Quote:
Originally Posted by SiriusHardware
If you go down the EPROM emulator route make sure you find one which can specifically emulate the MOS 6540 PROM because they have an unusual (I would go so far as to say - freakish) pinout. It could not be more different to the 2716, 2732, 2764, 27128 types which a typical EPROM emulator will emulate.
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Quite sensible from Commodore's point of view, though! You are always going to need a bunch of primitive "glue logic" -- AND / OR / NOT / EOR gates, flip-flops and the like -- to derive the output enable line for each memory IC from the higher-order address lines and the CPU clock, and you have your own chip factory; so why not bake in some general-purpose logic gates to provide multiple active-high and active-low select lines, right on the same die as the PROM? That way, you can easily pick up on various combinations of zeros and ones on the address bus to select the device -- for instance, if your chip has 4K of memory and you want it to respond to addresses 0800-0BFF then you need to connect A15, A14 and A12 to select inputs wanting a zero, A13 to a select input wanting a one, and just tie any redundant select inputs to a suitable power rail. If you wanted 0C00 to 0FFF, then A12 would need to go to an active-high select input instead.
Of course, it risks tying you to one manufacturer; but that's less of an issue when that manufacturer is also you.
(It's interesting to compare this idea of distributing the glue logic around the peripheral devices with the approach taken in the UK; which was to concentrate all the glue logic into a single device which could be placed close to the microprocessor and its bus signals .....)