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

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Article ID: 112298
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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.

Properties

Article ID: 112298 - Last Review: June 17, 2014 - Revision: 3.0
Keywords: 
kbcrt KB112298
Retired KB Content Disclaimer
This article was written about products for which Microsoft no longer offers support. Therefore, this article is offered "as is" and will no longer be updated.

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