Re: Why VAX Was the Ultimate CISC and Not RISC

Lawrence D'Oliveiro <ldo@nz.invalid> wrote:

On Fri, 7 Mar 2025 02:27:59 -0000 (UTC), Waldek Hebisch wrote:
 

VAX intstructions are very complex and much of that complexity is hard
to use in compilers.

 
A lot of them mapped directly to common high-level operations. E.g. MOVC3/
MOVC5 for string copying, and of course POLYx for direct evaluation of
polynomial functions.
 
In a way, one could say that, in many ways, VAX machine language was a
higher-level language than Fortran.

Trouble is that such "common" operations have rather low
frequency compared to simple stuff.  They are really library
functions.  String copies, if done well in microcode could
give some measurable speed gain.  Other probably not.

If they managed to make some simpler instruction faster,
there would be substantial gain.  RISC folks understood
this, but it is not clear if VAX folks were aware of this.
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.

--
                              Waldek Hebisch