Patch Information

Patch loading can be accomplished in multiple ways. Wisi’s patch loader used
in their dplibmod project copied the patch from EE memory to PPC-IOP mem and
then loaded an IOP module to trigger patch execution. This approach is similar
but does not require an IOP module to be loaded. It instead only requires an IOP reset.

Both of these methods were made under the assumption that an IOP module
could not read or write into the 0xA00000 - 0xAFFFFF address range. It’s since
been discovered that is not the case. An AIO IOP module patch loader would
probably be superior to both of these methods.

This patch loader copies the patch from EE mem to 0xA20000 PPC-IOP mem and then
places a branch instruction to 0xA20000 in the PPC-IOP intr handler. After
issuing an IOP reset this branch will be hit and PPC execution will divert to
the patch code.

From there the patch will resize and relocate the LUTs to create a continuous
region of free memory, approximately 966KB in size. Every entry in the LUTs
corresponds to a 4 word span. Example: 0x1F808300-0x1F808310 = one entry in
the table / one function will handle load or stores for 0x1F808300-0x1F808310
There are entries for the entire 0x1f801000 - 0x1f900fff range. As a result
a lot of space is unused/wasted in each LUT. Shrinking the load and store
LUTs is accomplished by removing entries for 0x1f808600 - 0x1f900fff and
inserting a branch before a function pointer is read from the LUT. At this
branch the address range is determined and the appropriate function is called
(see load_lut_handler & store_lut_handler in patch.c).

Before Patch:

Address Range	Function	Size
`A00000` - `A0E1A4`	DECKARD ELF code	(`0xE1A4`) 57.764 KB
`A0E1A8` - `A0FFFF`	DECKARD ELF data	(`0x1E58`) 7.768 KB
`A10000` - `A7FFFF`	Unused	(`0x70000`) 458.752 KB
`A80000` - `ABFFFF`	Load LUT	(`0x3FFFF`) 262.143 KB
`AC0000` - `AFFFFF`	Store LUT	(`0x3FFFF`) 262.143 KB

Graph

After Patch:

Address Range	Function	Size
`A00000` - `A0E1A4`	DECKARD ELF code	(`0xE1A4`) 57.764 KB
`A0E1A8` - `A0FFFF`	DECKARD ELF data	(`0x1E58`) 7.768 KB
`A10000` - `A11FFF`	Load LUT	(`0x2000`) 8.192kb
`A12000` - `A13FFF`	Store LUT	(`0x2000`) 8.192kb
`A14000` - `AFFFFF`	Unused	(`0xEBFFF`) 966.655KB

Graph

pie showData title PPC RAM Usage (bytes)
    "DECKARD ELF code" : 57764
    "DECKARD ELF data" : 7768
    "Load LUT" : 8192
    "Store LUT" : 8192
    "Free memory (gap + LUT recovered space)" : 966655

Additionally, the patch starts PPC-Monitor a simple serial debugging tool that runs in the background using the emulators event system.