RealView Compilation Tools Libraries and Floating Point Support Guide: 4.2.3. Microsoft compatibility functions

4.2.3. Microsoft compatibility functions

The following functions are implemented for compatibility with Microsoft products, to ease porting of floating-point code to the ARM architecture. They are defined in float.h.

_controlfp()

The function _controlfp() enables you to control exception traps and rounding modes:

unsigned int _controlfp(unsigned int new, unsigned int mask);

This function also modifies a control word using a mask to isolate the bits to modify. For every bit of mask that is zero, the corresponding control word bit is unchanged. For every bit of mask that is nonzero, the corresponding control word bit is set to the value of the corresponding bit of new. The return value is the previous state of the control word.

Note

This is not quite the same as the behavior of __ieee_status() (or __fp_status()), where you can toggle a bit by setting a zero in the mask word and a one in the flags word.

Table 4.8 describes the macros you can use to form the arguments to _controlfp().

Table 4.8. _controlfp argument macros

Macro	Description
`_MCW_EM`	Mask containing all exception bits
`_EM_INVALID`	Bit describing the Invalid Operation exception
`_EM_ZERODIVIDE`	Bit describing the Divide by Zero exception
`_EM_OVERFLOW`	Bit describing the Overflow exception
`_EM_UNDERFLOW`	Bit describing the Underflow exception
`_EM_INEXACT`	Bit describing the Inexact Result exception
`_MCW_RC`	Mask for the rounding mode field
`_RC_CHOP`	Rounding mode value describing Round Toward Zero
`_RC_UP`	Rounding mode value describing Round Up
`_RC_DOWN`	Rounding mode value describing Round Down
`_RC_NEAR`	Rounding mode value describing Round To Nearest

Note

The values of these macros are not guaranteed to remain the same in future versions of ARM products. To ensure that your code continues to work if the value changes in future releases, use the macro rather than its value.

For example, to set the rounding mode to round down, you would do:

_controlfp(_RC_DOWN, _MCW_RC);

To trap the Invalid Operation exception and untrap all other exceptions:

_controlfp(_EM_INVALID, _MCW_EM);

To untrap the Inexact Result exception:

_controlfp(0, _EM_INEXACT);

_clearfp()

The function _clearfp() clears all five exception sticky flags, and returns their previous value. The macros given in Table 4.8, for example _EM_INVALID, _EM_ZERODIVIDE, can be used to test bits of the returned result.

_clearfp() has the following prototype:

unsigned _clearfp(void);

_statusfp()

The function _statusfp() returns the current value of the exception sticky flags. You can use the macros shown in Table 4.8 to test bits of the returned result, for example, _EM_INVALID, _EM_ZERODIVIDE.

_statusfp has the following prototype:

unsigned _statusfp(void);

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