From: wayne@backbone.uucp (Wayne Schlitt) Subject: Re: Intel, the Pentium and Linux Date: Sat, 1 May 1993 03:21:11 GMT
In article <willmore.736143868@help.cc.iastate.edu> willmore@iastate.edu (David Willmore) writes:
> liljeber@hydra.Helsinki.FI (Mika Liljeberg) writes:
> >willmore@iastate.edu (David Willmore) wrote:
> >> Not likely. More likely a modern CISC processor would win because (at the
> >> same clock speed) there are fewer instruction fetches (i.e. none) in the
> >> CISC case and therefore less memory bandwidth wasted for I fetch.
>
> >This is not true. All modern processors have an onboard instruction
> >cache. A RISC processor doesn't waste any more memory bandwidth than a
> >CISC processor. [ ... ]
>
> You have to fetch them sometime. It's going to cost more than a CISC.
> Not much, sure, but it will cost more.
It won't cost very much more. The fetches are blocked together and
the extra cost of longer cache lines is not as important as the memory
latency.
> Also, A CISC has more exposed
> parralelism so that if you have a 2 access/cycle data cache, the CISC
> can take advantage of it more easily. A RISC processor would have to go
> superscalar or superpipelined to be able to do that.
Since the RISC chips are simpler, it is much easier for them to go to
superscalar or superpipelined with the same amount of transistors.
RISC chips tended to be full 32 bits before the CISC chips, they had
on chip caches first, they went superscalar/superpipelined first, and
now they are going to 64 bits first. That is the one of the
advantages of a RISC architecture, you have freed up transistors that
you can put to more effective use in other ways. So saying that a
RISC processor has to go superscalar in order to do the same thing is
kind of backwards. RISC chips _can_ go superscalar, so the CISC chips
_have_ to have much faster caches in order to keep up.
-wayne
-- Well intentioned, knowledgable people with shape minds can disagree on something without either one of them being unreasonable, or necessarily wrong. Too often people assume that other person has ill intentions, is unknowledgable or doesn't have a sharp mind.