NES 2.0 Mapper 304

NES 2.0 Mapper 304 is used for several ROM cartridge conversions of Famicom Disk Systems for which, along with the entirely-different INES Mapper 043 and NES 2.0 Mapper 311, UNIF MAPR UNL-SMB2J is used.

• Super Mario Bros. 2 (J) (Whirlwind Manu LE10, hangs after completing level 4-4 on real hardware)
• Volleyball (J) (Publisher and cartridge number unknown, GoodNES 3.23b calls it an "FDS hack", which may indicate that it does not exist as a physical cartridge)
• Zanac (J) (Whirlwind Manu LF11)

Banks

• CPU $6000-$7FFF: Switchable 8 KiB PRG-ROM bank (#4-#5)
• CPU $8000-$FFFF: Fixed 32 KiB PRG-ROM bank #0
• PPU $0000-$1FFF: Unbanked 8 KiB CHR-ROM or CHR-RAM.

Registers

PRG Bank Select ($4027)

Mask: Unknown

Bit 7654 3210
    ---------
    .... ...B
            +- 0: CPU $6000-$7FFF to 8 KiB bank #4
               1: CPU $6000-$7FFF to 8 KiB bank #5

On games with only 32 KiB ROM, $6000-$7FFF will (as usual) wrap to banks #0 and #1.

IRQ Control ($4068)

Mask: Unknown

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

According to FCEUX, when enabled, IRQ counter increases on every M2 cycle until it reaches 5750, upon which an IRQ is fired. While this does work nicely with the ROM image of LE10, the hardware video recording seems to indicate that the actual hardware does not have a functioning IRQ counter.