FP_CONTROL(3m,L)            AIX Technical Reference            FP_CONTROL(3m,L)



-------------------------------------------------------------------------------
fp_control, fp_restore, fp_exmask, fp_exunmask, fp_round,
fp_precision, fp_getcw, fp_getex, fp_getprecision, fp_getround,
fp_getsw



PURPOSE

Sets and reads the 80387 control word.

LIBRARY

Math Library (libm.a)

SYNTAX

#include <sys/fpcontrol.h>




                void fp_control(x)          void fp_restore()
                unsigned int(x)
                                            void fp_exunmask(x)
                void fp_exmask(x)           unsigned int(x)
                unsigned int(x)
                                            void fp_precision(x)
                void fp_round(x)            unsigned int(x)
                unsigned int(x)
                                            unsigned int fp_getex()
                unsigned int fp_getcw()
                                            unsigned int fp_getround()
                unsigned int fp_getprecision()
                                            unsigned int fp_getsw()



DESCRIPTION

The fp_control subroutine takes as an argument a new value for the 80387
control word.

The fp_exmask subroutine takes as an argument a bit mask corresponding to the
bits in the 80387 control word interrupt mask fields.  The bits are set in the
control word causing the corresponding exceptions to be masked.

The fp_exunmask subroutine takes as an argument a bit mask corresponding to the
bits in the 80387 control word interrupt mask fields.  The bits are cleared in
the control word causing the corresponding exceptions to be unmasked.





Processed November 7, 1990     FP_CONTROL(3m,L)                               1






FP_CONTROL(3m,L)            AIX Technical Reference            FP_CONTROL(3m,L)



The fp_round subroutine takes as an argument the bit patterns corresponding to
the bits in the 80387 control word rounding mode field.  The rounding mode is
set to the pattern specified.

The fp_precision subroutine takes as an argument the bit patterns corresponding
to the bits in the 80387 control word precision mode field.  The precision mode
is set to the pattern specified.

The fp_restore subroutine restores the 80387 control word to the value and mode
which existed before any of the above described subroutines were called.  If
none of the above described subroutines were called, then no action is taken.

The fp_getcw function returns the 80387 control word.

The fp_getex function returns the exception mask portion of the 80387 control
word.

The fp_getround function returns the rounding mode portion of the 80387 control
word.

The fp_getprecision function returns the precision mode portion of the 80387
control word.

The fp_getsw function returns the 80387 status word.

As an aid in specifying the exception mask bits and the rounding and precision
mode bit patterns, a set of definitions has been provided in the include file
sys/fpcontrol.h.

Exception masks:


  FPM_INV_OP     0x0001

  FPM_DENORM     0x0002

  FPM_DIVIDE_0   0x0004

  FPM_OVERFLOW   0x0008

  FPM_UNDERFLOW  0x0010

  FPM_PRECISION  0x0020

Rounding modes:










Processed November 7, 1990     FP_CONTROL(3m,L)                               2






FP_CONTROL(3m,L)            AIX Technical Reference            FP_CONTROL(3m,L)




  FPR_NEAR       0x0000

  FPR_DOWN       0x0400

  FPR_UP         0x0800

  FPR_CHOP       0x0c00


Precision modes:


  FPP_SINGLE     0x0000

  FPP_DOUBLE     0x0200

  FPP_EXTENDED   0x0300

These definitions can be used in a variety of ways.  They can be ORed together
to form an argument for the fp_control subroutine.  If this is the case, any
number of the exception mask symbols can be ORed with one of each of the
rounding and precision mode values.  For example:

  fp_control(FPP_SINGLE | FPR_UP | FPM_PRECISION | FP_OVERFLOW);

selects single-precision results of calculations, rounding up to positive
infinity, and mask exception interrupts for precision and overflow exceptions.

The values can be used in the appropriate subroutine to modify a specific part
of the 80387 control word.  For example:

  fp_precision(FPP_EXTENDED)

Note:  Using a value with a subroutine which is not appropriate could produce
       an undesired result.  The values can all be used with the fp_control
       subroutine as described above.

The values supplied in fpcontrol.h can be used with the fp_getcw, fp_getround,
fp_getex, and fp_getprecision subroutines to determine what is currently
selected.  For example:

  (fp_getex() & FPM_INV_OP)

returns a nonzero value if the invalid operation exception is masked.

  (fp_getround() == FPR_CHOP)

returns a nonzero value if chop (truncate towards zero) rounding mode is
currently selected in the 80387 control word.





Processed November 7, 1990     FP_CONTROL(3m,L)                               3