INES Mapper 042

iNES Mapper 042 was used for hacked FDS games converted to cartridge form. At least two examples are known: Ai Senshi Nicol and "Mario Baby" (really Bio Miracle Bokutte Upa). While they likely used different hardware, they can both be encompassed by a single mapper.

Overview

• PRG ROM size: 128 KiB
• PRG ROM bank size: 8 KiB
• PRG RAM: No
• CHR capacity: 128KiB ROM or 8KiB RAM
• CHR bank size: 8 KiB
• Nametable mirroring: Mapper controlled
• Subject to bus conflicts: ??

Banks

• CPU $6000-$7FFF: 8KiB switchable PRG ROM bank
• CPU $8000-$FFFF: Fixed to last 32 KiB of PRG ROM
• PPU $0000-$1FFF: 8KiB switchable CHR ROM bank or 8KiB fixed CHR RAM

Registers

Register addresses should be masked with $E003.

CHR Select ($8000)

7  bit  0
---- ----
xxxx CCCC
     ||||
     ++++- Select 8 KB CHR ROM bank for PPU $0000-$1FFF

This register is ignored on Bio Miracle Bokutte Upa, which uses 8KiB CHR RAM instead.

PRG Select ($E000)

7  bit  0
---- ----
xxxx PPPP
     ||||
     ++++- Select 8 KB PRG ROM bank for CPU $6000-$7FFF

Mirroring Control ($E001)

7  bit  0
---- ----
xxxx Mxxx
     |
     +---- Mirroring (0: Vertical; 1: Horizontal)

IRQ Control ($E002)

7  bit  0
---- ----
xxxx xxEx
       |
       +-- IRQ (0: Disable, Acknowledge, and Reset; 1: Enable)

The IRQ hardware counts M2 cycles and triggers after 24576 have elapsed.