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Path: utzoo!watmath!watdcsu!herbie
From: herbie@watdcsu.UUCP (Herb Chong [DCS])
Newsgroups: net.arch,net.micro.16k,net.micro.68k
Subject: Re: Using top eight bits of pc on 68000
Message-ID: <1111@watdcsu.UUCP>
Date: Thu, 14-Mar-85 10:01:47 EST
Article-I.D.: watdcsu.1111
Posted: Thu Mar 14 10:01:47 1985
Date-Received: Tue, 19-Mar-85 14:27:27 EST
References: <983@watdcsu.UUCP> <2385@nsc.UUCP> <730@amdcad.UUCP>
Reply-To: herbie@watdcsu.UUCP (Herb Chong [DCS])
Organization: U of Waterloo
Lines: 39
Xref: watmath net.arch:977 net.micro.16k:280 net.micro.68k:670
Summary: 

In article <281@calgary.UUCP> radford@calgary.UUCP (Radford Neal) writes:
>> > There are quite valid reasons for using
>> > the un-used top eight bits on a 68000.. 
>> >
>> > (Further quote from original posting)
>
>> I hear IBM used to use some of the otherwise unused bits in their address
>> fields on the 360.  I also hear they found out this was a big mistake.
>
>Now, tell me why I shouldn't use the top eight bits in this way?
>
>    Radford Neal

i think somehow, one of the points of this discussion dropped into a
discussion about hardware when the first digression was onto why the
unused address bits of hardware shouldn't be used.  

well, it all depends on the timeframe you're working in.  if you are
developing software that will probably never run on any other machine,
what does it matter whether you use the UNUSED address bits for your
own purposes?  they make dandy flags and other things.

on the other hand, if you plan to run the software (unchanged) on a
descendant of your current machine, you will NOT use the high order
bits under any circumstances because one of the first things that will
be changed is the addressing range.  once you have software tromping
all over address bits where it has no business being (on the new
machine) it becomes an EXPENSIVE proposition to re-write the software
to run.  (this applies to compiler writers, assembler programmers, and
hardware designers.)

Herb Chong...

I'm user-friendly -- I don't byte, I nybble....

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