INES Mapper 091

iNES Mapper 091 was used by the original release of Super Fighter III from an unknown publisher, but is more commonly known by a clone board from J.Y. Company.

INES Mapper 091 submapper table
Submapper #	Board	Games
0	JY830623C	Street Fighter 3, Mortal Kombat II, Dragon Ball Z 2, Mario & Sonic 2
0	YY840238C	1995 Super HIK 4-in-1 (JY-016), 1995 Super HiK 4-in-1 (JY-017)
1	EJ-006-1	Super Fighter III

Submapper 0 has a PA12-based IRQ counter and hard-wired mirroring; submapper 1 an M2-based IRQ counter and selectable mirroring.

Banks

CPU $8000-$9FFF: 8 KiB switchable PRG-ROM bank
CPU $A000-$BFFF: 8 KiB switchable PRG-ROM bank
CPU $C000-$FFFF: 16 KiB PRG-ROM bank, hard-wired to last bank
PPU $0000-$07FF: 2 KiB switchable CHR-ROM bank
PPU $0800-$0FFF: 2 KiB switchable CHR-ROM bank
PPU $1000-$17FF: 2 KiB switchable CHR-ROM bank
PPU $1800-$1FFF: 2 KiB switchable CHR-ROM bank

Registers

CHR-ROM Bank Select ($6000-$6003, write)

Mask: $F003/$F007 (submapper 0/1)

$6000: Select 2 KiB CHR-ROM bank at PPU $0000-$07FF
$6001: Select 2 KiB CHR-ROM bank at PPU $0800-$0FFF
$6002: Select 2 KiB CHR-ROM bank at PPU $1000-$17FF
$6003: Select 2 KiB CHR-ROM bank at PPU $1800-$1FFF

Nametable Mirroring Select ($6004-$6005, write, submapper 1 only)

Mask: $F007

$6004: Select horizontal mirroring, value ignored
$6005: Select vertical mirroring, value ignored

IRQ Counter Register ($6006-$6007, write, submapper 1 only)

Mask: $F007

$6006: Set IRQ Counter LSB
$6007: Set IRQ Counter MSB

PRG-ROM Bank Select ($7000-$7001, write)

Mask: $F003/$F007 (submapper 0/1)

$7000: Select 8 KiB PRG-ROM bank at CPU $8000-$9FFF
$7001: Select 8 KiB PRG-ROM bank at CPU $A000-$BFFF

IRQ Stop/Acknowledge ($7006, write)

Mask: $F003 (submapper 0)/$F007 (submapper 1)

Value is ignored.

IRQ Start/Reset ($7007, write)

Mask: $F003/$F007 (submapper 0/1)

Value is ignored.

Outer Bank register ($8000-$9FFF, write, submapper 0 only)

Mask: unknown

A~FEDC BA98 7654 3210
  -------------------
  .... .... .... .PPC
                  ||+- Select outer 512 KiB CHR-ROM bank (CHR A19)
                  ++-- Select outer 128 KiB PRG-ROM bank (PRG A17-A18)

Notes

Submapper 0: the IRQ counter is a binary counter clocked by PPU A12, and simply counts a fixed number of 64 unfiltered rises of PPU A12.
Submapper 1: the IRQ counter is a binary counter clocked by the M2 signal with a factor of 5/4, meaning counting down by five every fourth M2 cycle.
Submapper 1’s Super Fighter III runs a hardware test when resetting with Select+Start+B pressed. The test exists in J.Y. Company’s hacked versions as well, but was not adjusted for the simpler hardware and thus will fail some tests.
The commonly available ROM image of Super Fighter III is a mapper hack for INES Mapper 197.

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