INES Mapper 173
INES Mapper 173 is used to denote an unmarked board used by the original Idea-Tek releases (cartridge codes ET-xx) of several of their games:
- Puzzle (ET.01)
- 小瑪琍 (Xiǎo Mǎlí) (Nei-Hu and Idea-Tek releases, with identical cartridge shell and label) (ET.02)
- F-15 City War (ET.03)
- 撲克精靈 (Poker Jīnglíng) (ET.04)
- 戰國四川省 (Zhànguó Sìchuān Shěng) (ET.05), developed by Computer & Entertainment
- PPU $0000-$1FFF: 8 KB switchable CHR ROM bank
Mapper 173 uses a custom IC (real number 05-00002-010, fake marking "ITC20V8-10LP", falsely suggesting a particular type of programmable logic) serving as a latch, adder and inverter. There are six registers: "P", "R", and Output (three bits each); "S", inCrement, and and inVert (one bit each).
Mask: $E100 read $4100: [xxxx SRRR] |||| |||| |||| ++++- Copy internal registers 'RRR' and 'S' XOR 'V' to data bus. ++++------ open bus Mask: $E103 write $4100: If Increment is set, internal register 'RRR' <- 'RRR'+1 Otherwise, if Invert is clear, copy internal register 'PPP' to 'RRR' if Invert is set, copy '~PPP' to 'RRR' 'S' is not changed at all. write $4101: [.... ...V] - Invert Mode. This value is immediately inverted and used as CHR A14. write $4102: [.... SPPP] - Copy data bus to internal registers 'S' and 'PPP'. 'S' can be read back immediately; 'PPP' must be copied using $4100 first. write $4103: [.... ...C] - Increment Mode Mask: $8000 write $8000: copy bottom two bits of internal register 'RRR' to CHR A15 and CHR A13 banking pins, in order.
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.
Notes and Errata
- 小瑪琍 (Xiǎo Mǎlí) uses the Mapper 173 circuit board but replaces the 32 KiB CHR-ROM chip with a SGS M2764A 8 KiB UVEPROM. Its pin 27 function is /PGM instead of the normal A14. Setting Invert on this board just disables CHR-ROM. The game's protection involves reading from $4100 and disabling the PPU if the result does not match the expected value.
- 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 132.
- 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 TXC 01-22111-100 (INES Mapper 132) 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.
- 戰國四川省 (Zhànguó Sìchuān Shěng, original version of AVE's Tiles of Fate) is set to Mapper 132 in GoodNES 3.23b. That ROM image is actually a mapper hack with the PRG-ROM code unmodified but the CHR-ROM banks rearranged to work as Mapper 132; the correct mapper is INES Mapper 173.
- INES Mapper 173 is similar to INES Mapper 132 with only 32 KiB PRG-ROM except that Mapper 132 connects CHR A14 to Output bit 1 rather than the inverse of the Invert bit.