Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!mcsun!ukc!edcastle!aiai!richard
From: richard@aiai.ed.ac.uk (Richard Tobin)
Newsgroups: comp.arch
Subject: Re: MIPS, Compaq and Microsoft in bed - NYT story
Message-ID: <4109@skye.ed.ac.uk>
Date: 12 Feb 91 13:13:27 GMT
References: <29920@usc> <45758@mips.mips.COM> <3188@crdos1.crd.ge.COM>
Reply-To: richard@aiai.UUCP (Richard Tobin)
Organization: AIAI, University of Edinburgh, Scotland
Lines: 41

In article <3188@crdos1.crd.ge.COM> davidsen@crdos1.crd.ge.com (bill davidsen) writes:
>  What you are saying is that there are lots of poorly written
>non-portable programs around which do bizarre non-portable things with
>addresses, on the theory that because the {vax,sun,32k} has a linear
>address space starting with zero, that "all that ANSI crap about pointers
>doesn't mean much." (quote from one of my C students).

Non-portable, certainly.  But not necessarily badly-written.  Most
programs have no need to treat pointers in this way, but a very
important class do: those which implement other languages.  It's
extremely common for Lisp systems, for example, to represent objects
as a tagged word, consisting of a few bits (often 2, but sometimes as
many as 8) describing the type (integer, symbol, cons etc) and the
rest containing the address.  This involves treating addresses as
integers.

For many of us, the fact that it's hard to port such a system to an
8086 or other segmented architecture merely inclines us to reject such
processors.  We've weighed up the advantages of portability against
the disadvantage of having to use more cumbersome and probably less
efficient implementation techniques, and you'll find us using vaxes,
suns, mipses and sometimes 386s.

It's also worth noting that such languages are often used for the
implementation of programs requiring a huge address space.

I'm delighted to see that MIPS is going straight to 64 bits for both
pointers and integers, and hope that other manufacturers do the same.

>  And honestly, other languages which have pointers are not usually used
>to perform these atrocities. Other languages seem to be able to convince
>programmers that a pointer is not an int.

And that's one reason why you'll usually find Lisp and Prolog implemented
in C, not pascal.

-- Richard
-- 
Richard Tobin,                       JANET: R.Tobin@uk.ac.ed             
AI Applications Institute,           ARPA:  R.Tobin%uk.ac.ed@nsfnet-relay.ac.uk
Edinburgh University.                UUCP:  ...!ukc!ed.ac.uk!R.Tobin