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From: jones@pyrite.cs.uiowa.edu (Douglas W. Jones,201H MLH,3193350740,3193382879)
Newsgroups: comp.arch
Subject: Von Neumann Architectures
Message-ID: <5104@ns-mx.uiowa.edu>
Date: 28 Mar 91 18:29:01 GMT
Sender: news@ns-mx.uiowa.edu
Lines: 38

The claim that an architecture is or is not a Von Neumann architecture
is very frequently made in a misleading way.  Some people will use it
as a perjorative term in reference to "The Von Neumann" bottleneck
between processor and memory, others refer to the Von Neumann model of
memory addressing (all of memory is one big array).

Back in 1973, I remember a final exam question at Carnegie-Mellon
University; I think it was in Sam Fuller's class, but I'm not sure.

The question was:  Name six features common on modern machines which
were not present in the architectures proposed by Von Neumann.  (The
number six might have been something else, I forget).  I think the
point of this question was, at least in part, to get people to think
twice before characterizing an architecture as non-Von Neumann by
pointing out that essentially all modern architectures contain features
which are quite correctly described as such.

The next page of this note contains a list of popular non-Von Neumann
features in modern architectures:

1) Indexed addressing.
2) Indirect addressing.
3) Multiple accumulators.
4) Peripheral processors, I/O channels, DMA, or any similar feature.
5) Floating point.
6) Fractional or multiple word operands.
7) Procedure call instructions.
8) Stacks, whether used for call and return or anything else.
9) Any distinction between virtual and physical memory addresses.

It's worth noting that Von Neumann is supposed to have objected to the
introduction of floating point with the line "Any programmer worth his
salt should be able to keep the decimal point in his head."  It's also
worth remembering that Von Neumann may have anticipated the idea of
fractional word operands by packing multiple instructions per word.

					Doug Jones
					jones@cs.uiowa.edu