iNES Mapper 034 is used to designate both the BNROM and NINA-001 boards. These two boards are unrelated, but have unfortunately been grouped into the same mapper.
Many emulators implement both sets of registers simultaneously, which is safe for the two existing commercial games.
If the emulator wishes to separate the two implementations, they may check the number of CHR banks present: BNROM should be used when there is 0 to 8 KiB of CHR, and NINA-001 should be used otherwise. The NES 2.0 header with a submapper can be used to force use of specific board behavior.
Developers wishing to use mapper 34 should strictly use only the BNROM or only the NINA-001 registers in a single ROM, and be careful not to accidentally write to the other set of registers.
iNES Mapper 241 is a variation of BxROM with 8k PRG-RAM at $6000, which avoids having the conflicting NINA-001 registers in that location.
See: BNROM
Games:
Registers:
7 bit 0
---------
$8000-FFFF: .... ..PP - Select 32 KB PRG ROM bank
Emulators commonly support either a 4-bit or 8-bit register, enabling up to 512 KB or 8 MB PRG ROM. This variant may be referred to as BxROM.
See: NINA-001
Games:
Registers:
7 bit 0
---------
$7FFD: .... ...P - Select 32 KB PRG ROM bank
$7FFE: .... CCCC - Select 4 KB CHR bank at $0000
$7FFF: .... CCCC - Select 4 KB CHR bank at $1000
Categories: Nintendo licensed mappers, INES Mappers, Mappers with bus conflicts, Mappers with CHR RAM, NES 2.0 mappers with submappers