdat202/203/204
python3 tools/build_ac.py                           # expect 5x ": OK"

tools/build_ac.py preprocesses and compiles src/**/*.c through the PsyQ cc1 + maspsx pipeline, assembles the split asm, links each unit with its splat linker script plus the auto-detected symbols, and checks the SHA-1 against your disc's files:

build/USA/out/SCUS_941.82: OK
build/USA/out/FDAT_201.BIN: OK
build/USA/out/FDAT_202.BIN: OK
build/USA/out/FDAT_203.BIN: OK
build/USA/out/FDAT_204.BIN: OK

Contributing

1. Pick an INCLUDE_ASM function, write C that compiles to the same bytes, replace the stub.
2. Use proper structs, no pointer arithmetic with magic offsets.
3. Verify with python3 tools/build_ac.py. Only OK for every file counts as done.

Credits

Workflow and toolchain derived from the Digimon World 2 and Parasite Eve 2 decomps. Compiler builds from decompals/old-gcc and the PsyQ SDK archives; splat, m2c, maspsx, decomp-permuter, asm-differ, mkpsxiso.