Compared with the Apple II and the Atari 8-bits, I suspect that Commodore was a bit more tightly painted into a corner with compatibility. The range of differe

Compared with the Apple II and the Atari 8-bits, I suspect that Commodore was a bit more tightly painted into a corner with compatibility. The range of differences between any two C64s was likely smaller than between, say, an early 16K CTIA Atari 800 versus a late 800XL, or an vanilla II versus a highly pimped Extended IIe. This likely created a software ecosystem that played it much tighter in terms of poking magic registers and relying on specifics. I wonder if the reasoning at Commodore was a general philosophical hard-line for "100% compatibility at all costs" or more a researched analysis that said "once we start changing registers X, Y, and Z, we lose major software packages alarmingly fast." They already had been burned a few times with poor platform transitions (see: the attempt to foist the C16/+4 on the market, or even the need to rewrite anything but the most trivial BASIC from the VIC-20 to the 64), so the huge software base for the 64 was too precious to risk. OTOH, they could have definitely done much more to encourage 128-native stuff. I got a 128DCR a few years ago, but I haven't really broken out the Greaseweazle and prepared anything useful to do with it. All I have is some extremely heat-warped discs that came with some garage sale C64 setup.