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Q10826: Limitations on Writing Device Drivers in C

Article: Q10826
Product(s): Microsoft Programming Utilities
Version(s): 1.0,1.5,6.0,6.0a,6.0ax; MS-DOS:7.0
Operating System(s): 
Keyword(s): kb16bitonly
Last Modified: 06-MAY-2001

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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 
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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

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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

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