Path: utzoo!attcan!uunet!tut.cis.ohio-state.edu!pt.cs.cmu.edu!MATHOM.GANDALF.CS.CMU.EDU!lindsay
From: lindsay@MATHOM.GANDALF.CS.CMU.EDU (Donald Lindsay)
Newsgroups: comp.arch
Subject: Re: Why The Move To RISC Architectures?  ('386 vs. RISC)
Message-ID: <8512@pt.cs.cmu.edu>
Date: 20 Mar 90 20:32:07 GMT
References: <28012@cup.portal.com>
Organization: Carnegie-Mellon University, CS/RI
Lines: 33

In article <28012@cup.portal.com> Will@cup.portal.com (Will E Estes) writes:
>...since the 80386 has a more complex instruction set and does
>more work in a given instruction than does a typical RISC chip,
>does comparing MIPS figures between RISC and non-RISC
>architectures really tell you anything of worth?

Yes, but only if you understand the basic situation.  A MIPS rating
should be treated as give-or-take about a factor of two.  So, if one
machine has twice the MIPS of another (on a given compute-bound
task), the machines could still be about equal (on that task).  This
isn't true within a family, of course: different 386 boxes really can
be compared by their MIPS ratings.  Note, however, that I said "box".
Different boxes containing the same chip, at the same clock rate, can
still have different MIPS ratings.  (This is because of caches and
buses and other significant non-CPU items.) So, to ask for the MIPS
of a CPU chip is mostly to ask for an upper bound.

As for "complex" instructions, it is worth noting that the complexity
may be potential rather than actual.  Sometimes, a given machine runs
faster when the compilers avoid generating the more complex cases.
It was this observation that led us to explore the RISC idea.

>Finally, why is everyone so excited about RISC?  Why the move to
>simplicity in microprocessor instruction sets?  

The excitement is because the better RISC machines have genuinely
high throughput.  The simplicity is only relative - they're actually
quite complex machines.  The important point is that the designs are
carefully tuned, so that complexity is only used where it pays its
way.  So, rather than ask "Why simplicity?", it would be better to
ask about specific aspects, such as subroutine calling.
-- 
Don		D.C.Lindsay 	Carnegie Mellon Computer Science