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From: anton@mips.complang.tuwien.ac.at (Anton Ertl)
Newsgroups: comp.arch
Subject: Re: Why VAX Was the Ultimate CISC and Not RISC
Date: Wed, 12 Mar 2025 08:42:28 GMT
Organization: Institut fuer Computersprachen, Technische Universitaet Wien
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antispam@fricas.org (Waldek Hebisch) writes:
>Of course, it is possible that VAX designers understood
>performace implications of their decisons (or rather
>meager speed gain from complex instructions), but bet
>that "nice" instruction set will tie programs to their
>platform.

I don't think that they fully understood the performance implications,
but I believe that creating an appealing environment for software
developers was a major consideration of the architects: For the
assembly-language programmers, provide orthogonality; that also makes
it easy to write compilers (optimility in some form is a different
story).  The much-critized VAX CALL instruction is designed for a
software ecosystem where various languages can call each other, there
exists a common debugger for all of them, etc.  I am sure that they
were aware that this call instruction was expensive, but they expected
that it was worth the cost, and also expected that implementors would
reduce the cost to below what a sequence of simpler instructions would
cost (looking at REP MOVSB in many generations of Intel and AMD CPUs,
we see such expectations disappointed; I have not measured recent
generations, though).

- anton
-- 
'Anyone trying for "industrial quality" ISA should avoid undefined behavior.'
  Mitch Alsup, <c17fcd89-f024-40e7-a594-88a85ac10d20o@googlegroups.com>