NES 2.0 mapper 339 is used for a 21-in-1 multicart. Its UNIF board name is BMC-K-3006.
Mask: $E000
A~FEDC BA98 7654 3210
-------------------
0110 .... ..PO OIIi
|| |+++- Select 16 KiB inner PRG-ROM bank at CPU $8000-$BFFF and
|| | $C000-$FFFF (NROM-128) if P=0.
|| | If P=0 and II=6, i is replaced with CPU A14 (NROM-256).
|+-+---- Select 128 KiB outer PRG-ROM and CHR-ROM bank
+------- Select PRG banking mode
0: NROM, use OO and IIi bits
1: MMC3, use only OO bits
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: Mappers with scanline IRQs, MMC3-like mappers, Multicart mappers