Newsgroups: comp.arch
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: RISC  a short answer??
Message-ID: <1988May5.171444.849@utzoo.uucp>
Organization: U of Toronto Zoology
References: <1036@nusdhub.UUCP> <1988May3.224604.2252@utzoo.uucp>, <383@m3.mfci.UUCP>
Date: Thu, 5 May 88 17:14:44 GMT

> I consider this a good explanation of what RISC was, circa 1981-1983.
> But if that's ALL you consider RISC to be, then I think you're
> missing some very important things.

The trouble here is that we're increasingly in a situation where "RISC" is
considered a synonym for "good".  The term is rapidly losing any more
specific meaning because of persistent misuse.  I was attempting to strike
a blow for linguistic purity.  (Now and then I feel like supporting a lost
cause...)

Yes, it is true that optimizing compilers are a crucial part of many "RISC"
projects today.  It is also true that RISC architectures tend to be good
for optimization, since they give the compiler more control (and distract
it less with complex side issues).  This is a useful side effect of RISC
designs, which makes them more popular.  It is not a fundamental part of
the RISC concept.

> ...it's performance that drives RISC designs.

You mean "RISC" designs.

> And compilers that must manage at compile-time what hardware
> interlocks used to do at run-time are simple no longer...

Quite true.  But this is not what RISC (Reduced Instruction Set Computer,
remember) is all about.  Managing interlocks at compile time is a different
concept, albeit one that fits in well with RISC.  Much of the confusion
about the meaning of the term arises from just such goes-nicely-with-RISC
ideas.

Reduced Instruction Set means fewer and less complex instructions.  That
is all it means.  Many new designs that incorporate this concept also use
other somewhat-related concepts like software pipeline interlocks (delayed
branches being a simple form of this), overlapping register windows, heavy
reliance on optimizing compilers to maximize hardware performance, etc.
But proper usage (as opposed to what the marketing people do) does not
apply the name of one of these concepts indiscriminately to all the others.
-- 
NASA is to spaceflight as            |  Henry Spencer @ U of Toronto Zoology
the Post Office is to mail.          | {ihnp4,decvax,uunet!mnetor}!utzoo!henry