INES Mapper 100

iNES Mapper 100 was assigned by fwNES to denote mapper hacks for the MMC3 emulation of Nesticle, an influential NES emulator for MS-DOS in the late 1990s. Nesticle did not directly support many of the custom ASICs by Irem, Konami and Namco, but did incorrectly emulate the MMC3 in a way that made it useful to run modified ROM images. Most of them use 512-byte trainers that must be loaded to CPU address $7000.

Nestopia uses iNES mapper 100 as a magic number for UNIF-only boards. Nintendulator uses this as a general-purpose debugging mapper, allowing PRG/CHR banks to be selected arbitrarily using a control dialog.

Bankswitch Index Register ($8000)/Data Register ($8001)

Mask: $E001 (nominally)

$00: 1 KiB CHR-ROM bank at PPU $0000-$03FF := Value AND $FE
     1 KiB CHR-ROM bank at PPU $0400-$07FF := Value OR $01
$01: 1 KiB CHR-ROM bank at PPU $0800-$0BFF := Value AND $FE
     1 KiB CHR-ROM bank at PPU $0C00-$0FFF := Value OR $01
$02: 1 KiB CHR-ROM bank at PPU $1000-$13FF := Value
$03: 1 KiB CHR-ROM bank at PPU $1400-$17FF := Value
$04: 1 KiB CHR-ROM bank at PPU $1800-$1BFF := Value
$05: 1 KiB CHR-ROM bank at PPU $1C00-$1FFF := Value
$06: 8 KiB PRG-ROM bank at CPU $8000-$9FFF := Value
$07: 8 KiB PRG-ROM bank at CPU $A000-$BFFF := Value
$46: 8 KiB PRG-ROM bank at CPU $C000-$DFFF := Value
$47: 8 KiB PRG-ROM bank at CPU $A000-$BFFF := Value
$80: 1 KiB CHR-ROM bank at PPU $1000-$13FF := Value AND $FE
     1 KiB CHR-ROM bank at PPU $1400-$17FF := Value OR $01
$81: 1 KiB CHR-ROM bank at PPU $1800-$1BFF := Value AND $FE
     1 KiB CHR-ROM bank at PPU $1C00-$1FFF := Value OR $01
$82: 1 KiB CHR-ROM bank at PPU $0000-$03FF := Value
$83: 1 KiB CHR-ROM bank at PPU $0400-$07FF := Value
$84: 1 KiB CHR-ROM bank at PPU $0800-$0BFF := Value
$85: 1 KiB CHR-ROM bank at PPU $0C00-$0FFF := Value

On an actual MMC3, changing bits 6 and 7 in the index register would immediately apply the current content of all seven bank registers to their flipped meaning. Nesticle applies these bits only on the next $8001 write, which makes eight independent 1 KiB CHR-ROM banks available: Registers $82-$85 to set four 1 KiB banks at CPU $0000-$0FFF, and registers $02-$05 to set four more 1 KiB banks at CPU $1000-$1FFF.