MMC6

The Nintendo MMC6 is a mapper ASIC used in Nintendo's NES-HKROM and HVC-HKROM Game Pak boards. This board, along with most common TxROM boards (which use the Nintendo MMC3) are assigned to iNES Mapper 004. The MMC3 and the MMC6 are alike, except for how they handle WRAM. As such, this page only showcases the differences, see MMC3 for full details on the rest of the mapper.

Overview

• PRG ROM size: Up to 512 KB
• PRG ROM bank size: 8 KB
• PRG RAM: 1 KB (internal)
• CHR capacity: Up to 256 KB ROM
• CHR bank size: 1 KB and 2 KB
• Nametable mirroring: Controlled by mapper
• Subject to bus conflicts: No

Banks

• CPU $7000-$7FFF: 1 KB PRG RAM, mirrored
• CPU $8000-$9FFF (or $C000-$DFFF): 8 KB switchable PRG ROM bank
• CPU $A000-$BFFF: 8 KB switchable PRG ROM bank
• CPU $C000-$DFFF (or $8000-$9FFF): 8 KB PRG ROM bank, fixed to the second-last bank
• CPU $E000-$FFFF: 8 KB PRG ROM bank, fixed to the last bank
• PPU $0000-$07FF (or $1000-$17FF): 2 KB switchable CHR bank
• PPU $0800-$0FFF (or $1800-$1FFF): 2 KB switchable CHR bank
• PPU $1000-$13FF (or $0000-$03FF): 1 KB switchable CHR bank
• PPU $1400-$17FF (or $0400-$07FF): 1 KB switchable CHR bank
• PPU $1800-$1BFF (or $0800-$0BFF): 1 KB switchable CHR bank
• PPU $1C00-$1FFF (or $0C00-$0FFF): 1 KB switchable CHR bank

Registers

The MMC6 has 4 pairs of registers at $8000-$9FFF, $A000-$BFFF, $C000-$DFFF, and $E000-$FFFF - even addresses ($8000, $8002, etc.) select the low register and odd addresses ($8001, $8003, etc.) select the high register in each pair. Only $8000 and $A001 are covered here. For the rest of the registers, see MMC3.

Bank select ($8000-$9FFE, even)

7  bit  0
---- ----
CPMx xRRR
|||   |||
|||   +++- Specify which bank register to update on next write to Bank Data register
||+------- PRG RAM enable
|+-------- PRG ROM bank configuration (0: $8000-$9FFF swappable, $C000-$DFFF fixed to second-last bank;
|                                      1: $C000-$DFFF swappable, $8000-$9FFF fixed to second-last bank)
+--------- CHR ROM bank configuration (0: two 2 KB banks at $0000-$0FFF, four 1 KB banks at $1000-$1FFF;
                                       1: four 1 KB banks at $0000-$0FFF, two 2 KB banks at $1000-$1FFF)

PRG RAM protect ($A001-$BFFF, odd)

7  bit  0
---- ----
HhLl xxxx
||||
|||+------ Enable writing to RAM at $7000-$71FF
||+------- Enable reading RAM at $7000-$71FF
|+-------- Enable writing to RAM at $7200-$73FF
+--------- Enable reading RAM at $7200-$73FF

Hardware

The MMC6 exists in a 64-pin TQFP package. At least two revisions exist, though only MMC6B has been observed.

The PRG RAM protect bits control mapping of two 1K banks of RAM. If neither bank is enabled for reading, the $7000-$7FFF area is open-bus. If only one bank is enabled for reading, the other reads back as zero. The write-enable bits only have effect if that bank is enabled for reading, otherwise the bank is not writable. Both banks may be enabled for reading (and optionally writing) at the same time.

When PRG RAM is disabled via $8000, the mapper simply sets $A001 to $00 and keeps it there, ignoring all writes to $A001. While disabled in this way, all writes to $A001 are ignored.