Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sdd.hp.com!zaphod.mps.ohio-state.edu!rpi!crdgw1!crdos1!davidsen
From: davidsen@crdos1.crd.ge.COM (Wm E Davidsen Jr)
Newsgroups: comp.arch
Subject: Re: harvard architectures
Message-ID: <2773@crdos1.crd.ge.COM>
Date: 17 Oct 90 20:30:38 GMT
References: <9010160322.AA13808@lilac.berkeley.edu> <3468@bnr-rsc.UUCP> <7883@darkstar.ucsc.edu>
Reply-To: davidsen@crdos1.crd.ge.com (bill davidsen)
Organization: GE Corp R&D Center, Schenectady NY
Lines: 23

In article <7883@darkstar.ucsc.edu> haynes@ucscc.UCSC.EDU.UUCP (Jim Haynes) writes:

| 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.

  This has been applied "after the fact" in some cases. The old Z80 CPU
has only 64k addressing. One hack done by several people in many ways
was to use the "M1" bit (opcode fetch) as a bank selector. This allowed
64k code and 64k data. Alas, the popular o/s was CP/M, and it must have
messed with its own code, because it wouldn't run that way.

  I believe that there was a hack to have data store to memory go both
banks and read go one or the other, but it didn't buy much. I had a
version with the o/s and BIOS in one bank, user in the other, and 64k of
track buffer in a third. Ran really well for the times.
-- 
bill davidsen	(davidsen@crdos1.crd.GE.COM -or- uunet!crdgw1!crdos1!davidsen)
    VMS is a text-only adventure game. If you win you can use unix.