MMC2

MMC2
PxROM

Company	Nintendo
Games	1 in NesCartDB
Complexity	ASIC
Boards	PNROM, PEEOROM
Pinout	MMC2 pinout
PRG ROM capacity	128K
PRG ROM window	8K + 24K fixed
PRG RAM capacity	8K (PC10 ver.)
PRG RAM window	Fixed
CHR capacity	128K
CHR window	4K + 4K (triggered)
Nametable mirroring	H or V, switchable
Bus conflicts	No
IRQ	No
Audio	No
iNES mappers	009

The Nintendo MMC2 is an ASIC mapper, used on the PNROM and PEEOROM Nintendo Game Pak boards for Mike Tyson's Punch Out!!. The iNES format assigns Mapper 009 to PxROM. This chip appeared in November 1987.

Banks

• CPU $6000-$7FFF: 8 KB PRG RAM bank (PlayChoice version only; contains a 6264 and 74139)
• 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

The two 4 KB PPU banks each have two 4 KB banks, which can be switched during rendering by using the special tiles $FD or $FE in either a sprite or the background. See CHR banking below.

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)

CHR banking

The main interest of the MMC2 and MMC4 mappers is that they allow switching two pairs of 4 KB CHR-ROM banks at the same time, automatically alternating between banks within one pair during rendering. When the PPU reads from specific tiles ($FD or $FE) in the pattern tables, the MMC2/4 sets a latch that switches between two different 4 KB banks. This allows the tile limit to increase from 256 to 512 with bank splits, without involving the CPU or an IRQ.

• 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

Notice that latch 0 only responds to one address, but latch 1 responds to a range of addresses. This means that:

• The left ($0000-0FFF) pattern table only switches on the top row of the 8x8 tile
• The right ($1000-1FFF) pattern table switches on every row of the 8x8 tile

With this mapper, the left pattern table ($0000) is intended for use with sprites, and the right pattern ($1000) table for background. Backgrounds require a switch on every row. Because sprites aren't constrained to an 8x8 grid, triggering on only the first row allows switching sprites to be placed closer together if needed. This nuance is absent in the MMC4, where both pattern table switches from the entire tiles in a symmetrical fashion.

Note that the latch is updated after either pattern table byte is fetched, so the switching tiles $FD or $FE themselves are drawn using the old CHR-bank before the latch value is changed. As the PPU fetches 34 background tiles per scanline (and at most 33 are drawn), if vertical mirroring is used, background switching tiles can be placed past the edge of the screen where they will be unseen.

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.

The PEEOROM board is used in the re-issue of Mike Tyson's Punch-Out!!. Unlike PNROM, and unlike most other boards used in NES Game Paks sold to the public, it can be configured to support EPROM memory through jumpers on the board.

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

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 the MMC2 and MMC4, it is possible to make a circuit that simulates an MMC4 from an MMC2 with the help of a 7402 quad-NOR gate and a 7420 4-input NAND gate to decode PRG RAM. 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

See also

• Nintendo MMC2 01/29/98 by Jim Geffre.
• Comprehensive NES Mapper Document by \Firebug\, information about mapper's initial state and lates are inaccurate.
• NES Mapper list by Disch.