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.
Register addresses should be masked with $E003.
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.
7 bit 0 ---- ---- xxxx PPPP |||| ++++- Select 8 KB PRG ROM bank for CPU $6000-$7FFF
7 bit 0 ---- ---- xxxx Mxxx | +---- Mirroring (0: Vertical; 1: Horizontal)
7 bit 0 ---- ---- xxxx xxEx | +-- IRQ (0: Disable, Acknowledge, and Reset; 1: Enable)
The IRQ hardware counts M2 cycles and triggers after 24576 ($6000) have elapsed. FCEUX's source implies that the IRQ hardware is a 15-bit counter, and IRQ is asserted while the two MSBs are set, i.e. in the absence of CPU involvement, IRQ will be be not asserted for 24576 cycles, then asserted for 8192 cycles, and repeat endlessly.
Categories: INES Mappers, Discrete logic mappers, FDS conversion mappers, Mappers with CHR RAM