NES 2.0 Mapper 391
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 444 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:
- the MMC3’s CPU A14 input is held low, so MMC3 registers 6 and 7 apply to both $8000/$A000 and $C000/$E000,
- in NROM-256 mode, PRG A14 is additionally replaced with CPU A14.
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.