The consensus (?) proposal for submappers for iNES mappers 21/23/25 is as follows:
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3210 |||| |+++- Which address line corresponds is wired to the A1 in the VRC4a +---- 0: Use next lower address line for VRC4a A2; 1: use next higher line
According to the wiki page for iNES 2.0 submappers, "test ROMs" are needed before a proposal can be accepted. How exactly are these test ROMs supposed to work? I can hack up a VRC2/VRC4 test program that'll test a cartridge (or an emulator implementation) and report which address bits are connected to which lines on the mapper (including multiple bits ORed together), whether 1-screen mirroring is supported (VRC2/VRC4 difference), and whether IRQs are supported (another VRC2/VRC4 difference) if that's what the community is looking for.
As an aside, can someone explain exactly what this sentence (from the wiki VRC2 article) is supposed to mean?
Seriously, what the hell is it supposed to mean that a game is "canonically emulated" using a chip that's different from the one on the cartridge? This kind of nonsense makes me want to run screaming from the NESdev community.A third variant (similar to VRC2a but with full-width CHR registers and PRG-RAM) is canonically emulated as using the VRC4, and it's assigned to mapper 25.
ETA: Also, the fact that the ending in Ganbare Goemon Gaiden sets the CHR banks wrong on a real cartridge should probably be referenced somewhere on the wiki so emulator developers don't chase spurious bugs.