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From: mikeb@inset.UUCP (Mike Banahan)
Newsgroups: net.arch,net.micro.16k,net.micro.68k
Subject: Re: 24 bit vs. 32 bit address space
Message-ID: <523@inset.UUCP>
Date: Thu, 21-Mar-85 18:11:37 EST
Article-I.D.: inset.523
Posted: Thu Mar 21 18:11:37 1985
Date-Received: Tue, 26-Mar-85 16:14:36 EST
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Organization: The Instruction Set Ltd, London
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Xref: watmath net.arch:1038 net.micro.16k:298 net.micro.68k:691

To (probably mis-)quote a well known axiom of computing:
"There are only three sensible ways of allocating any resource:
	to have none of them
	to have only one of them
	or to have more than enough"

It goes for registers, segments, address bits or anything that you like.
A shortage of a resource forces concentration on why it is needed.
Sometimes this is a good discipline, sometimes bad. I remember an ALGOL60
compiler which supported the full language and ran in 4k (yes, no mistake)
bytes on a PDP11; that was because there were only 8k of them available
and it needed some headroom for the stack.
I also remember a FORTRAN compiler for the same machine that took some 50
odd k just because of the overlaying it had to use; without 
an arbitrary and unnatural constraint on its storage requirements
it would have got by in far less. The restraint was good, in that it
concentrated the mind of the ALGOL compiler writer who might otherwise
have got lazy, it was just a d****d nuisance for the FORTRAN hacker.

The UNIX (a very chauvinistic) community would do
well to look at MULTICS (or 4.xBSD and vread
vwrite) and to contemplate why files exist. Why are they there?
They are an unpleasant fix to the lack of an unconstrained segmented address
space. Read and Write are a hack around the `obvious' solution of
attaching to a named address space and assigning to or accessing the
value of the notional structures that it contains; be they bytes or
records. Why use lseek(2) when you could have pointers?

There is no solution. The best thing that could happen would be to have
ann infinite number of segments with an unlimited address space in each.
The feasability of this is nil. All other arguments are simply those
of which compromise is best; whether it is better to be rich and sick or poor
and well, or to be comfortable but with a bit of a twinge in the back
now and then...............


-- 
Mike Banahan, Technical Director, The Instruction Set Ltd.
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