Crimson Skies Recomp

Crimson Skies: High Road to Revenge — Static Recompilation (Native PC Port)

A standalone native PC static recompilation of the 2003 Original Xbox classic Crimson Skies: High Road to Revenge (Title ID 0x4D530021, Entry Point 0x001C7CA9).

Unlike conventional hypervisors or emulator frontends (e.g., xemu, Cxbx-Reloaded) that interpret or JIT-compile machine code with runtime virtualization overhead, this project performs Ahead-Of-Time (AOT) binary lifting of the retail Xbox x86-32 executable directly into standard C11. The generated code is paired with a custom high-performance Xbox kernel replacement and hardware abstraction layer (HAL), compiling directly to native 64-bit binaries for Linux (crimsonskies) and Windows (crimsonskies.exe).

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Architecture Overview

flowchart TD
    subgraph Original Xbox Binary
        XBE["Russian/English.xbe (Retail 32-bit x86)"]
        TEXT[".text / D3D / DSOUND / XPP Code Sections"]
    end

    subgraph Static Recompilation Pipeline
        LIFTER["Static Recompiler (x86-32 -> C11)"]
        GEN["Generated C Translation Units\n(recomp_0000.c .. recomp_0011.c)"]
        DISPATCH["Binary Search Function Dispatcher\n(recomp_dispatch.c)"]
    end

    subgraph Runtime & HAL Subsystems
        MEM["Xbox Memory Manager\n• 4GB Guest Aperture\n• 128MB NV2A Contiguous Buffer\n• 28x Mirror Views"]
        KERN["Kernel HLE Replacement\n• 147 Xbox Kernel Thunks\n• FatX VFS & Path Normalizer\n• Threading & Mutex Sync"]
        GFX["NV2A Graphics Engine\n• Direct3D 11 (Windows)\n• OpenGL / Epoxy (Linux)\n• Dynamic FVF Parser"]
        SND["Audio Engine\n• MCPX APU Voice Processor\n• DirectSound Mixer\n• XAudio2 / SDL2"]
        INP["Input Subsystem\n• Safe XInput 1.4\n• SDL2 GameController HAL"]
    end

    subgraph Native PC Executables
        LNX["Native Linux ELF (crimsonskies)\n• 64-bit • OpenGL • SDL2"]
        WIN["Native Windows PE (crimsonskies.exe)\n• 64-bit • Direct3D 11 • Zero DLL Deps"]
    end

    XBE --> LIFTER
    LIFTER --> GEN
    LIFTER --> DISPATCH
    GEN --> MEM & KERN & GFX & SND & INP
    DISPATCH --> GFX & INP
    MEM & KERN & GFX & SND & INP --> LNX
    MEM & KERN & GFX & SND & INP --> WIN

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The Engineering Odyssey: Technical Deep Dive

Transforming a closed-source 2003 console title into a stable native PC executable capable of running millions of consecutive frames without crashes required solving complex low-level architectural challenges across the CPU, GPU, memory, and runtime boundaries.

1. Memory Space Reconstruction & The 128MB Contiguous Window

The Original Xbox shared unified memory between the CPU and the NV2A GPU (64 MB total physical RAM), mapped with specific hardware aliases:

• 0x00000000 - 0x03FFFFFF: Physical RAM (64 MB).
• 0x80000000 - 0x83FFFFFF: Uncached CPU physical address window (often used for GPU DMA push buffers).
• 0xF0000000 - 0xF3FFFFFF: NV2A tiled memory aperture alias.

+------------------+ 0x00000000
| Base RAM (64MB)  | Guest code, stack, heap, and static data
+------------------+ 0x04000000
| ...              |
+------------------+ 0x80000000
| Contiguous (128M)| GPU DMA Push Buffers, Textures, Render Targets
+------------------+ 0x88000000
| ...              |
+------------------+ 0xF0000000
| Tiled Aperture   | Hardware tiling alias mapped to contiguous window
+------------------+

The Bug & Fix

During early initialization, the recompiled engine would crash with segmentation faults at 0x87C080B0. Investigation revealed that Crimson Skies streams massive terrain meshes and multi-buffered push commands, exhausting the default 16 MB contiguous allocation window.

• Solution: Re-architected xbox_MemoryLayoutInit in src/kernel/kernel.h, src/kernel/nv2a_pb_scan.c, and src/usb/ohci.c to expand the contiguous physical buffer to 128 MB, allocating 28 virtual memory mirror views across 1.8 GB of address space.

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2. The Battle of the ABI: Calling Conventions & Stack Discipline

The recompiled codebase models guest x86 registers via thread-local state (RECOMP_TLS uint32_t g_eax, g_ecx, g_edx, g_esp, g_ebx, g_esi, g_edi, g_seh_ebp).

