INES Mapper 151

iNES Mapper 151 represents the VRC1 on the Vs. System. Given that the iNES format has a bit to mark "For Vs. System", this assignment was erroneous and any games marked thus should be mapper 75 instead.

Several emulators treat this as "mapper 75 with hardwired mirroring", rather than letting the "alternative nametables" or "Vs. System" flags override mapper 75's normal ASIC-controlled mirroring. Since Vs. System by definition has 4K of NTRAM, and no games have hardwired mirroring without 4K of NTRAM, this separation is artificial.

FCEUX-2.1.5's and Nestopia-1.4.0's implementations of mapper 151 do not attach the 5th bit of CHR bank onto the lower 4 and instead emulate all registers as 8 bits wide because no Vs. System VRC1-using games have more than 64KiB of CHR.

Pictures of relevant Vs. System modules (here or here; scroll down to "Vs. Gradius" or "Vs. The Goonies") show the same VRC1 as in NesCartDB