INES Mapper 219
iNES Mapper 219, also known as "UNL A9746", is sort of like an MMC3 without fixed banks, and some odd CHR banking.
- PRG: 4x8KiB (none fixed), 4 bits = 128KiB max
- CHR: 8x1KiB, 12 bits = 4MiB max
Mask: $E003 $8000 Select Register 1 $8002 Select Register 2 $8001 Control Register Whichever of $8000, $8002 is last written decides what $8001 writes do.
It appears to have MMC3-like IRQ registers, as the implementation just falls through and uses those directly.
Mask $E001 $C000 IRQ latch $C001 IRQ reload $E000 IRQ disable $E001 IRQ enable
CHR banking is very odd. The following bits are ORed together: Latch: CBA98765.... [write << 4] Inner: .....7654321 [write >> 1] and, if the bank is $0400 or $0C00, the least significant bit is further ORed with 1.
This has the effect of allowing consecutive writes to adjacent bank regs to get "sort of" a 2k bank. Nestopia also ANDs the least significant remaining bit with 0 for the $0000 and $0800 pages, thereby effectively setting it to A10 for the left pattern table ($0000-$0FFF), but the coding appears to also mask off bits 765 from the lower write in this operation. As they can be set by the latch anyway, it is unclear if this is intentional.
|$8000 write||$8002 write||bits||$8000 effect|
|$26||-||..PPPP..||(reversed) bits of PRG bank $8000-$9FFF, central nibble only|
|$25||-||..PPPP..||(reversed) bits of PRG bank $A000-$BFFF, central nibble only|
|$24||-||..PPPP..||(reversed) bits of PRG bank $C000-$DFFF, central nibble only|
|$23||-||..PPPP..||(reversed) bits of PRG bank $E000-$FFFF, central nibble only|
|-||$08-$1e(even)||CBA98765||Outer CHR bit latch for any 1k bank, 8 bits|
|-||$09||765432*.||Inner 7 CHR bits for $0000-$03FF (*least bit ANDed to 0? See above.)|
|-||$0B||765432*.||Inner 7 CHR bits for $0400-$07FF (*least bit ORed to 1 regardless)|
|-||$0D||765432*.||Inner 7 CHR bits for $0800-$0BFF (*least bit ANDed to 0? See above.)|
|-||$0F||765432*.||Inner 7 CHR bits for $0C00-$0FFF (*least bit ORed to 1 regardless)|
|-||$11||7654321.||Inner 7 CHR bits for $1000-$13FF|
|-||$15||7654321.||Inner 7 CHR bits for $1400-$17FF|
|-||$19||7654321.||Inner 7 CHR bits for $1800-$1BFF|
|-||$1D||7654321.||Inner 7 CHR bits for $1C00-$1FFF|