Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!usc!zaphod.mps.ohio-state.edu!uakari.primate.wisc.edu!crdgw1!crdos1!davidsen
From: davidsen@crdos1.crd.ge.COM (Wm E Davidsen Jr)
Newsgroups: comp.arch
Subject: Re: harvard architectures
Message-ID: <2777@crdos1.crd.ge.COM>
Date: 19 Oct 90 12:54:15 GMT
References: <9010160322.AA13808@lilac.berkeley.edu> <3468@bnr-rsc.UUCP> <7883@darkstar.ucsc.edu> <2773@crdos1.crd.ge.COM> <1320@mpirbn.mpifr-bonn.mpg.de>
Reply-To: davidsen@crdos1.crd.ge.com (bill davidsen)
Organization: GE Corp R&D Center, Schenectady NY
Lines: 18

In article <1320@mpirbn.mpifr-bonn.mpg.de> p554mve@mpirbn.UUCP (Michael van Elst) writes:

| Oops. That seems to be difficult. The M1 signal is asserted during the
| opcode fetch only, any memory cycles for immediate data or addresses
| in the instruction can't be distinguished from other data fetches.
| You have to decode the opcode byte to guess the size of the instruction.

  Simple opcodes, only 256 and you could determine the length of the
instruction from it (may not be true of Z80, don't know). The part used
for the lookup in at least one was a small fast PROM. I don't remember
that part number, but it was an odd one, 74xx, sounded like 7400 TTL
logic chips, but was really a 50ns (or so) memory, available in 16, 64,
or 256 bytes flavors.

  There, I've shown that I'm an old guy...
-- 
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.