INES Mapper 218: Difference between revisions
(added starfight) |
(Magic Floor works on AxROM and NROM boards) |
||
| Line 22: | Line 22: | ||
Also note that the PPU.A13 variant can be adequately emulated as [[AxROM]] provided the program never writes to $8000-$FFFF and is marked as having 1024 bytes of CHR RAM in [[NES 2.0]]. | Also note that the PPU.A13 variant can be adequately emulated as [[AxROM]] provided the program never writes to $8000-$FFFF and is marked as having 1024 bytes of CHR RAM in [[NES 2.0]]. | ||
''Magic Floor'' itself is well behaved enough to be emulated as AxROM or NROM with vertical mirroring. | |||
* [//forums.nesdev.org/viewtopic.php?t=9342 nesdev forum, Post subject: Single Chip Cartridge] | * [//forums.nesdev.org/viewtopic.php?t=9342 nesdev forum, Post subject: Single Chip Cartridge] | ||
Revision as of 19:11, 5 February 2021
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 the PPU.A13 variant can be adequately emulated as AxROM provided the program never writes to $8000-$FFFF and is marked as having 1024 bytes of CHR RAM in NES 2.0. Magic Floor itself is well behaved enough to be emulated as AxROM or NROM with vertical mirroring.
