> 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

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