Path: utzoo!mnetor!uunet!seismo!sundc!pitstop!sun!decwrl!cadsys.dec.com!crabb
From: crabb@cadsys.dec.com (Charlie, SEG/CAD, HLO2-2/G13, (dtn 225)(617)568-5739)
Newsgroups: comp.arch
Subject: RE: Cycle stretching
Message-ID: <8802162251.AA20090@decwrl.dec.com>
Date: 16 Feb 88 22:51:17 GMT
Organization: Digital Equipment Corporation
Lines: 28




>>>>...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.  

 
>>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.
 
>The implementation of this variable period clock can be done with the
>AMD  AM2925  clock generator chip.
 
 
	I third the motion for non-dumbness.  The PR1ME (note
	old logo :-) ) did indeed have a clock field in the microcode
	word to adjust the clock on a per-instruction granularity.
	Time was spent tuning the machine for various (macro)
	instruction times in the 4xx-7xx series (pre-pipeline era). 
 
	/Charlie Crabb !decwrl!cadsys.dec.com!crabb