why address pin 14-15 tied for a 27c256 ROM ?

Maybe it has some advantages (I'm just a hobbyist and not a hardware guy), but I guess the same effect could have been achieved by tying the NC pin to GND?

If the reason for wanting to use a higher capacity chip is that these are more readily available, you could IMHO use a 27c512 if you keep within half the capacity (32K) and program those it twice into both "halves". That is contents from 0x0000 to 0x7fff should be the same as from 0x8000 to 0xffff.

Half of its memory would be wasted / redundant.

If the reason for wanting to use a 27c512 is the additional 32K, then you'll have to cut the A15->A14 trace and somehow get the A15 address line from the CPU bus to the chip. (Make sure that it is not used otherwise though, e.g. as a simple enable line for some other memory mapped hardware, to save address decoding hardware)

Regards, Gilles.

I forget the reason, it's been ages since I used an Eprom rather than parts with internal flash memory but there was some good reason for it IIRC.

Yes, it may stop a 27512 being used on that board.

Graham

Yes, you can use a 27C512, or a MTP equivalent such as SST27SF512. All you have to do, is duplicate the 256K code, in both halves. Then, when A15 flips, an identical code copy is found.

-jg