INES Mapper 218

iNES Mapper 218 is used by the homebrew game Magic Floor. Excluding the CIC, the cartridge board only contains a single PRG-ROM chip (up to 32KiB).

There is no CHR-ROM or CHR-RAM on the cartridge. Instead, the console's internal 2KiB nametable RAM is mapped as CHR-RAM. This RAM is permanently selected (CIRAM /CE wired to GND), and can be used in four configurations by wiring CIRAM A10 to one of the PPU A10..A13 address lines:

CIRAM A10 connection	Effect on PPU nametables	Effect on PPU pattern tables	iNES Flags 6	UNIF "MIRR" bit 7-0
PPU A10	Two-screen, Horizontal arrangement ("Vertical mirroring")	2KiB (128 tiles) CHR-RAM, also mapped to nametable data	$A1 (0001)	$01
PPU A11	Two-screen, Vertical arrangement ("Horizontal mirroring")	2KiB (128 tiles) CHR-RAM, also mapped to nametable data	$A0 (0000)	$00
PPU A12	Single-screen arrangement	BLK0: 1KiB (64 tiles) CHR-RAM, "swappable" via PPUCTRL	$A8 (1000)	$02
PPU A13	Single-screen arrangement	BLK1: 1KiB (64 tiles) CHR-RAM	$A9 (1001)	$03

Notes:

• The "alternative nametable layout" (bit 3) in Flags 6 of the iNES header is used to denote the two Single-screen configurations.
• If reads/writes to mirrored addresses are avoided, software for this mapper can also be made compatible with AxROM ($A8,$A9), BxROM ($A0,A1), 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.

Software

Games:

• Magic Floor ($A8)
• Starfight ($A8)

Tests:

• OAM corruption stress test ROM ($A0)

Miscellaneous:

• Port of Apple 1 Woz Monitor ($A9)

References

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