INES Mapper 027

This mapper is emulated in FCEUX. Not sure which game uses it.

Mask = F00F

Writes:
9002: ......x. = If bit 2 is set, next write to register 8000 affects PRG C000-BFFF instead of PRG 8000-AFFF
8000: xxxxxxxx = Switches 8k PRG bank 8000-9FFF (or C000-DFFF)
9000: ......xx = Select mirroring (0 = Vertical, 1 = Horizontal, 2 = Single screen 0, 3 = Single screen 1)
A000: xxxxxxxx = Switches 8K PRG bank A000-BFFF
B000-E003: CHR banks.  They appear to have 9 bits.  If address written to is even (least significant bit is zero), sets the low 4 bits of the value.  If address is odd (least significant bit is one), sets the 5 high bits of the value.
  CHR Bank number affected = Top nibble of address - B, times 2, plus 1 if address & 2.
  (B000 = 4 low bits of bank 0, B001 = 5 high bits of bank 0, B002 = 4 low bits of bank 1, B003 = 5 high bits of bank 1, C000 = 4 low bits of bank 2, etc...)
  Selects the 1K CHR bank.
F000: 4 low bits of IRQ latch
F001: 4 high bits of IRQ latch
F002: ......ER = 2 bit IRQ register, bit E = enable, bit R = enable on next acknowledgement (retrigger), then sets value of IRQ counter to IRQ latch - 1.
F003: Writing here acknowledges the IRQ, and sets bit E (enable) to bit R (retrigger).

IRQ behavior:
IRQ counter is triggered by A12, just like MMC3, and happens once per scanline.
If enabled, IRQ counter counts up to FF.  If it would count up when the counter is at FF, it triggers an interrupt, then resets the IRQ counter to IRQ Latch + 1.
If disabled, IRQ does not count at all.

PRG E000-FFFF is probably fixed to the last bank.