KnowledgeBase Archive

An Archive of Early Microsoft KnowledgeBase Articles

View on GitHub

Q63974: IOCtl Calls in Protected-Mode Microsoft Windows

Article: Q63974
Product(s): Microsoft Windows Device Driver Kit
Version(s): 3.0,3.1
Operating System(s): 
Keyword(s): 
Last Modified: 06-NOV-1999

-------------------------------------------------------------------------------
The information in this article applies to:

- Microsoft Windows Device Development Kit (DDK) for Windows, versions 3.0, 3.1 
-------------------------------------------------------------------------------

SUMMARY
=======

Generally, Microsoft Windows translates standard MS-DOS IOCtl calls in the two
protected modes (standard mode and enhanced mode). Therefore, in most cases an
application can make IOCtl calls regardless of the mode in which Windows is
running. However, if the buffer passed to the IOCtl call contains pointers to
additional data, the calling application must translate those pointers and the
data referenced by those pointers into a form that can be used in real mode,
before making the IOCtl call.

MS-DOS functions are executed only in real mode, and thus can only access data
that is located in the first 1 MB of memory. To work within this limitation, the
memory referenced by the pointers contained in the buffer must be allocated
below 1 MB. In addition, the buffer must contain the segment address of the
data, rather than the protected-mode selector.

For example, consider IOCtl call Write Control Data to Character Device Driver,
Interrupt 21h Function 44h Subfunction 03h. The parameters to this call are as
follows:

  AH = 44H
  AL = 03H
  BX = handle
  CX = number of bytes to write
  DS:DX = segment:offset address of data buffer

Because this interface is known to protected-mode Windows, Windows is able to
translate (map) the protected-mode address passed in DS:DX to a memory location
that is accessible in real mode. Because of this translation, in most cases the
application is not required to perform any additional work to make the call. The
caller should set the DS:DX register pair to the selector:offset address of the
data buffer.

If an MS-DOS device driver defines a special interface that requires pointers to
additional data be passed in the main data buffer, the caller must ensure the
pointers passed in the data buffer can be used in real mode. The caller should
allocate the memory referenced by these pointers using the GlobalDOSAlloc()
function.

GlobalDOSAlloc() returns a double-word. If the allocation is successful, the
high-order word contains the segment:paragraph address of the allocated buffer,
while the lower-order word contains the selector to the buffer. The selector is
used to access the buffer in protected mode; the segment:paragraph address can
be used in real mode. Use the GlobalDOSFree() function to free memory allocated
by GlobalDOSAlloc().

Additional query words: 3.00 3.10 DDKDPMI DDKTSR

======================================================================
Keywords          :  
Technology        : kbAudDeveloper kbWin3xSearch kbWinDDKSearch kbWinDDK300 kbWinDDK310
Version           : :3.0,3.1

=============================================================================

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.