Overlay Files [Hack64 Wiki]

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

<code C>
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
</code>

  * 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:
<code>ss -- rrrr oooooooooooooooooooooooo</code>

^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.
<code C>func_80081430(void *overlay_block, void *meta_section);</code>

The game keeps track of active overlays using an array of the following struct at 0x800DA840 in RAM.
<code C>
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
};
</code>