NeXTDoom — Static Recompilation

Static recompilation of NeXTDoom 1.2 — id Software's Doom as it shipped for NeXTSTEP 3.3, the platform Doom was written on — from the i386 slice of its fat Doom.app/Doom binary to native C. Every function in the game is lifted to C and compiled into a new program; the NeXTSTEP it expects (libsys, the Objective-C runtime, the slice of AppKit it touches) is a host runtime on SDL2. No emulator.

Built on the pcrecomp toolchain: its Mach-O front end (tools/macho) and NeXTSTEP host runtime (runtime/nextstep), both on pcrecomp's main since #25, and following its shared house style for recompilation projects.

Generated source is not distributed. You supply your own NeXTSTEP 3.3 install; the lifter runs on your machine and writes the C into a gitignored folder.

Status

v0.1.0 — alpha. It boots and plays the attract demo.

crt0, main, the nib, appDidInit: and D_DoomMain all run as recompiled code: the title screen comes up and the demos play. Everything on screen below is the game's own code, recompiled.

Part	State
Lift	676 functions, 0 errors, every body decodes onto the next start
3D renderer, sprites, status bar, palette effects	Working (attract demos)
R_DrawColumn / R_DrawSpan (hand-written assembly)	Working
Keyboard	Wired through VGAView keyDown:/keyUp:, not play-tested
Mouse, sound, window scaling, netgames	Not yet (roadmap)
Headless --record, conformance harness	Not yet (roadmap)

Screenshots

Title screen	Shotgun versus imp, demo 1
A fireball coming down the corridor	The blue-floored room

Getting Started

You need a NeXTSTEP 3.3 Intel install (a disk image is fine) with /LocalApps/Doom.app on it. Nothing else from it is used except /usr/shlib.

Step by step

Prerequisites, on Windows 10 or 11:

• MSYS2 with the mingw64 toolchain, SDL2, CMake and Ninja (tested with gcc 15.2, CMake 4.2, Ninja 1.13): pacman -S mingw-w64-x86_64-gcc mingw-w64-x86_64-SDL2 mingw-w64-x86_64-cmake mingw-w64-x86_64-ninja
• Python 3.11+ with capstone (tested with 3.13 and capstone 5.0.7): py -3 -m pip install capstone
• A checkout of pcrecomp next to this one (../pcrecomp), or anywhere, named by PCRECOMP for the Python scripts and -DPCRECOMP=<path> for CMake.

Commands are for a Git Bash or MSYS2 shell, from this folder:

1. Copy the game out of the disk image with pcrecomp's UFS reader:

   UFS=../pcrecomp/tools/macho/ufs.py
   MSYS_NO_PATHCONV=1 py -3 $UFS hd.img get /LocalApps/Doom.app original/Doom.app
   MSYS_NO_PATHCONV=1 py -3 $UFS hd.img get /usr/shlib original/shlib

2. Lift:

   py -3 run_lift.py

   Expected:

   [*] code=0x00003990-0x00022E50 entry=0x00003990 functions=676 imports=59
   ============================================================
     functions 676   errors 0   files 3   lines 92,838   1.3s
   ============================================================
   

3. Build:

   export PATH=/c/msys64/mingw64/bin:$PATH
   cmake -S . -B build -G Ninja -DCMAKE_C_COMPILER=gcc
   cmake --build build

   Expected: build/nextdoom.exe, about 1.4 MB.
4. Run: ./build/nextdoom.exe. The window opens on the title screen and the attract demos start.

Usual trip-ups: py vs python (use the py launcher; the Microsoft Store python alias is not a real interpreter), MSYS_NO_PATHCONV=1 (without it MSYS rewrites /LocalApps/... into a Windows path), and a PATH change that needs a new terminal.

A one-click Setup.cmd quick start is on the roadmap.

Usage

./build/nextdoom.exe runs the game from original/ (the path is baked in at configure time). Diagnostics are environment variables:

variable	effect
NS_TRACE=1	log every import bound and every message sent
NS_SHOT=first,count,path%05d.bmp	save frames first..first+count-1 (how these screenshots were made)

NS_SHOT=400,1,shot%05d.bmp ./build/nextdoom.exe

How it works

docs/architecture.md covers the design: how the function catalog is found, how shlib imports and Objective-C methods bind to host shims, the startup path, and video.

Building from source

See Step by step above. run_lift.py and CMakeLists.txt document each step.

License

MIT — see LICENSE. This covers the code in this repository only. Doom is © id Software and NeXTSTEP is © NeXT / Apple; no game or system code or data is included.