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From: gnu@l5.uucp (John Gilmore)
Newsgroups: net.arch
Subject: Are "discrete" CPUs faster than VLSI?  And why?
Message-ID: <277@l5.uucp>
Date: Sat, 23-Nov-85 15:16:44 EST
Article-I.D.: l5.277
Posted: Sat Nov 23 15:16:44 1985
Date-Received: Mon, 25-Nov-85 07:49:48 EST
References: <1795@peora.UUCP>
Organization: Nebula Consultants in San Francisco
Lines: 29

In article <1795@peora.UUCP>, jer@peora.UUCP (J. Eric Roskos) writes:
> More generally, ...CPUs built with discrete
> components (as vs. all on a single IC) tend to be faster.  This was
> something I was skeptical of myself back during my days of faith in
> the microprocessor, and I still tend to believe that is true largely due
> to practical considerations than theoretical ones.

There are plenty of counterexamples to this, too...e.g. the Vax 785 and
Sun-3, the IBM 1401 and the Apple ][, etc.  But let's restrict the
discussion to things that were designed at the same time, in which case
you're mostly right.

I think the reasons are complexity and generality.  VLSI chips can only
handle so much complexity -- once you exceed the number of gates you can
fit, you have to go to a board-level design.  This was the constraint
on Vax-on-a-chip for a number of years.  This is especially true in very
fast logic families like ECL.

The generality comes in when you are trying to make money at VLSI CPU
design.  You can't afford to design it exactly to one application's
specs, so it won't be as well suited for that application as something
that *was* designed down to the PALs and gates for that application.
On the other hand, due to hardware production volumes and software
availability, the "nonoptimal" VLSI CPU design may be a lot cheaper to
build, so cost/performance can come out ahead.

Another way to look at it is:  If you could get the speed you wanted by
designing in a VLSI CPU, why would you *bother* to design with discrete ICs?
That's why discrete CPUs are faster than VLSI CPUs.  :-)