INES Mapper 178

VirtuaNesEX sources call this mapper "Education / WaiXing / HengGe". However, virtuanes is among most other emulators in not supporting it fully; a game "Pet Family ES-1081" has a larger PRG than this mapper was originally supposed to handle, and only Nestopia 1.40u4 has been supporting it. The rest of the emulators will manifest a bug in the nametables when the main game engine starts. With r2536, FCEUX supports it; bizhawk does as well.

This is a simple mapper.

  $4800:  [.... ...M]
     M = Mirror mode
         %0 = Vertical
         %1 = Horizontal
  
  $4801:  [...P PPP.]
     P = Some bits of the selected 32K PRG bank
  
  $4802:  [pppp pppp]
     p = Some more bits of the selected 32K PRG bank

The selected 32K PRG bank (mapped to $8000) is P + (p<<2). The older emulators are masking this result by & 0xF which is what breaks the larger PRG games. It is unclear whether this mask is necessary for some games to work.

This addition is pretty weird. And, unlikely, I think. Pet Family seems to boot up and get past the nametable bug using an OR instead of a + which strikes me as much more likely.

The new PRG bank is only committed when $4801 is written. Therefore, you should write $4802 first and then $4801 to complete the remapping.