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From: Michael S <already5chosen@yahoo.com>
Newsgroups: comp.arch
Subject: Re: Computer architects leaving Intel...
Date: Thu, 12 Sep 2024 14:10:33 +0300
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On Wed, 11 Sep 2024 17:34:38 -0700
Tim Rentsch <tr.17687@z991.linuxsc.com> wrote:

> Michael S <already5chosen@yahoo.com> writes:
> 
> > On Wed, 11 Sep 2024 09:29:04 -0700
> > Tim Rentsch <tr.17687@z991.linuxsc.com> wrote:
> >  
> >> Josh Vanderhoof <x@y.z> writes:
> >>
> >> [how to write a portable, UB-free check if mempcy() intervals
> >> overlap] 
> >>> It is legal to test for equality between pointers to different
> >>> objects  
> >>
> >> Right.  This observation is the key insight.  
> >
> > Real mode x86 C compilers operating in Large and Compact Models that
> > were popular on IBM-compatible PCs 30-40 years ago could have more
> > than one representation for the pointer to the same memory
> > location.  If my memory serves me, the rules of pointers comparison
> > for equality were the same as rules of comparison for <>.  In both
> > cases for reliable result pointers had to be explicitly normalized
> > (i.e. converted from 'far' to 'huge' or something like that).
> >
> > It was long time ago and even back then I didn't use Large model
> > very often, so it's possible that I misremember.  But if I remember
> > correctly, does it mean that those C compilers now would be
> > considered non-compliant?  
> 
> The C standard was first ratified (by ANSI) in 1989.  The rules
> for pointer comparison were clarified in the C99 standard, but it
> has always been true that pointers to the same object have to
> compare equal.
> 
> C environments that have things like 'far' or 'huge' pointers,
> etc, are not standard C but must have extensions so that they can
> deal with the different kinds of pointers.  Depending on how the
> non-standard kinds of pointer worked, the implementation might or
> might not be conforming.  Most likely though it's a moot point
> because once a program starts using an extension all the rules
> can change, and the C standard allows that.  It's only programs
> that look like really standard C that have to do what the C
> standard says (for the implementation to be conforming);  any
> code that declares a 'far' pointer or 'huge' pointer certainly
> isn't standard C.

In Compact and Large models data pointers are 'far' by default. So,
the source doesn't have to use non-standard declarations.