[sillyquiet]

ok, I stand corrected, but I will say that the Apple II line represented smaller iterations of the generally same architecture, right? i.e., Commodore I guess didn't iterate within the same architecture as much, breaking that compatibility more easily.

[rbanffy]

One of the key difficulties of making a faster Apple II was the time-critical routines involved in reading/writing floppies. It was not sufficient to throttle back to 1 MHz when running time-critical loops, but one would need to emulate the timing of a 6502 doing that. Commodore made a better 6502 for the C65, but it'd wouldn't work on an Apple II because it wouldn't have the same cycle timings.

The IIgs and the //c+ was faster and very compatible thanks to a crazy number of hacks in them to acommodate Woz's brilliance.

In retrospect, Apple should have released a Disk II+ that isolated the timings from the CPU and let software break. Commodore should probably have done the same.

[timbit42]

There was a fair bit of compatibility within the PET/CBM line. Outside of that, every system had an incompatible memory map.