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From: Michael S <already5chosen@yahoo.com>
Newsgroups: comp.arch
Subject: Re: Computer architects leaving Intel...
Date: Thu, 12 Sep 2024 18:43:07 +0300
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On Thu, 12 Sep 2024 08:15:29 -0700
Tim Rentsch <tr.17687@z991.linuxsc.com> wrote:

> Michael S <already5chosen@yahoo.com> writes:
> 
> > On Wed, 11 Sep 2024 17:34:38 -0700
> > Tim Rentsch <tr.17687@z991.linuxsc.com> wrote:
> >  
> >> Michael S <already5chosen@yahoo.com> writes:
> >>  
> >>>
> >>> 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.  
> 
> In that case, if the defaulted 'far' pointers don't follow the
> rules given in the C standard for regular pointers, then the
> implementation is not conforming.  Extensions are allowed only if
> they don't change the behavior of any strictly conforming
> program.  If undecorated pointer declarations don't observe this
> condition then it's not a valid extension, which in turn causes
> the implementation to be non-conforming.


Thinking about it, there likely were no way to create aliases via using
only standard language constructs. That's assuming that any use of
preserved values of pointers to de-allocated heap storage, including
use for comparison, is non-standard.
So it probably was conforming implementation at the end.