> On Nov 22, 2023, at 6:06 PM, Fred Cisin via cctalk <cctalk@classiccmp.org>
> wrote:
>
> ...
> Motorola tended to redesign from scratch, whereas Intel would modify their
> previous design.
Which might explain why the x86 ISA is such a convoluted tangle.
Moto did those things too in various places, though: consider the 68k family.
The 68040 is clearly a derivative design but with a pile of things added.
Come to think of it, creating an ISA once and extending it N times is standard
industry practice, from PDP11 to VAX to Alpha to MIPS to Power, not to mention
earlier examples like CDC 6000/170/180 series, various IBM families,
Electrologica... I'd say it is probably harder to come up with singleton
designs, especially several of them from a single company, than families.
The value of N tends to depend on how well conceived the original architecture
is (which is why N is smaller for PDP11 and VAX and MIPS than it is for x86)
and also on the longevity of the family. So while Intel made a very large
family starting with the 4004, they made very small families out of the i960,
iAPX432, and various others.
paul
