iNES Mapper 045 denotes multicart PCBs using the GA23C ASIC in its standard configuration. It is an MMC3 clone with four outer bank registers.
The four outer bank registers are accessed at address $6000. The first write goes to the first register, the second write to the second, and so on; the fifth write goes to the first register again. The outer bank registers overlay WRAM and function regardless of the MMC3's WRAM bits' setting.
These registers function identically to the MMC3, with the PRG/CHR bank registers subject to masking by the outer bank registers.
Mask: $F001 D~7654 3210 --------- CCCC CCCC ++++-++++- Select CHR A10-A17, OR'd with MMC3's CHR A10-A17 masked according to CHR-AND
Mask: $F001 D~7654 3210 --------- ppPP PPPP ||++-++++- Select PRG A13-A18, OR'd with MMC3's PRG A13-A18 masked according to PRG-AND ++-------- Select PRG A19-A20
Mask: $F001 D~7654 3210 --------- PPCC cccc |||| ++++- Select number of CHR bits from MMC3 |||| ($F: 256 KiB, $E: 128 KiB ... $7-$0: 1 KiB) ||++------ Select CHR A18-A19 ++-------- Select CHR A20-A21 and PRG A21-22
Mask: $F001 D~7654 3210 --------- 1LPP PPPP |++ ++++- Select PRG-AND mask (inverted) ||| ($00: 512 KiB, $20: 256 KiB...) +-------- 1=Lock outer bank registers
Writing any value to $6001 (mask: $F001) resets the outer bank registers as a soft reset would, clearing the "Lock" bit and making the next write to $6000 go to register #0.
The 超强年度新卡 (Super New Year Cart) 15-in-1 multicart has a DIP switch whose setting causes the menu program to select one of eight different menus.
A~FEDC BA98 7654 3210 D~7654 3210
------------------- ---------
0101 AAAA AAAA .... .... ...D
++++-++++-----------------+- 1=If DIP switch is in the position selected by the corresponding A bit
This means that:
Some multicarts select between five different menus by connecting one of the higher address lines to PRG /CE. The menu code selects between menus by checking which of the higher address lines disables PRG-ROM when set.
Selection 0: No address line connected to PRG /CE Selection 1: PRG A20 connected to PRG /CE Selection 2: CHR A20 connected to PRG /CE Selection 3: PRG A19 connected to PRG /CE Selection 4: CHR A19 connected to PRG /CE
Cartridges with 1 MiB of PRG-/CHR-ROM cannot use selections 3/4, cartridges with 2 MiB of more of PRG-/CHR-ROM cannot use this menu selection mechanism.
Categories: Mappers with scanline IRQs, INES Mappers, MMC3-like mappers, Multicart mappers