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From: baskett@decwrl.UUCP (Forest Baskett)
Newsgroups: net.arch
Subject: Virtual versus Real address caches
Message-ID: <8@decwrl.UUCP>
Date: Mon, 12-Nov-84 12:38:03 EST
Article-I.D.: decwrl.8
Posted: Mon Nov 12 12:38:03 1984
Date-Received: Sat, 17-Nov-84 03:55:01 EST
Organization: DEC Western Research Lab, Los Altos, CA
Lines: 16

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.  This all works out with relative ease if your page size is
large enough compared to your cache size.  If you want to support multiple
virtual address spaces, particularly in the case of multiprocessing, the
extra complexity of a real address cache may be worth it.

Forest Baskett - Western Research Laboratory - Digital Equipment Corporation