Microsoft KB Archive/112298

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

Article ID: 112298

Article Last Modified on 11/18/2003

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APPLIES TO

 The C Run-Time (CRT), when used with:  Microsoft Visual C++ 1.0 Professional Edition

 Microsoft Visual C++ 2.0 Professional Edition 

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This article was previously published under Q112298



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).

