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From: ron@brl-sem.UUCP
Newsgroups: comp.arch
Subject: Re: Will caches ever become obsolete?
Message-ID: <671@brl-sem.ARPA>
Date: Wed, 4-Mar-87 13:14:38 EST
Article-I.D.: brl-sem.671
Posted: Wed Mar  4 13:14:38 1987
Date-Received: Fri, 6-Mar-87 06:14:52 EST
References: <3182@wateng.UUCP>
Distribution: comp
Organization: Electronic Brain Research Lab
Lines: 33
Keywords: cache, coherence problems

In article <3182@wateng.UUCP>, hmthaker@wateng.UUCP (Hemi M. Thaker) writes:

>       I have a question concerning caches.  Currently,
> caches are used, either in uniprocessors, or multiprocessors,
> because the time a microprocessor takes to execute an
> instruction is much smaller than the time to reference
> main memory.  Thus, a cache is used to match the speed
> difference (the cache is usually as fast as the microprocessor).

The advantage of the cache isn't because it is just faster main
memory, but in that it short circuits the address decoding that
must happen to access conventional RAM.   As main memories get
faster, so will caches.  Projects are underway to build processors
with large scale "content adderssable memories" which are essentially
machines whose main memory is like today's cache.
> 
>      My question is, then, with the current improvements in
> memory chips (ie. faster access, and greater densities), does
> anyone forsee a time in the distant future (> 3 or 4 years)
> that the speed of say, a 1Mb chip will be comparable to that
> of say a 1Kb ECL chip used in current caches?
> 

Like I said, faster memory, faster caches.  A few years ago, one of my
coworkers came in my office and told me that in a few years I would be
able to get a computer to sit on my desk the speed of a VAX (780) and
then I'd get one and I'd be happy.  I told him (and later this has been
referred to around here as "Ron's Rule of Computing") that when that day
came I'd still want a computer "this big" (arms outstretched indicating
the approximate size of a 780 CPU cabinet), whatever the technology of
the time can fit in that much space.  As the technology increases,
so do our needs.

-Ron