Path: utzoo!utgpu!water!watmath!clyde!rutgers!mailrus!umix!husc6!hao!ames!amdcad!philip
From: philip@amdcad.AMD.COM (Philip Freidin)
Newsgroups: comp.arch
Subject: Re: Cycle stretching
Message-ID: <20409@amdcad.AMD.COM>
Date: 16 Feb 88 19:00:11 GMT
References: <844@daisy.UUCP>
Reply-To: philip@amdcad.UUCP (Philip Freidin)
Organization: Advanced Micro Devices
Lines: 47

In article <844@daisy.UUCP> david@daisy.UUCP (David Schachter) writes:
>
>Here is a dumb question.  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?
>
>					-- David Schachter
>


This is not a dumb question, because I have the answer! The technique you
describe is a common one, and has been dealt with many times in various
ways in different implementations of assorted architectures. The ratio of
quick versus not so quick instructions though is not 99:1, but more in the
50:50 region (give or take 20%). The quick instructions are the logical
ops, because there is no interbit communications. The not so quick are the
primitive arith ops such as add, inc, sub, dec, etc, because the inter bit
communications slow things down (the carry chain). Longer instructions use
multiple cycles (and in some systems, a mix of long and short cycle instruc-
tions). 

SALES MODE ON  :-)

The implementation of this variable period clock can be done with the

AMD  AM2925  clock generator chip.

It is specifically designed to do exactly what you asked about. In a micro-
programmed system, each micro cycle execution duration is a function of
what the critical path will be for the specified opperation. This is
known at assembly time of the microcode. An extra field is added to the
microcode, which controls the Am2925, and thus sets the duration of the
clock for that microcycle. The chip has a 3 bit control field, so it can
generate 8 different clock periods. With wait states, this can be extended.

SALES MODE OFF :-) :-)  (double smily because I prefer to be out of sales
			 mode)



Philip Freidin @ AMD SUNYVALE on {favorite path!amdcad!philip)
Section Manager of Product Planning for Microprogrammable Processors
(you know.... all that 2900 stuff...)
"We Plan Products; not lunches" (a quote from a group that has been standing
				 around for an hour trying to decide where
				 to go for lunch)