C2C

This is a project to reverse engineer the game Crash Bandicoot 2: Cortex Strikes Back and eventually have a complete port to C. The game engine is converted directly from MIPS machine code into a massive block of C code which utilizes a playstation emulator for I/O. Portions of the converted code can then be decompiled by hand and used in place of the original code.

Technical Details

If you're only interested in compiling and running C2C, skip ahead past this section.

The Disassembler

Located in srcdisasm/ is a custom-built MIPS disassembler written in C#. This disassembler is given the game engine's executable file and outputs a massive C source file which contains equivalent code. For example, see the following series of instructions taken directly from the game:

	80012B0C: 00809821 !... ADDU $S3,$A0,$R0
	80012B10: 14530002 ..S. BNE $V0,$S3,80012B1C
	80012B14: 00000000 .... NOP
	80012B18: AC600000 ..`¬ SW $R0,0($V1)
ZZ_12AE4_38:
	80012B1C: 96620004 ..b. LHU $V0,4($S3)
	80012B20: 00000000 .... NOP
	80012B24: 2442FFEC ìÿB$ ADDIU $V0,$V0,-20
	80012B28: 2C420002 ..B, SLTIU $V0,$V0,2

This series of instructions becomes the following code in C:

	S3 = A0;
	if (V0 != S3)
	{
		ZZ_CLOCKCYCLES(13,0x80012B1C);
		goto ZZ_12AE4_38;
	}
	EMU_Write32(V1,R0);
	ZZ_CLOCKCYCLES(14,0x80012B1C);
ZZ_12AE4_38:
	V0 = EMU_ReadU16(S3 + 4); //+ 0x4
	V0 -= 20;
	V0 = V0 < 2;

As seen above, the disassembler's conversion is very direct and does little actual decompiling, only basic expression simplification. It is up to the programmer to do all necessary analysis to detect code structures such as loops or conditional execution. As an additional barrier, the disassembler does no analysis of indirect branches such as function pointer calls or jump tables for switch statements. Therefore, it is also up to the programmer to specify all code regions which have not been automatically detected by the disassembler. This can be seen in srcdisasm/Main.cs.

The Emulator

Located in srcemu/ is a modified version of the PCSX-R playstation emulator. The emulator has been repurposed into an external library to implement some important devices:

• System Control Coprocessor (COP0)
• Geometry Transformation Engine (COP2)
• HLE BIOS
• Interrupt System
• Root Counters (Timers)
• DMA Controller
• Graphics Processing Unit (with plugin)
• Sound Processing Unit (with plugin)
• CDROM Drive
• Gamepads (with plugin)

The Disassembled Code

Located in srczz/ is the output from the disassembler, one file for each function. Each destination of a direct "jump-and-link" is considered to be the start of a new function and thus begins a new file. At the top half of each file is a list of addresses occupied by the function, linking addresses to C labels as well as enabling warnings for two conditions:

1. Writing to code, which would signify broken or self-modifying code, a condition for which the disassembled code has no means of handling.
2. Branches to addresses not identified as potential branch destinations by the disassembler, which will not have an associated C label and as such cannot be handled.

Functions in the disassembled code are intended to be decompiled by hand and eventually moved out into "real" C functions within the main code base (src/) leaving the disassembled file as merely a call out to the new function.

As an example of hand-decompilation of code, the C code from above could be changed into:

	S3 = A0;
	if (V0 == S3)
		EMU_Write32(V1,0);
	V0 = EMU_ReadU16(S3 + 4) - 20U < 2;

Furthermore, V0 is actually used directly after this code as a condition for a branch and then discarded, so EMU_ReadU16(S3 + 4) - 20U < 2 could be placed directly into the expression and V0 removed. Eventually, a decompiled function will not even rely on MIPS registers and can use real C arguments and local variables.

Decompiled functions may call functions which have yet to be decompiled by calling EMU_Invoke. This allows decompilation to begin at non-leaf functions.

The Main Code

The main body of code is located in src/ and contains the core code of the project as well as interoperability code for accessing the emulator. Decompiled functions should be moved into this section of the code base and given proper function names.

Compiling

At the moment, C2C is only available for compilation and execution on Windows systems. You may be able to rig a non-windows build with some work, however.

C2C has an odd set of dependencies for compilation:

• For C2C: Cygwin
• GNU Make
• GCC (C99 and C++11)
• SDL2
• For the emulator: Microsoft Visual C++ 2005 (or later) Express (or Professional)
• The windows platform SDK may or may not be required

The disassembler is optional, and is unnecessary for any build operation.

• For the disassembler (optional): Microsoft Visual C# 2005 (or later) Express (or Professional)
• Probably also works with MonoDevelop

Building C2C

Under Cygwin, cd into the main directory (containing src/, srcemu/, Makefile, etc) and run make. Compilation of src/zz.c may take some time as it is incredibly large and filled with unusual control flow statements. The resulting executable is c2c.exe.

Building The Emulator

Under Microsoft Visual Studio or Visual C++, the solution file srcemu/hookemu.sln should build without issue. Newer versions of Visual Studio or Visual C++ may require the project file to be converted. This should not be an issue, however please do not submit pull requests containing the converted project file.

Building The Disassembler

Under Microsoft Visual Studio or Visual C#, the solution file srcdisasm/PSXDiscompile.sln should build without issue. See above for information on converting the project file to a newer format.

Running

To run C2C, under Cygwin, cd into the main directory (containing c2c.exe, psx.dll, etc) and run ./c2c.

You will need to have the following files present in the same directory:

• c2c.bin: An image of the Crash Bandicoot 2 (PAL) game disc, including complete MODE2/FORM1 headers. This is the BIN out of a CUE/BIN rip.
• psx.dll: The compiled emulator (see above).
• psx-gpu.dll: A playstation emulator GPU plugin.
• psx-spu.dll: A playstation emulator SPU plugin.
• psx-pad1.dll: A playstation emulator gamepad plugin.
• psx-pad2.dll: A playstation emulator gamepad plugin. This may need to be different from the other one.
• memcard1: A memory card file in the typical 128 KB format. This will be read but not written to.
• SCES_009.67: This is the original game executable, which can be found on the game disc (PAL).

For the plugins, I recommend the following:

• psx-gpu.dll: Pete's PSX OpenGL2 Renderer (gpuPeteOpenGL2)
• psx-spu.dll: P.E.Op.S. DSound PSX SPU Audio Driver (spuPeopsSound)
• psx-pad1.dll: Lilypad (comes with PCSX2)
• psx-pad2.dll: Harakiri PAD; you may need to configure a few buttons to get this one to run without erroring out

The second gamepad plugin is not actually utilized, but must be present for the emulator to run.

Reporting Issues

You may report any issues you have to chekwob@yahoo.com, however please understand that this software is currently not intended for regular use. The complex build and running requirements and should be a testament to this fact.