INES Mapper 172

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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

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.

See also

PCB images