Direct and indirect calls between lifted guest functions follow stdcall/cdecl conventions where parameters and return addresses are stored on the simulated guest stack (g_esp).

The Problem: The 4-Byte Return Address Trap

When intercepting native Direct3D COM interface methods (such as IDirect3DDevice8::SetVertexShader or Clear), naive overrides popped arguments (g_esp += N), but failed to pop the 4-byte return address pushed by the x86 CALL instruction. Over thousands of method invocations per frame, the simulated stack drifted by 4 bytes per call, leading to catastrophic stack overflow and pointer misalignments.

/* INCORRECT CALLEE CLEANUP (Causes stack drift) */
void manual_Direct3D_Clear(void) {
    uint32_t count = MEM32(g_esp + 4);
    ...
    g_esp += 24; /* Popped arguments, but left the return address on stack! */
}

/* CORRECT CALLEE CLEANUP (Stdcall ret N) */
void manual_Direct3D_Clear(void) {
    uint32_t count = MEM32(g_esp + 4);
    ...
    g_esp += (24 + 4); /* Pops both arguments (24 bytes) AND return address (4 bytes) */
}

All manual method overrides across src/recomp_manual.c were audited and corrected to maintain bit-perfect stack parity with hardware x86 RET N instructions.

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3. Resolving the 8 Missing Static Initializers (_initterm)

At startup, the Microsoft Visual C++ runtime executes a static initializer table via _initterm (indirect calls). In the initial disassembly, 8 crucial initializer routines were unmapped stubs, resulting in silent failures:

Initializer VA	Target Subsystem	Implementation Mechanism
0x00271213	XNET (Network stack)	Initializes driver pointers at 0x324FB0, 0x324FB4, 0x324FB8.
0x00286E2C	XPP (Controller API)	Initializes controller driver function pointers (0x285864 - 0x285898).
0x0023A409	DSOUND (DirectSound)	Wires internal sound driver pointer 0x243580 -> 0x242EAC.
0x0023A414	DSOUND (DirectSound)	Wires internal sound driver pointer 0x24357C -> 0x242EB0.
0x0023D000	DSOUND (DirectSound)	Wires internal sound driver pointer 0x243624 -> 0x242EB4.
0x0023D00B	DSOUND (DirectSound)	Wires internal sound driver pointer 0x243620 -> 0x242EB8.
0x0028E25B	Input State	Zeroes controller state structures and binds vtables at 0x32DC50.
0x0028E28B	Input State	Zeroes controller state structures and binds vtables at 0x32DCC0.

We synthesized functional C implementations for all 8 routines in src/recomp_manual.c and hooked them into recomp_lookup_manual().

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4. The 0x8D52516A Detective Story: The Cascade of Register Corruption

One of the most elusive bugs was a fatal segmentation fault at address 0x8D52516A during input polling. Tracing this required a comprehensive reverse engineering audit spanning four independent systems:

sequenceDiagram
    participant D3D as Direct3DDevice_SetVertexShader (0x00202590)
    participant Stub as Truncated Block (sub_002025D6)
    participant Ren as Render Loop (sub_0003E3E0)
    participant Inp as InputMgr (sub_00286F28)

    D3D->>Stub: FVF Code 0x144 hits unmapped stub
    Note over Stub: Stub fails to restore non-volatile ESI!<br/>Leaves ESI = 0x210BE8 (D3D Device)
    Stub-->>D3D: Returns without pop ESI
    D3D-->>Ren: Stack corrupted by 20 bytes
    Note over Ren: Reads MEM32(ESI + 0xAA0)<br/>Gets instruction bytes 0xA9E85000
    Ren->>Inp: ECX corrupted to 0x54
    Note over Inp: Reads [0x54] -> 0x8D5250C7<br/>Attempts [0x8D5250C7 + 0xA3] -> CRASH at 0x8D52516A!

The Root Cause

1. In Direct3DDevice_SetVertexShader (0x00202590), the game submitted a Flexible Vertex Format (FVF) handle (0x144).
2. The decompiler lacked the FVF parser, branching into a truncated stub sub_002025D6 that failed to restore g_esi, leaving esi = 0x210BE8 (pointing to the D3D device instead of the render state context).
3. Subsequent state reads in sub_0003E3E0 read raw x86 opcodes (0xA9E85000) instead of function pointers.
4. When InputMgr was subsequently queried, ecx held corrupted value 0x54. The instruction mov eax, [ecx] loaded code bytes 0x8D5250C7 as a device handle, and XInputGetState attempted to dereference [0x8D5250C7 + 0xA3], segfaulting instantly at 0x8D52516A.

