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Q112298: DOC: Port I/O with inp() and outp() Fails on Windows NT

Article: Q112298
Product(s): Microsoft C Compiler
Version(s): 1.00 2.00
Operating System(s): 
Keyword(s): kbCRT kbVC
Last Modified: 22-JUL-2001

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The information in this article applies to:

- The C Run-Time (CRT), included with:
   - Microsoft Visual C++, 32-bit Editions, versions 1.0, 2.0 
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SUMMARY
=======

Attempting to use the port I/O (input/output) functions such as inp() and outp()
from within an application for Windows NT running in user mode causes a
privileged instruction exception to occur. The sample code provided in the
OUTP.C online help sample for the outp() function and related functions can be
used to illustrate this behavior.

The documentation regarding the compatibility of the port I/O functions is
incomplete. Win32-based applications that call inp(), outp(), and so forth can
be successfully compiled and linked. However, these applications will generate
the privileged instruction exception because the port I/O functions cannot be
called from code running in user mode.

Do not call the following functions from within a Win32-based application
executing in user mode:

  _inp()
  _inpw()
  _inpd()
  _outp()
  _outpw()
  _outpd()

NOTE: This documentation error has been corrected for the Visual C++ 4.0 Books
Online. The descriptions of the various port I/O functions do not list "Win NT"
in the Compatibility section.


MORE INFORMATION
================

According to the documentation provided with Visual C++ 32-bit Edition, inp(),
outp(), and the other port I/O related functions are Win32 and Win32s
compatible. However, executing code that uses these functions causes a
privileged instruction exception on Win32 on Windows NT. The code that uses
inp(), outp(), and so forth will execute correctly on Win32s.

The inp(), outp(), and other I/O port related functions map to privileged
processor instructions. For example, on Intel processors, the inp() and outp()
functions end up calling the IN and OUT instructions. The privileged instruction
exception occurs when these instructions are executed because typical Windows NT
applications execute in a nonprivileged (user) mode. Only code executing in
kernel mode has the necessary rights to execute privileged instructions. Kernel
mode code is typically found in device drivers.

For more information on user mode and kernel mode in Windows NT, refer to the
Microsoft Press book "Inside Windows NT" by Helen Custer. For an example of a
kernel mode Windows NT device driver that allows user mode applications to
access hardware ports, refer to the GENPORT sample provided with the Microsoft
Windows NT Device Driver Kit (DDK).

REFERENCES
==========

The Online help for _inp, _inpw, _inpd, _outp, _outpw, and _outpd.
The Microsoft Press book "Inside Windows NT" by Helen Custer.
The Microsoft Press book "Microsoft's 80386/80486 Programming Guide" by Ross P.
Nelson.
The "Kernel-Mode Device Driver Guide" provided with the Microsoft Windows NT DDK.

Additional query words: 1.00 2.00

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Keywords          : kbCRT kbVC 
Technology        : kbVCsearch kbAudDeveloper kbCRT
Version           : 1.00 2.00

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