From: <SANDMANN AT clio DOT rice DOT edu>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: ERROR 0004
Date: Sat,  8 Mar 1997 23:44:51
Organization: Rice University, Houston, Texas
Lines: 14
Message-ID: <3321f9f3.SANDMANN@clio.rice.edu>
References: <199703031001 DOT KAA02454 AT bryggen>
Reply-To: SANDMANN AT clio DOT rice DOT edu
NNTP-Posting-Host: clio.rice.edu
To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp

>    CWSDPMI protects djgpp programs from reading/writing null-pointers.
>    QDPMI does not. Don't know about Windows/OS2.

Only CWSDPMI and 386MAX V7+ support DPMI 1.0 extensions such that null
pointers can be protected.

> The opposite mode (bit 2=0) is supervisor mode. AFAIK, the CPU is always
> in user-mode (except for when e.g. laptops/green-PC's shuts down).
> Maybe the real experts out there care to comment.

An excellent summary.  The CPU might not be in user mode if using CWSDPR0
or PMODE, or if inside the DPMI provider itself.  Little hints like what
mode you are in sometimes pinpoints what's gone wrong.  The bit 2=0 is
an indication it's probably your bug and not something in the DPMI provider.