INES Mapper 119
iNES Mapper 119 is used to designate the TQROM board, which uses the Nintendo MMC3 like other TxROM boards but uses the CHR bank number in such a way so as to allow both CHR ROM and CHR RAM to be used simultaneously.
Registers
See MMC3
Bank data ($8001-$9FFF, odd)
Bit 6 of the CHR bank number is used to switch between the CHR ROM chip and the CHR RAM chip.
7 bit 0
---- ----
xCDD DDDD
||| ||||
|++-++++- New bank value, based on last value written to Bank select register
| 0: Select 2 KB CHR bank at PPU $0000-$07FF (or $1000-$17FF);
| 1: Select 2 KB CHR bank at PPU $0800-$0FFF (or $1800-$1FFF);
| 2: Select 1 KB CHR bank at PPU $1000-$13FF (or $0000-$03FF);
| 3: Select 1 KB CHR bank at PPU $1400-$17FF (or $0400-$07FF);
| 4: Select 1 KB CHR bank at PPU $1800-$1BFF (or $0800-$0BFF);
| 5: Select 1 KB CHR bank at PPU $1C00-$1FFF (or $0C00-$0FFF);
| 6, 7: As standard MMC3
+-------- Chip select (for CHR banks)
0: Select CHR ROM; 1: Select CHR RAM
Nintendo could have used bit 7 instead of bit 6, allowing 128 KiB of CHR ROM instead of 64 KiB. However, using bit 7 would have precluded a hypothetical "TQSROM" board combining TQROM-style ROM/RAM mixing with TLSROM-style single-screen mirroring. Rare developed both games on this board, and Rare was also a big fan of single-screen mirroring on the AxROM boards.
Hardware
A 74HC32 IC controls the CHR ROM's enable line. PPU A13 is ORed with CHR A16 in order to create a /CE signal for the CHR ROM. CHR A16 also directly connects the positive CE pin on the 6264 SRAM used as CHR RAM.
For some totally unknown reason (board space?) official TQROM boards doesn't allow PRG ROMs greater than 128 KB. However, a third party board could expand this mapper up to 512 KB of PRG ROM, like any other MMC3 boards, and split a 128 KiB CHR ROM between banks 0-63 and banks 128-191.
Variants
iNES Mapper 074 and iNES Mapper 192 are similar to mapper 119 except they replace fewer CHR ROM banks with RAM.
References
- NES-TQROM by Kevin Horton
