INES Mapper 218

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     Effect on                         iNES Byte 6   UNIF "MIRR"
 to       Name Tables                       Bit3.Bit0     Bit7-0
 PPU.A10  Two-Screen, Vertical Mirroring    0.1           01h
 PPU.A11  Two-Screen, Horizontal Mirroring  0.0           00h
 PPU.A12  One-Screen, BLK0                  1.0           02h
 PPU.A13  One-Screen, BLK1                  1.1           03h
 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).

Also note that the PPU.A13 variant can be adequately emulated as AxROM.

• http://forums.nesdev.org/viewtopic.php?t=9342 - nesdev forum, Post subject: Single Chip Cartridge
• http://forums.nesdev.org/viewtopic.php?t=5156 - nesdev forum, Post subject: Using the NES's PPU RAM as CHR RAM?
• http://nocash.emubase.de/magicflr.htm - Magic Floor game