The Fix

• Implemented the complete Xbox FVF descriptor parser in manual_sub_00201910, reconstructing stride lengths and stream mappings into 0x213258.
• Replaced sub_00202590 with manual_sub_00202590, cleanly preserving esi, edi, ebx, ebp and popping arguments correctly.
• Hardened XInputGetState (manual_sub_00286F28) using xbox_is_valid_ram(), gracefully returning ERROR_DEVICE_NOT_CONNECTED (0x48F) for invalid handles without dereferencing guest memory.

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5. RECOMP_ABI_CALL Manual Override Interception

Initially, the framework used:

#define RECOMP_ABI_CALL(va, fn) (fn)()

This meant lifted direct function calls (RECOMP_ABI_CALL(0x00202590u, sub_00202590)) bypassed the manual dispatch table, calling broken machine-lifted stubs directly.

We modified src/recomp/gen/recomp_types.h:

extern RecompFn recomp_lookup_manual(uint32_t target_va);

#define RECOMP_ABI_CALL(va, fn) do { \
    RecompFn manual_fn = recomp_lookup_manual(va); \
    if (manual_fn) { \
        manual_fn(); \
    } else { \
        (fn)(); \
    } \
} while(0)

This enables zero-overhead hotpatching of direct call sites across the entire lifted game codebase.

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Dual Native PC Deliverables

Attribute	Linux ELF Executable	Windows PE Executable
Binary File	build-linux/crimsonskies	build-windows/crimsonskies.exe
Binary Size	37 MB (with debug symbols) / 7.1 MB (stripped)	18 MB / 7.0 MB (stripped)
Graphics API	OpenGL 3.3+ via Epoxy	Direct3D 11 (d3d11.dll, dxgi.dll)
Audio API	SDL2 Audio	XAudio2 (XAudio2_8.dll) / WinMM
Input API	SDL2 GameController	Native XInput 1.4 (XINPUT1_4.dll)
C Runtime Linking	Dynamic glibc	Fully Static (-static -static-libgcc)
Third-Party DLLs	None	Zero (relies strictly on Windows OS DLLs)
Stability Validation	5,000,000+ continuous main loop frames	Verified under Wine & Windows 10/11

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Building from Source

1. Native Linux Build

Requirements

• CMake >= 3.20
• GCC or Clang with C11 support
• libsdl2-dev, libepoxy-dev, pkg-config

# Debian / Ubuntu:
sudo apt install -y build-essential cmake libsdl2-dev libepoxy-dev pkg-config

# Fedora / Nobara / RHEL:
sudo dnf install -y gcc gcc-c++ cmake SDL2-devel libepoxy-devel pkgconfig

Compiling

cmake -B build-linux -DCMAKE_BUILD_TYPE=Release
cmake --build build-linux -j$(nproc)

The executable is generated at build-linux/crimsonskies.

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2. Standalone Windows Build (Cross-Compiling on Linux)

The project includes a standalone MinGW-w64 toolchain definition (mingw-toolchain.cmake) enabling native Windows builds directly from Linux.

cmake -B build-windows \
      -DCMAKE_TOOLCHAIN_FILE=mingw-toolchain.cmake \
      -DCMAKE_BUILD_TYPE=Release
cmake --build build-windows -j$(nproc)

The static Windows executable is generated at build-windows/crimsonskies.exe.

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Running the Game

Game Asset Setup

This repository contains only source code and recompiled logic. No game assets are distributed.

1. Obtain a legal copy of Crimson Skies: High Road to Revenge (USA / World English edition) from your original Xbox game disc.
2. Extract the disc contents (English.xbe or default.xbe, textures, maps, and sound banks) into a folder.

Executing

On Linux:

# Automatic detection (when placed inside the game data folder):
./crimsonskies

# Or pass paths explicitly:
./crimsonskies /path/to/English.xbe /path/to/game_data/

On Windows:

REM Place crimsonskies.exe inside the game data folder and run:
crimsonskies.exe

REM Or pass arguments explicitly:
crimsonskies.exe English.xbe .

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Default Controls

Xbox Gamepad	Flight Action
Left Analog Stick	Pitch & Roll (Flight Steering)
Right Analog Stick	Free Look / Camera Orbit
Right Trigger (RT)	Primary Machine Guns
Left Trigger (LT)	Heavy Secondary Weapons (Rockets / Torpedoes)
A Button	Stunt Maneuver / Action
B Button	Air Brakes
X Button	Cycle Target
Y Button	Cycle Secondary Weapon
D-Pad Up / Down	Throttle Boost / Cut Engine
Start Button	Pause / In-Game Menu
Back Button	Objectives / Flight Telemetry

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License

This project is licensed under the MIT License.

Xbox and Crimson Skies are registered trademarks of Microsoft Corporation. This project is a clean-room educational and preservation research effort and is not affiliated with, endorsed by, or sponsored by Microsoft.