Start and end: ResetMMC1_b4

       Name: ResetMMC1_b4                                            [Show more]
       Type: Subroutine
   Category: Start and end
    Summary: The MMC1 mapper reset routine at the start of the ROM bank
  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:
             * Vectors_b4 calls ResetMMC1_b4


 When the NES is switched on, it is hardwired to perform a JMP ($FFFC). At this
 point, there is no guarantee as to which ROM banks are mapped to $8000 and
 $C000, so to ensure that the game starts up correctly, we put the same code
 in each ROM at the following locations:

   * We put $C000 in address $FFFC in every ROM bank, so the NES always jumps
     to $C000 when it starts up via the JMP ($FFFC), irrespective of which
     ROM bank is mapped to $C000.

   * We put the same reset routine (this routine, ResetMMC1) at the start of
     every ROM bank, so the same routine gets run, whichever ROM bank is mapped
     to $C000.

 This ResetMMC1 routine is therefore called when the NES starts up, whatever
 the bank configuration ends up being. It then switches ROM bank 7 to $C000 and
 jumps into bank 7 at the game's entry point BEGIN, which starts the game.


.ResetMMC1_b4

 SEI                    ; Disable interrupts

 INC $C006              ; Reset the MMC1 mapper, which we can do by writing a
                        ; value with bit 7 set into any address in ROM space
                        ; (i.e. any address from $8000 to $FFFF)
                        ;
                        ; The INC instruction does this in a more efficient
                        ; manner than an LDA/STA pair, as it:
                        ;
                        ;   * Fetches the contents of address $C006, which
                        ;     contains the high byte of the JMP destination
                        ;     below, i.e. the high byte of BEGIN, which is $C0
                        ;
                        ;   * Adds 1, to give $C1
                        ;
                        ;   * Writes the value $C1 back to address $C006
                        ;
                        ; $C006 is in the ROM space and $C1 has bit 7 set, so
                        ; the INC does all that is required to reset the mapper,
                        ; in fewer cycles and bytes than an LDA/STA pair
                        ;
                        ; Resetting MMC1 maps bank 7 to $C000 and enables the
                        ; bank at $8000 to be switched, so this instruction
                        ; ensures that bank 7 is present

 JMP BEGIN              ; Jump to BEGIN in bank 7 to start the game