Utility routines: SetupMMC1

       Name: SetupMMC1                                               [Show more]
       Type: Subroutine
   Category: Utility routines
    Summary: Configure the MMC1 mapper and page ROM bank 0 into memory at $8000
  Deep dive: Splitting NES Elite across multiple ROM banks
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * ResetVariables calls SetupMMC1


.SetupMMC1

 LDA #%00001110         ; Set the MMC1 Control register (which is mapped to
 STA $9FFF              ; $8000-$9FFF) as follows:
 LSR A                  ;
 STA $9FFF              ;   * Bit 0 clear, Bit 1 set = Vertical mirroring (which
 LSR A                  ;     overrides the horizontal mirroring set in the iNES
 STA $9FFF              ;     header)
 LSR A                  ;
 STA $9FFF              ;   * Bits 2,3 set = PRG-ROM bank mode 3 = fix ROM bank
 LSR A                  ;     7 at $C000 and switch 16K ROM banks at $8000
 STA $9FFF              ;
                        ;   * Bit 4 clear = CHR-ROM bank mode 0 = switch 8K at
                        ;     a time

 LDA #0                 ; Set the MMC1 CHR bank 0 register (which is mapped to
 STA $BFFF              ; $A000-$BFFF) to map the first 4K of CHR-RAM to $0000
 LSR A                  ; in the PPU, so pattern table 0 is writable RAM
 STA $BFFF
 LSR A
 STA $BFFF
 LSR A
 STA $BFFF
 LSR A
 STA $BFFF

 LDA #0                 ; Set the MMC1 CHR bank 1 register (which is mapped to
 STA $DFFF              ; $C000-$DFFF) to map the second 4K of CHR-RAM to $1000
 LSR A                  ; in the PPU, so pattern table 1 is writable RAM
 STA $DFFF
 LSR A
 STA $DFFF
 LSR A
 STA $DFFF
 LSR A
 STA $DFFF

 JMP SetBank0           ; Page ROM bank 0 into memory at $8000, returning from
                        ; the subroutine using a tail call

IF _NTSC

 EQUB $F5, $F5, $F5     ; These bytes appear to be unused
 EQUB $F5, $F6, $F6
 EQUB $F6, $F6, $F7
 EQUB $F7, $F7, $F7
 EQUB $F7, $F8, $F8
 EQUB $F8, $F8, $F9
 EQUB $F9, $F9, $F9
 EQUB $F9, $FA, $FA
 EQUB $FA, $FA, $FA
 EQUB $FB, $FB, $FB
 EQUB $FB, $FB, $FC
 EQUB $FC, $FC, $FC
 EQUB $FC, $FD, $FD
 EQUB $FD, $FD, $FD
 EQUB $FD, $FE, $FE
 EQUB $FE, $FE, $FE
 EQUB $FF, $FF, $FF
 EQUB $FF, $FF

ELIF _PAL

 EQUB $FF, $FF, $FF     ; These bytes appear to be unused
 EQUB $FF, $FF, $FF
 EQUB $FF, $FF, $FF
 EQUB $FF, $FF, $FF
 EQUB $FF, $FF, $FF
 EQUB $FF, $FF, $FF
 EQUB $FF, $FF, $FF
 EQUB $FF

ENDIF