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Q119458: Result of _fpreset() Is Not _CW_DEFAULT

Article: Q119458
Product(s): Microsoft C Compiler
Version(s): winnt:
Operating System(s): 
Keyword(s): kbCRT kbVC
Last Modified: 29-JUL-2001

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The information in this article applies to:

- The C Run-Time (CRT), included with:
   - Microsoft Visual C++, versions 1.0, 1.50 
   - *EDITOR Please do not choose this product*Microsoft Visual C++ 32-bit Edition* use 241, 265, 225, versions 1.0, 2.0, 2.1 
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SUMMARY
=======

Calling _fpreset() reinitializes the floating-point math package. The header
file FLOAT.H contains the following definition of _CW_DEFAULT:

      /* initial Control Word value */ 

     #ifdef  _M_IX86

     #define _CW_DEFAULT ( _RC_NEAR + _PC_64 + _EM_INVALID + _EM_ZERODIVIDE +
        _EM_OVERFLOW + _EM_UNDERFLOW + _EM_INEXACT )

     #endif

You might expect that if you call _fpreset() and then call _controlfp(0,0) to get
the control word, that the control word will be _CW_DEFAULT; however, that is
not the case. _CW_DEFAULT does not actually correspond to the default value of
the x87 control word. The difference is that _CW_DEFAULT does not mask the
denormal exception. The _fpreset() routine masks all exceptions, including
denormal.

The denormal exception does not belong to the set of floating-point exceptions
specified by the IEEE Standard. You cannot mask or unmask the exception using
_controlfp(). The abstract control word used by the C run- time reserves the
upper bits for machine-specific features. Therefore, you should not compare full
control-word encodings; instead, you should compare the bits that you are
interested in.

Additional query words: kbinf 1.00 1.50 2.00 2.10

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Keywords          : kbCRT kbVC 
Technology        : kbVCsearch kbAudDeveloper kbCRT
Version           : winnt:

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