I was looking into some VRC2\4 stuff today. One thing that struck me was the supposed 9 bit chr regs on VRC4. All of the VRC4 carts on nescartdb are only 256kB CHR or less, only requiring 8 bit chr regs. Are we sure that this is a behavior that's not only for some pirate VRC4 clone carts? If 9 bit chr is confirmed, could anyone tell me what game(s) it's used on?
No games rely on it. Please see
this thread where l_olivera traced the pins and found something that could have been connected to what would have been CHR A18 on a 23C4000 mask ROM. I subsequently asked Bootgod to verify the pin's functionality: it
does work.
Interesting and thanks. Making a note of it.
Looks like they abandoned the idea of 512kB chr rom as uneconomical, as neither VRC6 nor VRC7 supports it.
Coming back to this: actually, the VRC6 does support it! In a very funny way. We only recently learned the full weirdness of the VRC6's $B003 register. It supports a mode with four 2KiB slices for CHR, where either the VRC6 emulates connecting PPU A10 to CHR A10 for a maximum of 256KiB CHR, or else all the full latched value is presented and PPU A10 has to be manually connected to CHR A10 for a maximum of 512KiB CHR.
That said, they never used it there, either.
I had heard of the "awesome new modes" in the VRC6, but I hadn't sat down to look at it until now. I think I'm understanding it. One question that would help me: Where on the VRC6 is the CIRAM A10 pin signal connected?
CHR A10. I'll update the pinout on the wiki.