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).

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:

• Magic Floor ($A8)
• Starfight ($A8)
• OAM corruption stress test ROM ($A0)

References

• nesdev forum, Post subject: Single Chip Cartridge
• nesdev forum, Post subject: Using the NES's PPU RAM as CHR RAM?