View source for MMC4

[[Category:ASIC mappers]]
The '''Nintendo MMC4''' is an [[:Category:ASIC mappers|ASIC]] [[MMC|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 conflict]]s: 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. 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 $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 [[MMC4 pinout|44-pin TQFP package]].
Only one revision is known to exist.

== Variants ==
Nintendo's [[MMC2]], used in [[PxROM]] boards, is a similar mapper with 8 KB switchable PRG ROM banks, a 24 KB fixed PRG ROM bank, no PRG RAM, and a slightly different behaviour in auto-switching on the left pattern table.