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From: bcase@uiucdcs.UUCP
Newsgroups: net.arch
Subject: Re: Virtual versus Real address caches
Message-ID: <27800036@uiucdcs.UUCP>
Date: Wed, 21-Nov-84 17:29:00 EST
Article-I.D.: uiucdcs.27800036
Posted: Wed Nov 21 17:29:00 1984
Date-Received: Sun, 25-Nov-84 03:30:01 EST
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Nf-From: uiucdcs!bcase    Nov 21 16:29:00 1984

[bug lunch]

> Peter Deutsch implies that a virtual address cache is the fastest kind, other
> things being equal (they never are).  While there may be some truth to such an
> assertion, there are a sufficient number of interesting cases where it is
> possible for a real address cache to match the required performance of a
> virtual address cache that it is probably safer to assert that a virtual
> address cache is the simpliest fast cache.  If you do your real address
> lookup in parallel with your cache lookup, you have only one level of
> latching just before the tag comparison to differentiate the two cases.
> Since the translation cache is often smaller than the instruction/data cache,
> technology will often make it faster so the extra level of latching may be
> covered up, too.

I am not ready to dispute your argument, but I don't think I understand:
Doesn't the virtual address (in a real-address-taged cache) have to go
through one level of mapping before it can be looked up in the cache
tags?  If so, then this would seem to say that a real address cache is
slower than a virtual cache, but like I said, I don't think I understand
your argument yet.  Are you saying that the translation can be done very
quickly with a small TLB (I agree here), and that it is so fast that
the cache isn't slowed down (I'll have to think about this one)?  Or
are you saying that with a slight modification of the cache tag lookup
procedure, the cache can lookup the real address and the cache tag at
the same time (this sounds like what you are saying)?  If you can, would
you elaborate? -- Always looking for new ideas....