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From: aryeh@eddie.mit.edu (Aryeh M. Weiss)
Newsgroups: comp.unix.wizards
Subject: Re: What machines core dump on deref of NULL?
Message-ID: <1990Jun30.134000.24937@eddie.mit.edu>
Date: 30 Jun 90 13:40:00 GMT
References: <444@mtndew.UUCP> <31079@cup.portal.com> <13226@smoke.BRL.MIL> <412@minya.UUCP> <1990Jun29.232656.20695@virtech.uucp>
Reply-To: aryeh@eddie.MIT.EDU (Aryeh M. Weiss)
Organization: MIT EE/CS Computer Facilities, Cambridge, MA
Lines: 14

Under SCO Xenix V/386, 386 native 32-bit (`small' model) programs dump core 
on NULL deref.  This is because location 0 is not allocated to the data space.
Actually, stack grows down from 0x1880000, while static and heap storage grow
up from this location (although this offset can be changed by a linker option).

The situation is drastically different for 286 16-bit programs under Xenix 386
or Xenix 286.  Small model 16-bit programs do not core dump because location 0
IS in the memory map.  On the other hand, large and compact model 16-bit
programs, where POINTERS are 32-bits (or 32-bit `far' pointers in medium/small
programs) will cause a core dump because the most significant 16 bits of the
pointer is actually a selector for the segment table and segment 0 cannot
exist.

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