NES 2.0 Mapper 274

NES 2.0 Mapper 274 denotes the Cartridge Story series of multicarts from RCM Group. Its UNIF board name is BMC-80013-B.

Cartridge Story (72-in-1)
Cartridge Story II (80-in-1)
Cartridge Story III (90-in-1, a.k.a. 90-in-1 Hwang Shinwei)

Cartridge Story only contains a single main 2 MiB PRG-ROM chip; II and III both add an extra PRG-ROM chip.

Banks

CPU $8000-$BFFF: Switchable inner and outer PRG-ROM bank, main or extra PRG-ROM chip
CPU $C000-$FFFF: Fixed inner and switchable outer PRG-ROM, main PRG-ROM chip only
PPU $0000-$1FFF: 8 KiB of CHR-RAM

Registers

Inner Bank and Mirroring ($8000-$9FFF, write)

Mask: $E000

D~7654 3210
  ---------
  ...M BBBB
     | ++++- Select PRG A17..A14 if CPU A14=0
     +------ Select nametable mirroring
             0: Vertical
             1: Horizontal

Outer Bank and Chip Select ($A000-$FFFF, write)

Mask: $E000

D~7654 3210  A~FEDC BA98 7654 3210
  ---------    -------------------
  .OOO oooo    1C1. .... .... ....
   ||| ||||     +------------------ Select PRG-ROM Chip if CPU A14=0
   ||| ||||                          0: Extra (default)
   ||| ||||                          1: Main
   ||| ++++- Select PRG A17..A14 if CPU A14=1
   +++------ Select PRG A20..A18

Notes:

Since the extra PRG-ROM chip only has 16 KiB (Cartridge Story II)/64 KiB (Cartridge Story III), PRG A16-A20 are ineffective when C=1. Because this yields a PRG-ROM section of the NES 2.0 file that is not a power of two, emulators must mask them off explicitly.
The main PRG-ROM chip is always selected when CPU A14=1.
Because C is set to 0 upon reset, the cartridge will boot with the extra PRG-ROM chip mapped, if present.

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