Why does the Famicom connector have two CHR A10s? Thanks in advance!
I think one A10 is the PPU's address, and the other is A10 of the "CIRAM" (the NES PPU's 2 KB VRAM), used for holding background maps. A cart can connect CIRAM A10 to PPU A10, giving V-mirroring, or it can connect CIRAM A10 to PPU A11, giving H-mirroring, or it can connect all three lines to the mapper, giving selectable mirroring, or it can connect both PPU A10 and PPU A11 to VRAM on the cartridge and disable CIRAM, giving 4-screen mirroring.
But why doesn't NES do the same? That's what threw me off. I thought /CIRAM was what got tied to an address line to do mirroring?
The NES does do the same. A cart will output CHR /A13 to the /CIRAM enable independently of the mirroring. I think it's pretty much there so you can disable the internal chip and use the full space like I did with Squeedo.
I just consider CHR /A13 to be "nametable /CE".
But the NES doesn't bring two A10s to the connector?
It does have both A10s. The 'normal' A10 is an output from the PPU (to drive the cart memory), that's pin 62 on the edge. The other A10 (pin 22) is the actual address input to the CIRAM (and only that).
To name them properly, one pin is 'PPU A10' (output) and the other one is 'VRAM A10' (input).
Excellent! thank you! Siudym's pinout should be updated ;)