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From: mshute@cs.man.ac.uk (Malcolm Shute)
Newsgroups: comp.arch
Subject: Re: Historical architectural advances??
Message-ID: <1914@m1.cs.man.ac.uk>
Date: 12 Nov 90 17:02:13 GMT
References: <8553@scolex.sco.COM> <1888@m1.cs.man.ac.uk> <1990Nov7.155719.24413@dg-rtp.dg.com>
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Reply-To: mshute@cs.man.ac.uk (Malcolm Shute)
Organization: Department of Computer Science, University of Manchester UK
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In article <1990Nov7.155719.24413@dg-rtp.dg.com> hamilton@dg-rtp.dg.com (Eric Hamilton) writes:
>I would like to see the term "tightly coupled" reserved for
>systems in which memory is shared and coherent for all
>processors *and* all the processors occupy the same physical
>address space.  The distinguishing characteristic of these
>machines is that the C construct *p=... can be used to
>change shared data.  A bunch of microprocessors on the
>same bus qualifies as tightly coupled in this sense.

I worked for some time on a research project in which the machine
used a low-level form of some of the Manchester Dataflow ideas
combined with some of the Wafer Scale ideas involved in Ivor Catt's
Property-1A machine.

Since it was originally a dataflow machine (later converted to being
a fully-lazy reduction machine), the test for use of the *p= assignment
doesn't really work (though being fully-lazy, the concept of pointers
is relevant).

Since it uses a token-passing machine (packet-based), it could
not be described as a shared-memory machine.  Indeed, in my favourite
form of the project, there was no memory at all other than the
storage involved in buffering the tokens from one processor to another,
and to the ALU.  There was no microcode, no scratch area; just
a handful of random combinatorial logic, and a few buffers.
So, the clause on the left hand side of your *and* (which I have
highlighted with ***s in the quote above), is not applicable either.

However, the clause on the r.h.s. of the *and* is applicable to the
machine... so maybe I can agree with your definition if you change
the *and* into an *or*.

Admittedly, the machine was never built.  It fell foul of the
bain of all multiprocessor computer architecture design: communications
costs, and the harnessing of locality.  So, maybe you could argue
that you are not upset that this machine does not qualify under
your definition as it stands.


As a last comment, I would be interested to here from anyone who knows of
any other non-von Neumann machines which are made entirely from
non-von Neumann machine components.  I know that certain cellular
automata can be shown to be equivalent to a Turing machine...
but they make my project look truely efficient!!
--

Malcolm SHUTE.         (The AM Mollusc:   v_@_ )        Disclaimer: all