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From: kyriazis@iear.arts.rpi.edu (George Kyriazis)
Newsgroups: comp.arch
Subject: Re: Totally asynchronous computers
Message-ID: <6HT^$$-@rpi.edu>
Date: 2 Jan 91 20:18:14 GMT
References: <3523@bruce.cs.monash.OZ.AU>
Distribution: comp
Organization: Rensselaer Polytechnic Institute, Troy NY
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In article <3523@bruce.cs.monash.OZ.AU> zik@bruce.cs.monash.OZ.AU (Michael Saleeba) writes:
>Still, the basic concept still applies. Why not design a processor without
>any sort of clock at all? 
>
This is already done at Caltech.  There is a team (I don't remember names,
but I can give the reference later) that produced a microprocessor made
out of asunchronous logic.  It worked quite fast, and when you cooled it
down, it worked even faster.  It was built from the top down, using 
communicating processes to simulate the different design blocks and 
message exchange between them.  They claim it was a successful design.

>Now I'm aware of quite a few reasons why totally asynchronous machines haven't
>been made much, but I can think of work-arounds to nearly all of them. Would 
>anyone like to offer a concrete reason why this system is so little used? Or
>mention some machines that have used similar systems?
>
Such systems are a pain to design and eat up a lot of wire area.  You don't
only have to say that is the value of a signal, but also if the signal has
arrived or not.  This is usually done by 2-rail logic and/or some additional
2-cycle or 4-cycle handshaking protocols.  There is a chapter is Mead &
Conway's book "Introduction to VLSI Systems", that is devoted to asynchronous
VLSI circuits.  You might want to take a look.

Disclaimer:  I don't know too much about asynchronous circuits, but I know
the above were true a while back (1 to 2 years).  Things might have changed
by now.


----------------------------------------------------------------------
  George Kyriazis                 kyriazis@rdrc.rpi.edu
 				  kyriazis@iear.arts.rpi.edu