Path: utzoo!attcan!uunet!nbires!ico!rcd
From: rcd@ico.ISC.COM (Dick Dunn)
Newsgroups: comp.arch
Subject: Re: RISC  a short answer??
Summary: if pigs could fly, RISC would be dead
Message-ID: <5206@ico.ISC.COM>
Date: 20 May 88 00:04:26 GMT
References: <1036@nusdhub.UUCP> <1988May3.224604.2252@utzoo.uucp> <770@l.cc.purdue.edu>
Organization: Interactive Systems Corp, Boulder, CO
Lines: 36

In article <770@l.cc.purdue.edu>, cik@l.cc.purdue.edu (Herman Rubin) writes:
> The philosophy of RISC seems to be, to quote one of the above articles, that
> one shoul not worry about the "slow" instructions to speed up 70% of the 
> instructions...

This is Rubin's creation.  I don't know where the 70% comes from.  Rubin
goes on to construct a hypothetical situation where, if you use the slow
instructions often enough, and they're slowed down a lot, you can actually
make things worse.

Fine.  So what?  It is certainly true that it would be possible to mis-
apply the RISC approach and make a machine which is slower than an
otherwise-comparable CISC in comparable technology.  Why would you do that?

Look, the whole RISC game relies heavily on studying what REALLY happens in
REAL programs--people finally figured out that at least some CISCs spent
enormous amounts on hardware that didn't do much for program execution
speed; the silicon was better spent elsewhere.  You can't construct a valid
counter-argument to empirical results with "what-if" arguments...you're
just setting up a straw man and knocking it down.  When Rubin can show us
some REAL results that illustrate a real (or at least realistic) RISC
falling down compared to a comparable-cost real CISC on something that
approximates a real problem, then we should sit up and listen.

>...It seems that, instead, there is great merit in a VCISC design, in which
> useful instructions are included to decrease the number of instructions needed
> in a program...

I see no evidence of any merit whatsoever for a VCISC design.  It's a
Herman Rubin wish, but he doesn't run my programs for me.  His arguments in
favor of CISC can be intellectually appealing--it's not that the ideas are
inherently wrong.  It's just that the ideas, if they are to work, require
empirical results to come out differently than they really do.
-- 
Dick Dunn      UUCP: {ncar,cbosgd,nbires}!ico!rcd       (303)449-2870
   ...Never attribute to malice what can be adequately explained by stupidity.