MMC2

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The Nintendo MMC2 is an ASIC mapper, used on the PxROM Nintendo Game Pak boards. The iNES format assigns mapper 9 to PxROM. This chip appeared in November 1987.

Contents

Overview

  • PRG ROM size: 128 KB
  • PRG ROM bank size: 8 KB
  • PRG RAM: none by itself; a PlayChoice game contains a 6264 and 74139
  • CHR capacity: 128 KB
  • CHR bank size: 4 KB
  • Nametable mirroring: Vertical or horizontal, controlled by program
  • Subject to bus conflicts: No

Banks

  • CPU $8000-$9FFF: 8 KB switchable PRG ROM bank
  • CPU $A000-$FFFF: Three 8 KB PRG ROM banks, fixed to the last three banks
  • 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 MMC2 sets a latch that tells it to use a different 4 KB bank number. On the background layer, this has the effect of setting a different bank for all tiles to the right of a given tile, virtually increasing the tile count limit from 256 to 512 without monopolising the CPU.

  • PPU reads $0FD8: latch 0 is set to $FD for subsequent reads
  • PPU reads $0FE8: latch 0 is set to $FE for subsequent reads
  • PPU reads $1FD8 through $1FDF: latch 1 is set to $FD for subsequent reads
  • PPU reads $1FE8 through $1FEF: latch 1 is set to $FE for subsequent reads

Contrary to popular belief, the MMC2 does implement this latch behavior on both CHR ROM banks. However the behaviors for the two banks differ! The left pattern table only switches on the top row of the 8x8 tile, whereas the right pattern table switches on every row of the 8x8 tile. For a "normal" usage, sprites should use the left table, and background the right table.

Note that the latch is updated after both pattern table bytes are fetched, so the tiles $fd and $fe are always entirely fetched from the old CHR-bank, then a new latch value is set. Also since the PPU fetches 34 background tiles per scanline (and at most 33 are drawn), it is possible to rely on fetches of tiles that won't show up on the screen to set latches to a known value on the next scanline. If vertical mirroring is used, these switch tiles can therefore be completely invisible.

Registers

PRG ROM bank select ($A000-$AFFF)

7  bit  0
---- ----
xxxx PPPP
     ||||
     ++++- Select 8 KB PRG ROM bank for CPU $8000-$9FFF

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 MMC2 is implemented in a 40-pin shrink-DIP package. At least two revisions are known to exist, the MMC2 and the MMC2-L.

A pirate clone that exclusively uses discrete logic has been found and reverse-engineered. [1]

Variants

Nintendo's MMC4, used in the FxROM board set, is a similar mapper with PRG RAM support and PRG bank sizes of 16kb instead of 8kb. It also suppresses the different banking behavior of the left pattern table.

Because of the extreme similarity between both chips (MMC2 and MMC4), it is possible to make a circuit that simulates a MMC4 from a MMC2, although, it is doubtful whether Nintendo ever exploited this publicly.

For example, with the help of a 7402 quad-NOR gate and a 7420 4-input NAND gate to decode PRG RAM, one can make the MMC2 act like an MMC4. The following circuit "tricks" the MMC2 into thinking the program is still in the $8000-$9fff range when reading from $A000-$BFFF, but doesn't affect mapper writes. It also shifts all addresses left one bit so that it switches 16kB instead of 8kB banks, and it shortcuts around the different behavior for pattern tables at $0000 and $1000.

MMC2 A16  ----------------------------------  PRG A17

MMC2 A15  ----------------------------------  PRG A16

MMC2 A14  ----------------------------------  PRG A15
                ____              ___
MMC2 A13  -----\    `.       ,---\   `.
                )     )o-----+    )    )o---  PRG A14
CPU A14   -----/____,'       `---/___,'  

CPU A13   ---+------------------------------  PRG A13
             |    ___
             +---\   `.         ___
             |    )    )o------\   `.
             `---/___,'         )    )o-----  MMC2 A13
                          ,----/___,'
R/W       ----------------'

GND       --------------------+-------------  MMC2 PA2
                              |
                              +-------------  MMC2 PA1
                              |
                              `-------------  MMC2 PA0
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