Q120079: PRB: INT 2Fh 1684h May Return ES == 0 and DI != 0

Article: Q120079
Product(s): Microsoft Windows Device Driver Kit
Version(s): 3.1
Operating System(s): 
Keyword(s): 
Last Modified: 03-NOV-1999

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The information in this article applies to:

- Microsoft Windows Device Development Kit (DDK) for Windows, version 3.1 
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SYMPTOMS
========

A DLL that calls INT 2Fh function 1684h (Get Device API Entry Point) to
communicate with a VxD may receive ES == 0 and DI != 0. This is contrary to the
documentation, which indicates that ES and DI will be zero on failure -- meaning
if either is non-zero, the function succeeded. However, zero is not a valid code
selector, so attempting to call the returned address results in a general
protection (GP) fault.

CAUSE
=====

This function uses the VMM service Allocate_PM_Call_Back to obtain an address
for the protected-mode DLL to call. Under low memory conditions, it is possible
for this service to be unable to create a valid code selector. Internally in
VMM, the resulting code selector value is not checked, resulting in it blindly
returning a code selector value of zero to the caller of Allocate_PM_Call_Back.
In turn, an invalid callback address is returned to the DLL with ES containing
zero.

RESOLUTION
==========

Code calling INT 1Fh function 1684h should check to see if ES contains zero on
return. If ES is zero and DI is zero, the VxD is not loaded or does not export a
protected mode entry point. If ES is zero and DI is non-zero, the VxD is loaded
and exports a protected mode entry point, but the entry point address cannot be
obtained due to low memory conditions.

Additional query words: 3.10 docerr

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Keywords          :  
Technology        : kbAudDeveloper kbWin3xSearch kbWinDDKSearch kbWinDDK310
Version           : :3.1

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