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From: guy%gorodish@Sun.COM (Guy Harris)
Newsgroups: comp.arch
Subject: Re: The 360 was a design landmark  (360 vs vax)
Message-ID: <26603@sun.uucp>
Date: Wed, 26-Aug-87 15:27:01 EDT
Article-I.D.: sun.26603
Posted: Wed Aug 26 15:27:01 1987
Date-Received: Fri, 28-Aug-87 06:39:24 EDT
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> >seem to be doing quite well for themselves.
> Hoding, maybe....many "VAX-labs" are buying into non-DEC stuff
> (MIPS, Sun, Pyramid, Sequent, et.al.)

Yes, but note that a lot of VAX customers are not "VAX labs"; I believe VAXes
are going about 50% to commercial and 50% to technical applications.

The question is "is there any market in which the VAX is still increasing its
market share?"  If the answer is "yes", then in that market they are neither
"losing ground" nor "holding".

I think, obviously, that it would be Truly Wonderful if all those VAXes out
there could be replaced by Suns, but I'm not going to assume that VAXes and VMS
aren't still strong competitors and aren't going to remain so, at least in the
short term.

> The 8600 overlaps operand-decode with operand-fetch, and uses
> multiple functional (execution) units, but **UNLIKE** IBM and any
> other true pipe-line design, can *NOT* have multiple instructions
> in the decode phase simultaniously!

I.e., a machine that has, say, a fetch/decode unit and an execute unit, with
the fetch/decode unit being one instruction ahead of the execute unit, is not
pipelined?  In other words, your definition of "pipelined" is "has multiple
instructions in the decode phase simultaneously"?  Is this the only definition
of "pipelined" commonly used?

You may not be able to get *as much* parallelism from a pipelined
implementation of the VAX architecture as you can out of a pipelined
implementation of the 370 architecture, but this is very different from "you
can't pipeline VAXes".

> >So?  By itself, that may merely indicate that DEC hasn't pushed raw hardware
> >technology as hard as IBM has.
> While the 370 is not nice, at least with it, the first
> operand-byte **ALLWAYS** tells you where the next instruction
> starts!  The vax could be *MUCH* faster if it did this too, but
> then it would lose its code-density!

OK, so if you implement a VAX using the same technology as a top-of-the line
IBM mainframe, how fast would it be?  I can well believe it would not be as
fast as an equivalent 370, but would it top out at 8 MIPS or would it be, say,
more like 12 MIPS?  The fact that the top of the line VAX is only 8 MIPS, while
the top of the line 370 is 20 MIPS, does not *in and of itself* indicate that
this is due solely to architectural problems with the VAX.
	Guy Harris
	{ihnp4, decvax, seismo, decwrl, ...}!sun!guy
	guy@sun.com