Path: utzoo!utgpu!water!watmath!clyde!burl!codas!uflorida!gatech!bloom-beacon!athena.mit.edu!wesommer
From: wesommer@athena.mit.edu (William E. Sommerfeld)
Newsgroups: comp.arch
Subject: Re: Cycle stretching
Summary: the first LISPM's did it
Message-ID: <2982@bloom-beacon.MIT.EDU>
Date: 16 Feb 88 10:31:12 GMT
References: <844@daisy.UUCP>
Sender: daemon@bloom-beacon.MIT.EDU
Reply-To: wesommer@athena.mit.edu (William E. Sommerfeld)
Organization: Massachusetts Institute of Technology
Lines: 15

In article <844@daisy.UUCP> david@daisy.UUCP (David Schachter) writes:
>Say I have a CPU where 99 percent 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?

Rumour has it that the original Lisp Machines (the `CADR's) did just
this; there were two clocks, and one bit of the microcode selected
which clock would be used.  The story goes that both clocks were
hand-adjustable, and that special microcode diagnostics were used to
tune each one (speed it up until it crashes, and then back off a 1/4
turn..) so each machine would run as fast as possible..

				-- Bill Sommerfeld
				   wesommer@athena.mit.edu