INES Mapper 172
INES Mapper 172 is used to denote the Super Mega P-4070 board, used by the following games:
- 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, and nametable mirroring is changed; 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 as a copy-protection measure. Unlike most of the other boards using the JV001/TXC 05-00002-010 ICs, this board allows nametable mirroring to be controlled by software. When writing to $8000-$FFFF, nametable mirroring is changed to Horizontal if Invert was clear, and to Vertical if Invert was set.
Errata
- The TXC re-release of 麻将方块 (Mahjong Block) uses INES Mapper 132.
Similar Mappers
The board is identical to INES Mapper 136 except that the CPU bits D0-D5 are connected in reverse order, and that Mapper 136 has hard-wired nametable mirroring.
