INES Mapper 172

INES Mapper 172 is used by games from Super Mega.

• 1991 賭馬 Racing (1991 Dǔmǎ Racing, "Enjoyable Horse Racing")
• 麻将方块 (Mahjong Block) (original release)
• Venice Beach Volley (Super Mega release)

Banks

• PPU $0000-$1FFF: 8 KB switchable CHR ROM bank

Registers

Mapper 172 uses a custom IC (marked "JV001") serving as a latch, adder and inverter. There are five registers: Input (6 bits), Output (6 bits), Register (6 bits), Mode (1 bit) and Invert (1 bit).

Mask: $E103
Read $4100-$4103: [..RR RRRR]: Read Register. Bits 4-5 are inverted if Invert==1. Bits 6-7 are open bus. Note that the bit order D0-D5 is reversed.
Write $4100: When Mode==0: Bits 0-5 of Register := Input, bits 0-3 being inverted if Invert==1.
             When Mode==1: Bits 0-3 of Register incremented by one, bits 4-5 unaffected.
Write $4101: Invert := Written value bit 5.
Write $4102: Input := Written value bits 0-5. Note that the bit order D0-D5 is reversed.
Write $4103: Mode := Written value bit 5.
Write $8000-$FFFF: Output := Register; written value is ignored.

In Mapper 172, bits 0/1 of the 8 KiB CHR ROM bank number (CHR A13/A14) come from Output 5/4, or 0/1 after taking the reverse bit order into account:

8 KiB CHR-ROM bank number := (Output &3);

Games will check the lower six bits of $4100 for the correct value after several increment and inversion operations, again as a copy-protection measure.

Errata

• The TXC re-release of 麻将方块 (Mahjong Block) uses INES Mapper 136.

Similar Mappers

The board is identical to INES Mapper 136 except that the CPU bits D0-D5 are connected in reverse order.