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From: scott@mcs-server.gac.edu (Scott Hess)
Newsgroups: comp.sys.next
Subject: Re: RISC vs. CISC -- SPECmarks
Message-ID: <SCOTT.91Apr29123159@mcs-server.gac.edu>
Date: 29 Apr 91 17:31:59 GMT
References: <1991Apr10.155032.14786@data.com><1991Apr15.165540.14270@agate.berkeley.edu><1
 991Apr22.044553.16805@mp.cs.niu.edu><1991Apr24.170804.25670@kithrup.COM><1991Apr24.181932.17810@cs.cornell.edu>
Organization: Gustavus Adolphus College
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In-reply-to: wayner@CS.Cornell.EDU's message of 24 Apr 91 18:19:32 GMTLines: 35

In article <1991Apr24.181932.17810@cs.cornell.edu> wayner@CS.Cornell.EDU (Peter Wayner) writes:
   sef@kithrup.COM (Sean Eric Fagan) writes:

   >Nope, not really. That's the problem:  with a "CISC" instruction set, it's
   >really very difficult to go superscalar, at least compared to some "RISC"
   >instruction sets.   Why?  Because not enough registers, too many memory
   >references in a single instruction, or other small, niggling details.

   In another sense, going "superscalar" is much easier with CISC
   machines.  I think the Intel 486 does a PUSH instruction in one cycle.
   In RISC land, this is a decrement and a load. The CISC designer just
   needs to use enough silicon to pipeline the important instructions.
   There is no need for complex logic to handle all the possible cases of
   two instructions coming down the pipe. The RISC designer needs to
   worry about generality.

Not!  The CISC instruction set is the one that requires the complex logic
to handle all the possible cases.  That's because CISC gives you much
more to work with.  A basic example of the problem is a sequence where
the first instruction does a post-increment on one of the registers,
and the very next one uses that register.  In most RISC machines, this
is not possible, as there is no post-increment instruction.

Of course, this is not to say that RISC machines don't have data hazards.
But, the RISC data hazards are necessarily a subset of CISC hazards,
and thus should be _easier_ to handle (I didn't say "easy", I said
easier!).

Later,
--
scott hess                      scott@gac.edu
Independent NeXT Developer	GAC Undergrad
<I still speak for nobody>
"Simply press Control-right-Shift while click-dragging the mouse . . ."
"I smoke the nose Lucifer . . . Banana, banana."