Limitations on Writing Device Drivers in C

ID: Q10826

6.00 6.00a 6.00ax 7.00 | 1.00 1.50

MS-DOS                 | WINDOWS

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The information in this article applies to:

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

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.

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