INES Mapper 254
iNES Mapper 254 has been allocated for Pikachu Y2K of crypted ROMs which is one of the pirate hacks of Felix the Cat.
This is basically MMC3 but WRAM read is XOR’ed in certain condition. (TODO: what is the source hardware?)
These registers are overlaid on regular MMC3 registers and works in parallel.
According to MesenCE emulation source, the PRG-RAM read value is XOR’ed with $A001 until the game writes to $8000 at least once.
It is unknown if the original hardware uses reset-detection for clearing the flag when reset. However there should be reset detection otherwise the game won’t work.
This register sets the XOR mask value to apply to PRG-RAM read value, applied until the game writes to $8000 at least once.