PS2 Code Overlay

The PS2 version of San Andreas uses code overlays to save memory and swap code in only when it is actually needed. The files have the extension 'nm' for NTSC and 'pm' for PAL respectively.

The following code overlays are used: clothes, credits, cutscene, fileload, init, jpeg, mc, mpeg

In the SLES version the files are loaded by the function at 0x3C6A80 which takes as its first argument the name of the file containing the code and as second argument the address where it is to be loaded (0x88D980 typically).

The files have a 64 byte long header:

 4b - fourcc (MWo3)
 4b - unknown (number of segments?)
 4b - load address/entry point?
 4b - size of text segment
 4b - size of data segment
 4b - size of bss segment
 4b - start address of overlay load callbacks
 4b - end address of overlay load callbacks
 32b - file name

After that follow the text and data segment.

1. 1. Overlay load callbacks

The start and end address of overlay load callbacks forms an array of 32bit pointers. The addresses have to aligned to 4 byte boundaries and have to either point inside the game executable or the code overlay. The addresses are absolute memory offsets, not relative to the code overlay base address. The end address delimits the array and thus it does not point to a valid routine address.

 numfuncs = ( end-addr - start-addr ) / 4