Programming MMC1

MMC1 was Nintendo's first ASIC mapper for the NES.

Quick setup for UNROM style

If you are using SGROM or SNROM to provide an environment similar to UNROM, with 8 KB of CHR RAM, a fixed PRG ROM bank at $C000, and a 16 KB switchable PRG ROM bank at $8000, do this in your init code:

  lda #$80   ; reset the mapper
  sta $E000
  lda #$0E   ; vertical mirroring, fixed $C000, 8 KB CHR pages
  sta $8000  ; (use $0F instead for horizontal mirroring)
  lsr a
  sta $8000
  lsr a
  sta $8000
  lsr a
  sta $8000
  lsr a
  sta $8000

Very early revisions of the MMC1 IC might power up in a mode other than fixed-$C000, requiring that the vectors and the start of the init code be placed in all banks, much as in BxROM or AxROM or GxROM. Later revisions guarantee that the fixed bank is loaded at power on.

Then to switch banks, load the bank number into A and call this subroutine:

mmc1_load_prg_bank:
  sta $E000
  lsr a
  sta $E000
  lsr a
  sta $E000
  lsr a
  sta $E000
  lsr a
  sta $E000
  rts