Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.2 9/18/84; site calgary.UUCP
Path: utzoo!watmath!clyde!burl!ulysses!mhuxr!ihnp4!alberta!calgary!radford
From: radford@calgary.UUCP (Radford Neal)
Newsgroups: net.arch,net.micro.16k,net.micro.68k
Subject: Re: Reply on using the top eight bits of pointers on a 68000
Message-ID: <335@calgary.UUCP>
Date: Tue, 26-Mar-85 15:37:26 EST
Article-I.D.: calgary.335
Posted: Tue Mar 26 15:37:26 1985
Date-Received: Thu, 28-Mar-85 03:11:26 EST
References: <306@calgary.UUCP> <191@redwood.UUCP>
Organization: University of Calgary, Calgary, Alberta
Lines: 32
Xref: watmath net.arch:1042 net.micro.16k:302 net.micro.68k:694

RE: Rob Warnock's extensive analysis of pro's and con's to using the top
    eight bits of a 68000 interrupt vector to allow identification of
    exception type without 256 interrupt routines.

I have to agree that nowdays 1K of memory for interrupt routine stubs is
not all that much, though it does offend my sense of thrift (perhaps not
a practical sense in this context).

Regarding the use of the top eight bits of addresses for type flags in,
e.g. a Lisp interpreter, which Mr. Warnock still things is a BIG mistake:
I think there's an element of "making of a processor what you want" to all
this. A 68000 can be seen as

    1) A machine with 24-bit addresses. You get to use the top 8 bits
       of pointers for any old data. (Short-sighted, yes, but sometime
       one might have to be.)

    2) A machine with 32-bit addresses. You get to address 4 Gigabytes
       of memory - great! (At least once you can get a 68020 :-)

    3) A machine with 32-bit pointers, the top 8 bits of which 
       hold information on the type of the object addressed by the
       low 24-bits. What an innovation in machine architecture! I 
       can see the glossies now - "The only processor with hardware
       support for object-oriented programming", "Use the languages
       of the future on a machine available today", etc. The fact
       that it can only handle 16 Megabytes of memory is a minor 
       limitation not likely to deter anyone wishing to enter the
       realm of fifth-generation hardware...

    Radford Neal
    The University of Calgary