MMC4

The Nintendo MMC4 is an ASIC mapper, used on the FxROM board set. The iNES format assigns mapper 10 to these boards. The chip first appeared in August 1988.

Overview

• PRG ROM size: 256 KB
• PRG ROM bank size: 16 KB
• PRG RAM: 8 KB + battery
• CHR capacity: 128 KB
• CHR bank size: 4 KB
• Nametable mirroring: Vertical or horizontal, controlled by program
• Subject to bus conflicts: No

Banks

• CPU $6000-$7FFF: 8 KB fixed PRG RAM bank
• CPU $8000-$BFFF: 16 KB switchable PRG ROM bank
• CPU $C000-$FFFF: 16 KB PRG ROM bank, fixed to the last bank
• PPU $0000-$0FFF: Two 4 KB switchable CHR ROM banks
• PPU $1000-$1FFF: Two 4 KB switchable CHR ROM banks

When the PPU reads from specific tiles in the pattern table during rendering, the MMC4 sets a latch that tells it to use a different 4 KB bank number. This has the effect of setting a different bank for all tiles to the right of a given tile.

• PPU reads $0FD0 through $0FDF: latch 0 is set to $FD
• PPU reads $0FE0 through $0FEF: latch 0 is set to $FE
• PPU reads $1FD0 through $1FDF: latch 1 is set to $FD
• PPU reads $1FE0 through $1FEF: latch 1 is set to $FE

Registers

PRG ROM bank select ($A000-$AFFF)

7  bit  0
---- ----
xxxx PPPP
     ||||
     ++++- Select 16 KB PRG ROM bank for CPU $8000-$BFFF

CHR ROM $FD/0000 bank select ($B000-$BFFF)

7  bit  0
---- ----
xxxC CCCC
   | ||||
   +-++++- Select 4 KB CHR ROM bank for PPU $0000-$0FFF
           used when latch 0 = $FD

CHR ROM $FE/0000 bank select ($C000-$CFFF)

7  bit  0
---- ----
xxxC CCCC
   | ||||
   +-++++- Select 4 KB CHR ROM bank for PPU $0000-$0FFF
           used when latch 0 = $FE

CHR ROM $FD/1000 bank select ($D000-$DFFF)

7  bit  0
---- ----
xxxC CCCC
   | ||||
   +-++++- Select 4 KB CHR ROM bank for PPU $1000-$1FFF
           used when latch 1 = $FD

CHR ROM $FE/1000 bank select ($E000-$EFFF)

7  bit  0
---- ----
xxxC CCCC
   | ||||
   +-++++- Select 4 KB CHR ROM bank for PPU $1000-$1FFF
           used when latch 1 = $FE

Mirroring ($F000-$FFFF)

7  bit  0
---- ----
xxxx xxxM
        |
        +- Select nametable mirroring (0: vertical; 1: horizontal)

Hardware

The MMC4 is implemented in a 44-pin TQFP package. Only one revision is known to exist.

Variants

The Nintendo MMC2, used in PxROM boards, is a similar mapper with 8 KB switchable PRG ROM banks, a 24 KB fixed PRG ROM bank, and no PRG RAM.

In theory it would be possible to make the MMC4 behave similar to a MMC2 with the help of 2 chips, a 74HC32 quad-OR gate and a 74HC02 quad-NOR game with the following diagram. It creates an hardwired bank at $a000-$bfff, and shifts all bits right so that the mapper switches 16KB banks instead of 8KB. However, the MMC2 has different behavior for the left and right banks that this does not compensate for. It is unknown if Nintendo ever exploited this possibility.

                            ___
MMC4 A17 ------------------\   `.
                            )    )------------------------ PRG A16
             +-------------/___,'
             |
             |              ___
MMC4 A16 ----(-------------\   `.
             |              )    )------------------------- PRG A15
             +-------------/___,'
             |
CPU A13 -----+----------------+---------------.
                              |   ___         |
                              `--\   `.       |   ___
                     ___          )    )o-----(--\   `.
                ,---\   `.    ,--/___,'       |   )    )--- PRG A14
MMC4 A15 -------+    )    )o--'           ,---(--/___,'
                `---/___,'                |   |
	                                  |   |
CPU A14  --------------------+------------'   |   ___
                             |     ___        `--\   `.
                             `----\   `.          )    )--- PRG A13
                     ___           )    )o-------/___,'
                ,---\   `.   ,----/___,' 
MMC4 A14  ------+    )    )o-'
                `---/___,'