[bell-cot]

> a lot of people who hand coded assembler on 68k processors regard the ISA as especially elegant, powerful and fun to develop for. In many ways I think of it as peak CISC, thanks to its orthogonal instruction set and wildly flexible addressing modes.

I definitely agree...but I'd say Motorola really got carried away with those wildly flexible addressing modes. Which lead them into implementation, power draw, and gate-delay hells by the late 1980's and the 68040. The future was ever-rising transistor counts and clock speeds - and their 68k architecture just couldn't go there.

[mrandish]

> I'd say Motorola really got carried away with those wildly flexible addressing modes.

Yeah, while they could certainly be extremely powerful, I'll admit the edges of my 68000 programmer's reference card quickly got dog-eared from how often I'd need to remind myself exactly how some program-counter relative indexed redirection+offset instruction worked. Almost made me miss the days of simple 8-bit loads, stores, compares and branches being all we had.

> The future was ever-rising transistor counts and clock speeds - and their 68k architecture just couldn't go there

I've always wanted to understand more about why Motorola abandoned the 68k architecture. I understand the broad factors cited in the Wikipedia article and on RetroStackExchange but I don't recall anyone citing supporting the addressing modes specifically (though it makes sense). I never programmed x86 assembler but my sense was that ISA also had its own oddball complexities. I never understood if there was some fundamental conceptual difference between the 68k and x86 ISAs that prevented one from being able to scale into the future while the other could. Would love any more info or links if you have them handy.

[bell-cot]

Not seriously detailed, but try this:

https://userpages.umbc.edu/~vijay/mashey.on.risc.html

Another way to view it: Motorola did not have a senior 68k implementation engineer, who could look down the road and push back against cool- or easy-sounding ideas for making 68k programmers happy.

(I once heard that, with virtual memory, a single 68040 instruction could generate 16 page faults. No, that'll never happen in the real world - but once the spec' is final, the CPU implementation team has to lay out a chip that can handle every situation correctly. And if you're pipelining a sequence of "tough" instructions against corner-case data - yeah, that can be factorial hell.)

[mrandish]

Thanks for the link to that John Mashey post. It's meaty, so I'll need some time to chew through it :-)

> Motorola did not have a senior 68k implementation engineer, who could look down the road...

Yeah, this makes sense. Having started out as a complete newbie user and fanboy on 8-bit micros and then leaping to the brand new Amiga 1000 as my first 68k (because it was just so awesome), I've realized the perspective I had back then on Moto and the 68k line wasn't very complete. Reading some of the first-hand oral history from insiders in recent years shows that Motorola management made key strategic mistakes like not realizing the potential of what they had at various inflection points. The 68k was created by a new team with very little experience but which had some unusually brilliant people on it. That yielded a bold and expansive design with lots of deeply powerful aspects (like that addressing) but it may have been "too expansive" (or maybe over-complete) for what would be the first part in a long product line.

Moto also didn't seem to realize they were in an all-out, high-stakes drag race to advance silicon fabrication faster and farther than their competitors. Moto was quite the laggard both in leading edge process technology and in perfecting their leading edge manufacturing reliability/predictability. This may have just been due to Moto being a huge conglomerate with lots of divergent businesses, like selling millions of 8-bit 6803 derivatives to General Motors every year. Whereas in the 1980s, Intel could still adopt a startup mindset and singular focus when they decided it was crucial. Maybe that's the over-arching meta here. Intel bet the company on figuring out some way to scale the x86 into the future and Moto treated the 68k CPUs like they were just another line of business.

Personally, I now think of the 68k line as a beautiful swan born to distracted, mediocre parents who never really understood its potential, while the x86 was a bit of an ugly duckling born to committed, passionate, smart parents who were determined to find a way to make it successful. Maybe things would have turned out differently if the Moto board of directors knew that in 30 years the most valuable corporations in the world would all be based around silicon fabrication and IP. :-)