If it's for CHR-RAM, 128K would be enough for any practical use. For CHR-ROM though, 512K would be very useful.
I downloaded WinCupl from Amtel, I'm slowly figuring that out. First I think I'll get the timings for the memory access figured out, and write down some notes on how I think it would work with some mappers.
I'm looking at this doc http://nesdev.parodius.com/rom.txt, and there's a few things I need to be sure about on the cartridge connector, if any would care to explain.
pin 14: PRG R/W
when it's set, the CPU is reading or writing to memory?
pin 50: PRG /CE (/A15 & /M2)
I do not yet know what /CE is. Or /M2. A15 would be for accessing memory higher than $7FFF, right? Because the NES only 'sees' 32K of PRG-ROM, I wouldn't think it's state would ever change, as far as a mapper is concerned.
pins 21 & 56
CHR's /RD & /WR lines are seperate? I'm guessing it's that way so CHR-RAM can be used. (as opposed to having them on the same pin like PRG, which was always read-only)
With PRG R/W on the same pin, how then is a read access to ROM distinguished from a write? By looking for specific conditions on the address and data bus, perhaps?
Yeah, that a lot of questions.. But I think I'm starting to make sense of this stuff.