> That said, Commodore was not nearly as good at Apple and Atari at keeping backward compatibility on their 65xx 8-bits, and they did something weird with the P

> That said, Commodore was not nearly as good at Apple and Atari at keeping backward compatibility on their 65xx 8-bits, and they did something weird with the Plus/4 type machines, which bizarrely had almost no compatibility at all. You could easily run software from the late 70s on a mid/late 80s Apple IIgs, or on an Atari XE. It's interesting to compare the approaches of the Apple IIgs and Apple III in this respect. The Apple III had backward compatibility, but it was deliberately limited to a no-frills Apple II compatibility mode, supposedly because Apple didn't want developers to continue targeting new software to Apple II. Apple IIgs, whose "native" architecture diverges much more from the original Apple II, had better backward compatibility (thanks in large part to the 65C816's emulation mode and the Mega II chip).