Path: utzoo!mnetor!uunet!husc6!cmcl2!brl-adm!umd5!purdue!gatech!ukma!psuvm.bitnet!bjj@psueclb.BITNET
From: bjj@psueclb.BITNET
Newsgroups: comp.arch
Subject: Re: Cycle stretching
Message-ID: <983@PSUECLB>
Date: 19 Feb 88 00:51:29 GMT
Organization: Engineering Computer Lab, Pennsylvania State University
Lines: 29


> Hey, I saw an old PDP (was it 8?) with a knob on the front panel, regulating
> the clock frequency!

That was a PDP-10.  KA10 processor.  Sorry, that know only changes the speed
front panel repeat function.  Like when you turn on repeat, hit "deposit next",
and have the CPU fill all of memory.  Handy for memory tests when the CPU
won't run anything.

>        If you're going to do this, why not take it all the way and
> make your computer "event driven" instead of clocked?

The KA10 is asynchronous.  There really is no clock.  All timing is
determined by delay lines (and wire length).  It has subroutines
(like the memory cycle subroutine) which accept parameters (read
and write).  All done by sending the pulse off in various directions
and gating the returning pulse with a flag to indicate who's waiting
for the subroutine to finish.  Very nice for doing things in parallel
as you can have separate steams executing independently and wait
for the last to finish.

>        Bunches of asynchronously executing components... I wonder
> what it would be like to diagnose the microcode :-<.

Who knows.  The microcode works, why diagnose it?  Fixing it is easy,
just takes a scope.  Occasionally our KA10 will have a pulse amplifier
go bad resulting in a lost pulse.  You just check the state of the
machine (there are lights on nearly every register) to get a good
idea where the pulse was lost.