Path: utzoo!utgpu!water!watmath!clyde!rutgers!gatech!hao!ames!pasteur!ucbvax!hplabs!pyramid!voder!apple!baum
From: baum@apple.UUCP (Allen J. Baum)
Newsgroups: comp.arch
Subject: Re: Cycle stretching
Message-ID: <7415@apple.UUCP>
Date: 16 Feb 88 17:15:15 GMT
References: <844@daisy.UUCP>
Reply-To: baum@apple.UUCP (Allen Baum)
Organization: Apple Computer, Inc.
Lines: 17

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[]
>In article <844@daisy.UUCP> david@daisy.UUCP (David Schachter) writes:
>
>Here is a dumb question.  Say I have a CPU where 99% of the instructions
>take, say, one clock.  The remaining instructions need just a little longer--
>one clock plus a few nanoseconds.  Why not stretch the clock a bit when exec-
>uting those instructions, instead of wasting most of a second clock period?

Not a dumb question. Lots of older microcoded minis did exactly this in their
microcode. They had a control field to slow down the clock (from 150ns. to
180ns., for instance) when something slow came up, like a branch. I believe
the first Prime was a machine that did this.
This does complicate the world, especially synchronizing to the outside world.
Its easier to just take a full cycle in the 1% cases.

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