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).

• http://forums.nesdev.org/viewtopic.php?f=2&t=9342 - nesdev forum, Post subject: Single Chip Cartridge
• http://nocash.emubase.de/magicflr.htm - Magic Floor game