NES 2.0 Mapper 357

NES 2.0 Mapper 357 is used for a 4-in-1 multicart (cartridge ID 4602) from Bit Corp. The first game is Bit Corp’s hack of the YUNG-08 conversion of Super Mario Brothers 2 (J) named Mr. Mary 2, the other three games are UNROM games.

Unlike most multicarts, there is no menu; instead, the desired game is selected via two DIP Switches that protrude through the cartridge shell. The DIP switches directly select the outer 128 KiB PRG-ROM bank, game-specific nametable mirroring and mapper mode — outer bank 0 selects SMB2J mode, all others select UNROM mode. All four games use 8 KiB of unbanked CHR-RAM. Mirroring is Vertical for banks 0-2 and Horizontal for bank 3.

Banks in SMB2J mode

CPU $5000-$5FFF: 4 KiB PRG-ROM bank, fixed to #16
CPU $6000-$7FFF: 8 KiB PRG-ROM bank, switchable to #2 or #0
CPU $8000-$9FFF: 8 KiB PRG-ROM bank, fixed to #1
CPU $A000-$BFFF: 8 KiB PRG-ROM bank, fixed to #0
CPU $C000-$DFFF: 8 KiB PRG-ROM bank, switchable
CPU $E000-$FFFF: 8 KiB PRG-ROM bank, switchable to #10 or #8
PPU $0000-$1FFF: unbanked 8 KiB CHR-RAM

Registers in SMB2J mode

PRG Bank Select 1 ($4022, write)

Mask: probably $71FF

Bit 7654 3210
    ---------
    .... .CCC
          +++- Select 8 KiB PRG-ROM bank at CPU $C000-$DFFF.

The actual bank number is:

Value  Bank#
------------
0      4
1      3
2      5
3      3
4      6
5      3
6      7
7      3

PRG Bank Select 2 ($4120, write)

Mask: probably $71FF

Bit 7654 3210
    ---------
    .... ...I
            +- 0: CPU $6000-$7FFF to bank 2,
                  CPU $E000-$FFFF to bank 10
               1: CPU $6000-$7FFF to bank 0,
                  CPU $E000-$FFFF to bank 8

IRQ Control ($4122)

Mask: probably $F1FF

Bit 7654 3210
    ---------
    .... ...I
            +- 0: Acknowledge and disable IRQ, reset counter
               1: Enable IRQ

When enabled, the 12-bit IRQ counter increases on every M2 cycle until it overflows, upon which an IRQ is fired.