INES Mapper 056

iNES Mapper 056 reportedly represents a specific unlicensed reproduction of Super Mario Bros. 3

Banks

• CPU $6000-$7FFF: 8 KB PRG RAM bank (not battery backed!)
• CPU $8000-$9FFF: 8 KB switchable PRG ROM bank
• CPU $A000-$BFFF: 8 KB switchable PRG ROM bank
• CPU $C000-$DFFF: 8 KB switchable PRG ROM bank
• CPU $E000-$FFFF: 8 KB semi-fixed PRG ROM bank (banks 15 or 31)
• PPU $0000-$03FF: 1 KB switchable CHR bank
• PPU $0400-$07FF: 1 KB switchable CHR bank
• PPU $0800-$0BFF: 1 KB switchable CHR bank
• PPU $0C00-$0FFF: 1 KB switchable CHR bank
• PPU $1000-$13FF: 1 KB switchable CHR bank
• PPU $1400-$17FF: 1 KB switchable CHR bank
• PPU $1800-$1BFF: 1 KB switchable CHR bank
• PPU $1C00-$1FFF: 1 KB switchable CHR bank

Registers

IRQ Reload value ($8000-$BFFF)

Writes set IRQ reload value, in four four-bit words, same as VRC3

IRQ Enable ($C000-$CFFF)

The only game writes $FF to enable, and $00 to disable. But it's credible that this register is also like VRC3.

IRQ Acknowledge ($D000-$DFFF)

Any write to this register will acknowledge the pending IRQ, like VRC3

Bank select ($E000-$EFFF)

Select one of three bank registers to update on next write to $F000.

7  bit  0
---- ----
.... ..RR
       ||
       ++- Specify which bank register to update on next write to Bank Data register
           0: Writes to $F000 have no effect
           1: Select 8 KB PRG bank at CPU $8000-$9FFF
           2: Select 8 KB PRG bank at CPU $A000-$BFFF
           3: Select 8 KB PRG bank at CPU $C000-$DFFF

Bank data ($F000-$FFFF)

Note that this register is logically superimposed on the next thirteen registers.

7  bit  0
---- ----
.... DDDD
     ||||
     ++++- New bank value, based on last value written to Bank select register (mentioned above)

PRG bank upper bits ($F000-$F3FF)

Note that these registers are superimposed on the Bank data register.

Mask: $FC03

$F000: [...P ....] - select PRG ROM A17 during reads from $8000-$9FFF
$F001: [...P ....] - same, for reads from $A000-$BFFF
$F002: [...P ....] - same, for reads from $C000-$DFFF
$F003: [...P ....] - same, for reads from $E000-$FFFF

The game relies on these four registers powering up holding 1.

The original hardware dummies out the upper three bits, but they could have been connected.

The above six registers are intended be used together as

 LDX #2
 STX $E000
 LDA bankA000
 STA $F001

Mirroring control ($F800-$FBFF)

Note that this register is superimposed on the Bank data register.

Mask: $FC00

$F800: [.... ...M] - 0: H (PPU A11), 1: V (PPU A10)

CHR banking control ($FC00-$FFFF)

Note that these registers is superimposed on the Bank data registers.

Mask: $FC07

 $FC00 [.CCCCCCC] - select 1kB CHR for $0000-$03ff
 $FC01 [.CCCCCCC] - select 1kB CHR for $0400-$07ff
 $FC02 [.CCCCCCC] - select 1kB CHR for $0800-$0bff
 $FC03 [.CCCCCCC] - select 1kB CHR for $0c00-$0fff
 $FC04 [.CCCCCCC] - select 1kB CHR for $1000-$13ff
 $FC05 [.CCCCCCC] - select 1kB CHR for $1400-$17ff
 $FC06 [.CCCCCCC] - select 1kB CHR for $1800-$1bff
 $FC07 [.CCCCCCC] - select 1kB CHR for $1c00-$1fff

The original hardware dummies out the top bit, but it could have been connected.

See also

• krzysiobal's reverse-engineering notes
• nocash's documentation