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From: howard@metheus.UUCP (Howard A. Landman)
Newsgroups: net.arch
Subject: Re: Use Of Multiple Register Sets / Re: RISC perspective - (nf)
Message-ID: <218@metheus.UUCP>
Date: Thu, 22-Mar-84 02:17:46 EST
Article-I.D.: metheus.218
Posted: Thu Mar 22 02:17:46 1984
Date-Received: Sun, 18-Mar-84 08:12:27 EST
References: <2747@fortune.UUCP>
Organization: Metheus, Portland Oregon
Lines: 14

The question in the referenced article was whether interprocess context
switches were more or less common than procedure call context switches.

Actually, we ASKED that question before designing RISC I.  The measurements
taken indicated that for VAX UNIX, interprocess switches were about 100 times
less frequent than procedure calls.  Thus the payoff for handling them in
special hardware is also much less.  We decided it wasn't worth the effort.
I still think that was the correct decision, especially considering that some
interprocess switching (e.g. simple interrupts) can be coerced into using the
register window mechanism much like a procedure call.  (One interrupt which
CAN'T be so coerced is the window overflow interrupt, for obvious reasons.)

	Howard A. Landman
	ogcvax!metheus!howard