Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!lll-winken!uunet!tektronix!sequent!mntgfx!mbutts
From: mbutts@mntgfx.mentor.com (Mike Butts)
Newsgroups: comp.arch
Subject: Re: EXACTLY what is Superscalar?
Message-ID: <1989Mar27.103240.371@mntgfx.mentor.com>
Date: 27 Mar 89 18:32:38 GMT
References: <22975@ames.arc.nasa.gov>
Organization: Mentor Graphics Corporation, Beaverton Oregon
Lines: 33

From article <22975@ames.arc.nasa.gov>, by lamaster@ames.arc.nasa.gov (Hugh LaMaster):
> For quite a while, I have heard superscalar used, and I think the term was
> defined in a paper in IEEE Computer a while back, but I am still a little
> fuzzy on it.  Is "superscalar" an exact concept, or is it a buzzword like 
> "RISC"?  Is a Multiflw machine a superscalar machine, or the i860, or
> the Weitek XL-8064?

In "Superscalar vs. Superpipelined Machines" (Comp. Arch. News, ACM SIGARCH, 
v.16, #3, June 1988, p. 71-80), Norman P. Jouppi of DEC West in Palo Alto 
offers this definition: "A superscalar machine of degree n can issue n 
instructions per cycle."  VLIW machines are similar.

It's a very interesting paper, which I recommend to anyone interested in 
this subject.  He discusses superpipelined machines, which "can issue only one 
instruction per cycle, but have cycle times shorter than the time required 
for any operation", compares the alternatives, and discusses limits to 
instruction-level parallelism.

In particular, let me quote from his concluding comments: "The most important 
point to emphasize is that significant improvements in uniprocessor 
performance via internally parallel processors will only occur for 
applications with large amounts of instruction-level parallelism.  These are 
applications that are often of more importance to physical scientists than to 
computer scientists.  Many applications of importance to computer scientists 
and computer engineers, such as compilers, operating systems, and programs
involving manipulation of linked data structures, will not benefit from
highly parallel uniprocessors."

-- 
Mike Butts, Research Engineer         KC7IT           503-626-1302
Mentor Graphics Corp., 8500 SW Creekside Place, Beaverton OR 97005
...!{sequent,tessi,apollo}!mntgfx!mbutts OR  mbutts@pdx.MENTOR.COM
These are my opinions, & not necessarily those of Mentor Graphics.