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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