256, when i flash the stock 256k image onto the 512k chip it will not boot. No clue as to why. This could be a function of my programmer, not sure.
Duplicate the image into both of halfs of the 512k chip. Whats probally happening is that the additional address line on the 512k chip is not pulled down on the board so its floating and your chip is selecting addresses in the unprogrammed area of the chip. If you dup the image then it won't matter what the floating address line is doing.
That is as long as the line dosn't float into the "undefined" range and cause a latchup.