From: "Charles Sandmann" <sandmann AT clio DOT rice DOT edu>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Getting cr2 in exception handler
Date: Mon,  8 Mar 1999 15:20:45
Organization: Aspen Technology, Inc.
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Message-ID: <36e3eacd.sandmann@clio.rice.edu>
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To: djgpp AT delorie DOT com
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> I have an application where I need to set up an exception handler for
> page faults and have access to the value of cr2 (linear address to which
> access faulted).  But:

> * mov cr2 is a privileged instruction.
> * There is a DPMI 1.0 exception handling function that passes it, but it
> will be tricky to make it interoperate with DJGPP's exception handlers,
> and anyway CWSDPMI doesn't support it.

Yes, this is all correct.

> I could, I suppose, use CWSDPR0 and read cr2 directly, but that's
> difficult, since virtual memory will probably be required.  (It's a
> malloc debugger whose memory usage is extremely inefficient, and I want
> it to run on small boxes if possible.)

> So: Does anyone know of another way to find the value of cr2?  I don't
> care about it working in anything but CWSDPMI.

There are some tricks that can probably be done to get the value:

1) Pull it out of the memory data structure using some tricks to find it.
2) Set up a ring-0 call gate and point it to some routine in your code.
   This is a hole in the CWSDPMI ring-3 security on purpose to allow
   such things.  You will need to have an Intel reference manual and 
   probably the CWSDPMI source to know what needs to be done, however.