INES Mapper 027

GoodNES thinks this is used for World Hero.

We should discover whether this mapper number is used for any other unlicensed/pirate/multicart VRC2/4 games.

This is supported by BizHawk as of r3975 and FCEUX for some time prior but definitely as of 26-nov-2012.

Formerly FCEUX emulated this with unique code, but as of r2722 cah4e3 asserts that it is a dupe for INES Mapper 023. However, FCEUX's VRC2/4 emulation is filled with hacks for pirate and multicart games which are otherwise similar to VRC2/4.

In particular, World Hero is a VRC4-compatible but depends on the 9-bit CHR banking described below, where the most significant nibble of the CHR banks are actually 5 bits instead.

The below notes are left until the author agrees they are describing VRC4.

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.