NROM and CHR RAM

[coinheaven]

so if a NROM has CHR RAM, anybody know how to put it on a cart? i tried just putting a WRAM chip on the CHR and it didn't work. is it like when making FF3, does it need to be rewired? i just did a straight solder.

[Bregalad]

Yeah exactly you'd just need to add the /WE pin so that it works as expected.
By the way it should be easier to use an UNROM devcart or something in the like if you already have one.

[tepples]

Bregalad's solution would work fine for a 16 KiB ROM (NROM-128), as long as it's properly doubled to fill the ROM. For a 32 KiB ROM (NROM-256), the PRG ROM bank loaded into $8000-$BFFF at power-on is undefined. To use it on an UNROM cart, you'll need to add a stub to the reset handler somewhere in $C000-$FFFF that maps the first bank into $8000-$BFFF:

Code: Select all

reset_stub:
  ldx #0
  stx reset+1
  jmp real_reset

Then your vectors would be nmi, reset_stub, and irq.

[Bregalad]

This isn't even necessarly, what I meant is that if you have a 32kb EPROM and a UNROM devcart, puting the 32kb EPROM on the UNROM devcart has a 6/7 probability to work. It won't work if you are unlucky enough so that bank 7 is swapped at $8000-$bfff at power up.

[qbradq]

Hey, maybe you could use that to seed your PRNG Put seven different seed values as the first byte of your low seven banks, then on power up lda $8000 and use that as your seed.

Seriously though, you should just be able to solder a wire from the Write Enable pin on the edge connector to the Write Enable pin on the RAM chip.

[Bregalad]

This won't work as I think a 74HC161 will reset in the same state. However different 74HC161's will reset in different states.

Note that this is completely untested. Maybe they all always reset to $0 or to $f, I have no way to know currently I should do more testing.

[coinheaven]

ill try it out with the an unrom board this weekend and let you know how it goes