INES Mapper 218

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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: The "alternative nametables" bit 3 in Byte 6 of iNES header is commonly thought as meaning four screens, but for this mapper it is used to choose between the two one-screen layouts.

The VA10 connection also affects 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).

Also note that with if reads/writes to mirrored addresses are avoided, software for this mapper can also be made compatible with AxROM ($A8,$A9), BxROM ($A0,A1) or NROM + CHR-RAM, or other suitable mappers. Magic Floor itself is well behaved enough to be emulated as AxROM or NROM + CHR-RAM with vertical mirroring.

It has been used in two games, and one test ROM: