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From: eachus@spectre.mitre.org (Robert I. Eachus)
Subject: Re: null pointer representation (was: Boolean Representation)
Date: 1998/10/02
Message-ID: <EACHUS.98Oct2143045@spectre.mitre.org>#1/1
X-Deja-AN: 397124067
References: <yecemt0kp9f.fsf@king.cts.com>
 <Ezw8Et.57H.0.-s@inmet.camb.inmet.com> <yec1zoyd2xa.fsf@king.cts.com>
 <evans-2709981430360001@ppp23.s9.pgh.net> <6umkbn$pv5$1@nnrp1.dejanews.com>
 <yecu31su3gj.fsf@king.cts.com> <6uo7p6$dk4$1@nnrp1.dejanews.com>
 <yec1zotnlor.fsf@king.cts.com>
Organization: The Mitre Corp., Bedford, MA.
Newsgroups: comp.lang.ada
Date: 1998-10-02T00:00:00+00:00
List-Id: <comp.lang.ada>

In article <yec1zotnlor.fsf@king.cts.com> Keith Thompson <kst@king.cts.com> writes:

  > Even if there weren't any such machines, I don't believe that the
  > assumption that System.Null_Address is all zero bits would be
  > justified -- though admittedly my argument would be a bit weaker.

   Some years back at Stratus, we used one not zero as the value for
null pointers, since it trapped if dereferenced.  When the 68020 came
along, we temporarily switched to xFFFFFFFF (I don't think that
version ever shipped.)  Then to a version of the OS where page zero
was inaccessable in user mode so that we could use zero as a null
address in user code.
--

					Robert I. Eachus

with Standard_Disclaimer;
use  Standard_Disclaimer;
function Message (Text: in Clever_Ideas) return Better_Ideas is...