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From: haynes@ucscc.UCSC.EDU (99700000)
Newsgroups: comp.arch
Subject: Re: harvard architectures
Message-ID: <7883@darkstar.ucsc.edu>
Date: 16 Oct 90 17:54:01 GMT
References: <9010160322.AA13808@lilac.berkeley.edu> <3468@bnr-rsc.UUCP>
Sender: usenet@darkstar.ucsc.edu
Reply-To: haynes@ucscc.UCSC.EDU.UUCP (Jim Haynes)
Organization: University of California, Santa Cruz  CATS
Lines: 32

In article <3468@bnr-rsc.UUCP> bcarh185!schow@bnr-rsc.UUCP (Stanley T.H. Chow) writes:
>Incidentally, I have always wondered why this busing arrangement is
>known as "Harvard". Anyone know?
Yeah.  The "von Neumann" architecture is that in which programs and data
are stored in the same memory, and specifically allows the processor to
operate on the instruction stream.  Self-modifying code was a feature.
The computer group at Harvard, under Aiken, had built the Mark I
electromechanical computer, and then Mark II and III and IV, some of
which were electronic and which followed the philosophy of keeping
instructions and data in separate memories.

Since those early days various things have clouded the distinction between
the two architectures.
1)  Even in pure "von Neumann" architectures the idea of self-modifying
code has become rather passe'
2)  The Burroughs machines from the B6500 on have used tag bits to
distinguish instructions from data when both are living in the same
memory.  So it can't fetch instructions from data words.
3)  The PDP11/45 and other large 11s had separate sets of memory
mapping registers for code and data (and some architectural difficulties
where data might be embedded in the code); so you could put code and data
in disjoint areas of memory if you wanted to.
4)  A lot of microprocessor systems, particularly embedded micros, have
absolutely read-only instruction code, with read-write memory used only
for data.  Although these are in the same address space the code
certainly can't modify itself.
haynes@ucscc.ucsc.edu
haynes@ucscc.bitnet
..ucbvax!ucscc!haynes

"Any clod can have the facts, but having opinions is an Art."
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