NES 2.0 Mapper 391 denotes the MMC3-based BS-110 PCB. It is a variation of INES Mapper 052 that adds support for GNROM-like PRG bankswitching. Note that the UNIF MAPR BMC-BS-110 is incorrectly-named and refers to NES 2.0 Mapper 334 instead.
Mask: $E000 A~FEDC BA98 7654 3210 D~7654 3210 ------------------- --------- 011. ...C .... .... LcGC pNPP | |||| ||++- PRG A17..A18 | |||| |+--- NROM mode type if G=1 | |||| | 0=NROM-128 | |||| | 1=NROM-256 | |||| +---- Outer PRG bank size | |||| 0=256 KiB (PRG A17 from MMC3) | |||| 1=128 KiB (PRG A17 from outer bank register) | |||+------ CHR A17 | ||+------- PRG banking mode | || 0=MMC3 PRG banking mode | || 1=GNROM-like PRG banking mode | |+-------- Outer CHR bank size | | 0=256 KiB (CHR A17 from MMC3) | | | 1=128 KiB (CHR A17 from outer bank register) | +--------- 1=Lock outer bank register +------------------------ CHR A18
GNROM mode means that:
As it uses the MMC3 clones's WRAM interface, writing to the outer bank register requires enabling and not write-protecting WRAM in the MMC's $A001 register.
Mask: $E001 See MMC3.
Categories: MMC3-like mappers, Mappers with scanline IRQs, Multicart mappers