INES Mapper 173

INES Mapper 173 is used by the original Idea-Tek releases of several of their games:

• Puzzle (ET-01)
• 小瑪琍 (Xiǎo Mǎlí) (Nei-Hu and TXC releases) (ET-02)
• F-15 City War (ET-03)
• 撲克精靈 (Poker Jīnglíng) (ET-04)

Banks

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

Registers

Mapper 173 uses a custom IC (with a fake marking "ITC20V8-10LP", falsely suggesting a particular type of programmable logic) serving as a latch, adder and inverter. There are five registers: Input (4 bits), Output (4 bits), Register (4 bits), Mode (1 bit) and Invert (1 bit).

Mask: $E103
Read $4100-$4103: [.... RRRR]: Read Register. Bit 3 is inverted if Invert==1. Bits 4-7 are open bus.
Write $4100: When Mode==0: Bits 0-3 of Register := Input, bits 0-2 being inverted if Invert==1.
             When Mode==1: Bits 0-2 of Register incremented by one, bit 3 unaffected.
Write $4101: Invert := Written value bit 0.
Write $4102: Input := Written value bits 0-3.
Write $4103: Mode := Written value bit 0.
Write $8000-$FFFF: Output := Register; written value is ignored.

In Mapper 173, bit 0 of the 8 KiB CHR ROM bank number (CHR A13) comes from Output bit 0, while bit 1 of the 8 KiB CHR ROM bank number (CHR A14) is the inverted value of the Invert register:

8 KiB CHR-ROM bank number := (Output &1) | (~Invert <<1);

Games therefore set Invert (writing $FF or $01 to $4101) to choose CHR banks 0 and 1, and clear Invert (writing $00 to $4101) to choose CHR banks 2 and 3. 小瑪琍 (Xiǎo Mǎlí) uses this for copy-protection purposes, having 32 KiB of CHR-ROM with the first 16 KiB empty, always clearing Invert and writing $0E/$0C/$08/$06/$04/$02/$00 to Input and expecting CHR A14 to remain 1. Games will also check the lower nibble of $4100 for the correct value after several increment and inversion operations, again as a copy-protection measure.

Errata

• 麻将方块 (Mahjong Block) (TXC re-release, headerless CRC32 0ACFC3CD) is commonly set to mapper 173, but since it expects Output bit 1 to select CHR A14, it actually uses INES Mapper 136.
• 撲克精靈 (Poker Jīnglíng) writes the actual 8 KiB bank number to $8000 to copy Register into Output, making it potentially CNROM-compatible. It will not run as Mapper 3 on emulators that emulate bus conflicts (such as FCEUX) though, nor will it run on an actual Nintendo CNROM board, for the same reason.
• Rad Racket - Deluxe Tennis II (TXC release, MGC-011) uses a INES Mapper 136 board but sets/clears Invert for 8 KiB CHR-ROM banks 0 and 1/2 and 3, making it compatible with Mapper 173 as well.

Similar Mappers

The custom IC was first analyzed for INES Mapper 036. INES Mapper 136 is similar to Mapper 173 except that CHR A14 is connected to Output bit 1.

See also

http://forums.nesdev.org/viewtopic.php?f=3&t=15961&start=15#p210297