INES Mapper 218
iNES Mapper 218 is used by the homebrew Magic Floor game. The cartridge contains only one single PRG-ROM chip (and a CIC, if required).
There's no CHR-ROM or CHR-RAM. Instead, the console's internal 2Kbyte Name Table RAM is mapped as CHR-RAM. The 2K RAM is permanently selected (/VCS wired to GND), and can be used in four modes by wiring VA10 to one of the PPU.A10..A13 address lines:
|VA10 connection||Effect on nametables||iNES Flags 6||UNIF "MIRR" bit 7-0|
|PPU.A10||Two-Screen, Vertical Mirroring||$A1 (0001)||$01|
|PPU.A11||Two-Screen, Horizontal Mirroring||$A0 (0000)||$00|
|PPU.A12||One-Screen, BLK0||$A8 (1000)||$02|
|PPU.A13||One-Screen, BLK1||$A9 (1001)||$03|
Note: Bit 3 in Byte 6 of iNES header would be usually Four-Screen flag, but, for this mapper it is used as One-Screen flag.
The VA10 connection does, of course, also affect the CHR-RAM mapping at 0000h-1FFFh. BLK1 would be the most common case (1K NT plus 1K CHR-RAM). BLK0 would allow to swap CHR RAM via Port 2000h.Bit3-4. Two-Screen would allow to use two NTs (and to squeeze CHR data into unused NT areas). Two-Screen would also allow to use 2K OBJ tiles (when leaving BG unused).
Note: 1K CHR-RAM allows to use as much as 64 tiles of 2bpp (or, with suitable color attributes, 128 monochrome tiles of 1bpp).