When I did this, I used a couple of 8Kbyte EPROMs and fitted them into the spare sockets on the motehrboard. I then added a bit more logic (I think just one IC) to disanble the top 8K words of the origian BIOS ROMs and enable the extra EPROMs. This was relatively easy, since the enable lines are o nthe DIP shunt at the rear edge of the motherboard. There was no real reason to do this, it just meant I could use 2764s from my junk box, whereas I would have had to order the 27256s. I didn't bother with any program other than a hex editor. I simply editied the values in the ROM image, recalculated the checksum (OK, I think I wrote a program for that), then took the last 8K words of the new image, split them into odd and even bytes, then programmed the ERPOMs. I had no trouble at all. -tony