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_control87

Syntax

 
#include <float.h>

unsigned int _control87(unsigned int newcw, unsigned int mask);

Description

This function sets and retrieves the FPU's control word.

The control word is a special 16-bit register maintained by the math coprocessor. By setting and clearing bit fields in the control word, you can exercise control of certain aspects of coprocessor operation. The individual bits of the x87 control word are defined by macros in float.h, and shown in this table:

 
---- ---- --XX XXXX = MCW_EM - exception masks (1=handle exception
                                                internally, 0=fault)
---- ---- ---- ---X = EM_INVALID - invalid operation
---- ---- ---- --X- = EM_DENORMAL - denormal operand
---- ---- ---- -X-- = EM_ZERODIVIDE - divide by zero
---- ---- ---- X--- = EM_OVERFLOW - overflow
---- ---- ---X ---- = EM_UNDERFLOW - underflow
---- ---- --X- ---- = EM_INEXACT - rounding was required
---- --XX ---- ---- = MCW_PC - precision control
---- --00 ---- ---- = PC_24 - single precision
---- --10 ---- ---- = PC_53 - double precision
---- --11 ---- ---- = PC_64 - extended precision
---- XX-- ---- ---- = MCW_RC - rounding control
---- 00-- ---- ---- = RC_NEAR - round to nearest
---- 01-- ---- ---- = RC_DOWN - round towards -Inf
---- 10-- ---- ---- = RC_UP - round towards +Inf
---- 11-- ---- ---- = RC_CHOP - round towards zero
---X ---- ---- ---- = MCW_IC - infinity control (obsolete,
                                                 always affine)
---0 ---- ---- ---- = IC_AFFINE - -Inf < +Inf
---1 ---- ---- ---- = IC_PROJECTIVE - -Inf == +Inf

_control87 uses the value of newcw and mask variables together to determine which bits of the FPU's control word should be set, and to what values. For each bit in mask that is set (equals to 1), the corresponding bit in newcw specifies the new value of the same bit in the FPU's control word, which _control87 should set. Bits which correspond to reset (zero) bits in mask are not changed in the FPU's control word. Thus, using a zero value for mask retrieves the current value of the control word without changing it.

The exception bits MCW_EM (the low-order 6 bits) of the control word define the exception mask. That is, if a certain bit is set, the corresponding exception will be masked, i.e., it will not generate an FP exception (which normally causes signal SIGFPE to be delivered). A masked exception will be handled internally by the coprocessor. In general, that means that it will generate special results, such as NaN, Not-a-Number (e.g., when you attempt to compute a square root of a negative number), denormalized result (in case of underflow), or infinity (e.g., in the case of division by zero, or when the result overflows).

By default, DJGPP startup code masks all FP exceptions.

The precision-control field MCW_PC (bits 8 and 9) controls the internal precision of the coprocessor by selecting the number of precision bits in the mantissa of the FP numbers. The values PC_24, PC_53, and PC_64 set the precision to 24, 53, and 64-bit mantissa, respectively. This feature of the coprocessor is for compatibility with the IEEE 745 standard and only affect the FADD, FSUB FSUBR, FMUL, FDIV, FDIVR, and FSQRT instructions. Lowering the precision will not decrease the execution time of FP instructions.

The MCW_PC field is set to use the full-precision 64-bit mantissa by the DJGPP startup code.

The rounding-control field MCW_RC (bits 10 and 11) controls the type (round or chop) and direction (-Inf or +Inf) of the rounding. It only affects arithmetic instructions. Set to round-to-nearest state by the DJGPP startup code.

The infinity-control bit MCW_IC has no effect on 80387 and later coprocessors.

Return Value

The previous control word.

(Note that this is different from what _control87 from the Borland C library which returns the new control word.)

Portability

ANSI/ISO C No
POSIX No

Example

 
  /* mask all exceptions, except invalid operation */
  _control87 (0x033e, 0xffff);


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