NES 2.0 Mapper 331

NES 2.0 mapper 331 is used for the 7-in-1 (NS03) multicart. Its UNIF board name is BMC-12-IN-1.

Inner Bank Register A ($A000-$BFFF)

Mask: probably $E000

D~7654 3210
  ---------
  CCCC CPPP
  |||| |+++- Select 16 KiB inner PRG-ROM bank at CPU $8000-$BFFF and CPU $C000-$FFFF
  |||| |     active when the PPU is rendering from $0000-$0FFF, subject to A14-A16 modification
  ++++-+---- Select 4 KiB inner CHR-ROM bank at PPU $0000-$0FFF

Inner Bank Register B ($C000-$DFFF)

Mask: probably $E000

D~7654 3210
  ---------
  CCCC CPPP
  |||| |+++- Select 16 KiB inner PRG-ROM bank at CPU $8000-$BFFF and CPU $C000-$FFFF
  |||| |     active when the PPU is rendering from $1000-$1FFF, subject to A14-A16 modification
  ++++-+---- Select 4 KiB inner CHR-ROM bank at PPU $1000-$1FFF

Outer Bank and Mode Register ($E000-$FFFF)

Mask: probably $E000

D~7654 3210
  ---------
  .... PMOO
       ||++- Select 128 KiB outer PRG-ROM and CHR-ROM bank
       |+--- Select nametable mirroring type
       |      0: Vertical
       |      1: Horizontal
       +---- Select PRG A14-A16 mode
              0: Force PRG A14-A16 to 111b if CPU A14=1 (UNROM-like)
              1: PRG A14=CPU A14 (NROM-256)
              

It's probably a good idea to initialize the PPP bits of both inner bank registers to the same value to prevent the PRG-ROM bank at $8000-$BFFF from changing back and forth as the PPU is rendering.