Overlay Files

Yoshi's Story uses overlays to save memory. The structure of an overlay file is as follows:

u8  text[textSize]
u8  data[dataSize]
u8  rodata[rodataSize]
meta {
    u32 textSize
    u32 dataSize
    u32 rodataSize
    u32 bssSize
    u32 numRelocations
    u32 relocations[numRelocations]
}
u32 metaSize

The position of the meta section is calculated by subtracting metaSize from the total size of the overlay file.
The .bss section is allocated immediately after the overlay file in memory.

Relocation bitfields:

ss -- rrrr oooooooooooooooooooooooo

Bitfield	Description
s	Section (0 = invalid?, 1 = .text, 2 = .data, 3 = .rodata)
-	Unused?
r	Relocation type (4 = R_MIPS_26, 5 = R_MIPS_HI16, 6 = R_MIPS_LO16)
o	Offset

The dynamic linker routine is located at 0x80081430 in RAM.

func_80081430(void *overlay_block, void *meta_section);

The game keeps track of active overlays using an array of the following struct at 0x800DA840 in RAM.

struct OverlayEntry
{
    /*00*/ void *location;     // actual location in memory
    /*04*/ u16   objectId;     // id of the object that this overlay belongs to
           u8    _pad[2];
    /*08*/ void *fakeMemStart; // fake start address
    /*0C*/ void *fakeMemEnd;   // fake end address
};