NES 2.0 Mapper 291

NES 2.0 Mapper 291 is used for at least two Kasheng 2-in-1 multicarts both involving the game Mortal Kombat 6, a title screen hack of 餓狼伝説 Special (Garou Densetsu Special) and another game. Nestopia Plus! for some inexplicable reason reuses INES Mapper 047 for them. It's an MMC3 clone with an outer bank register.

Outer Bank Register ($6000), write

Mask: Probably $E000

7654 3210
---------
?OM. .PP.
|||   ++-- Select 32 KiB Inner PRG-ROM bank if Bit 5=1
||+------- Select PRG-ROM banking mode
||          0: Use MMC3 inner PRG-ROM bank (128 KiB)
||          1: Select 32 KiB inner PRG-ROM bank using $6000 Bits 1 and 2
|+-------- Select 128 KiB outer PRG and 256 KiB outer CHR-ROM bank
|           0: First
|           1: Second
+--------- Unknown function

The Outer Bank Register responds even when the MMC3 clone's WRAM bit is clear.

Errata

The 2-in-1 (Mortal Kombat 6, Samurai Spirits) ROM image in GoodNES 3.23b has the outer 128 KiB and 256 KiB CHR-ROM banks swapped. To run that image unmodified using the above description, the value written to $6000 would have to be XORed with 0x40.