DOCUMENT:Q10826  06-MAY-2001  [utilities]
TITLE   :Limitations on Writing Device Drivers in C
PRODUCT :Microsoft Programming Utilities
PROD/VER::1.0,1.5,6.0,6.0a,6.0ax; MS-DOS:7.0
OPER/SYS:
KEYWORDS:kb16bitonly

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

 - Microsoft C for MS-DOS, versions 6.0, 6.0a, 6.0ax 
 - Microsoft C/C++ for MS-DOS, version 7.0 
 - Microsoft Visual C++, versions 1.0, 1.5 
-------------------------------------------------------------------------------

SUMMARY
=======

You cannot develop a complete device driver only in C or C++. You must write
some assembly code and you cannot use the majority of functions in the C
run-time library.

The three major problems that occur when you develop a device driver in C are as
follows:

 - The .EXE file loader is in COMMAND.COM which is not available when MS-DOS
   loads a device driver. Therefore, a device driver must be the binary image of
   a single-segment program. Microsoft C/C++ version 7.0 introduced the ability
   to generate code in the tiny memory model in which the application stores its
   code and data in one memory segment.

 - The C run-time initialization code requires an appropriate PSP (program
   segment prefix) and requires MS-DOS to be available. Even though the source
   code to the startup code is provided with the C compiler, most of the startup
   code is in assembler and would require major modifications to be useful in a
   driver.

 - The entry-exit sequences used by C functions are inappropriate for use in an
   MS-DOS driver.

You can work around some of these limitations by using the C compiler to generate
assembly code. The -Fa switch directs the compiler to produce an assembly code
listing that you can edit and use as part of the driver source code. Depending
on the complexity of the MS-DOS device driver, you could write most of the
driver in C and modify and maintain the code relatively easily.

Additional query words: kbinf 6.00 6.00a 6.00ax 7.00 1.00 1.50

======================================================================
Keywords          : kb16bitonly 
Technology        : kbVCsearch kbAudDeveloper kbPTProdChange kbvc150 kbvc100 kbCCompSearch kbZNotKeyword3 kbCComp600DOS kbCComp600aDOS kbCComp600axDOS kbCVC700DOS
Version           : :1.0,1.5,6.0,6.0a,6.0ax; MS-DOS:7.0

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

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.

Copyright Microsoft Corporation 2001.