INES Mapper 218: Difference between revisions
Rainwarrior (talk | contribs) (we should not describe fantastical AxROM header configurations, this sentence is a wild goose chase. revising to be about subset compatibility instead) |
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| PPU.A13 || One-Screen, BLK1 || $A9 (1001) || $03 | | PPU.A13 || One-Screen, BLK1 || $A9 (1001) || $03 | ||
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Note: | Note: The "alternative nametables" bit 3 in Byte 6 of iNES header is commonly thought as meaning four screens, but for this mapper it is used to choose between the two one-screen layouts. | ||
The VA10 connection | The VA10 connection also affects the CHR-RAM mapping at 0000h-1FFFh. BLK1 would be the most common case (1K NT plus 1K CHR-RAM). BLK0 would allow to swap CHR RAM via Port 2000h.Bit3-4. Two-Screen would allow to use two NTs (and to squeeze CHR data into unused NT areas). Two-Screen would also allow to use 2K OBJ tiles (when leaving BG unused). | ||
Note: 1K CHR-RAM allows to use as much as 64 tiles of 2bpp (or, with suitable color attributes, 128 monochrome tiles of 1bpp). | Note: 1K CHR-RAM allows to use as much as 64 tiles of 2bpp (or, with suitable color attributes, 128 monochrome tiles of 1bpp). | ||
Also | Also note that with if reads/writes to mirrored addresses are avoided, software for this mapper can also be made compatible with [[AxROM]] ($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: | It has been used in two games, and one test ROM: | ||
Latest revision as of 10:10, 1 February 2024
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:
- Magic Floor ($A8)
- Starfight ($A8)
- OAM corruption stress test ROM ($A0)
