setfp_hw(3M) — UTek 4.0



SETFP(3M)               COMMAND REFERENCE               SETFP(3M)



NAME
     setfp, setfp_hw - set m68881 floating point unit to desired
     state.


SYNOPSIS
     #include <math.h>

     setfp (round, precision, trapmask)

     setfphw (value, clearit)

DESCRIPTION
     Setfp is the high-level user interface used to set the
     rounding mode, precision, and exception trap vector of the
     m68881.  Additionally, the trap_mask can be used to request
     a clear of the floating point status register and/or
     floating point instruction address register.  A value of -1
     in any of the parameters means no change for that feature.

     Setfphw is the low-level user interface used to set the
     m68881.  Value is the vector that is or'ed into the m68881
     to enable the desired features.  Clearit is the vector that
     is and'ed into the m68881 to disable previously enabled
     features.  Additionally, clearit can be used to request a
     clear of the floating point status register and/or floating
     point instruction address register.

     The rounding modes affect numbers which can not be
     represented exactly in IEEE floating point format; that is,
     numbers for which the binary representation of a decimal
     number requires more mantissa bits than are available.  Many
     familiar numbers (such as 0.3) require infinitely long
     binary mantissas to be represented exactly.  The choice of
     rounding mode selects which method of rounding shall be
     used.  Only one rounding mode may be selected at a time.
     The default rounding mode is FP_RND_NEAREST.

          Rounding modes:  Meaning:
          FP_RND_NEAREST   round to nearest number, if tie use even number
          FP_RND_TRUNCATE  round toward 0 by truncating
          FP_RND_DOWN      round toward negative infinity
          FP_RND_UP        round toward positive infinity

     The precision modes affect the number of mantissa bits used
     to represent numbers stored in the floating point registers
     of the m68881.  The default precision mode is
     FP_PREC_DOUBLE.  Only one precision mode may be selected at
     a time.  For programs or applications which heavily depend
     on repititive floating point calculations, it is strongly
     recommended that the precision mode be set to
     FP_PREC_EXTENDED.  Otherwise, the cumulative error resulting



Printed 4/6/89                                                  1



SETFP(3M)               COMMAND REFERENCE               SETFP(3M)



     from the lessened precision will cause the m68881 to yield
     incorrect results.

          Precision modes: Meaning:
          FP_PREC_EXTENDED extended precision, 64 bits
          FP_PREC_SINGLE   single precision, 24 bits
          FP_PREC_DOUBLE   double precision, 53 bits

     The trap_mask is used to choose which classes of floating
     point exceptions will be ignored, and which will cause
     exceptions. Any, all, or none of the exception classes may
     be enabled at any given time by or'ing the desired exception
     classes together.  Previous enabled traps are cleared when a
     new trap_mask (other than -1) is selected. The default
     setting for trap_mask clears the floating point status and
     instruction address registers, and enables the FP_TRP_BSUN,
     FP_TRP_OPERR, FP_TRP_OVFL, and FP_TRP_DZ exception classes.

          Exception classes:               Meaning:
          FP_TRP_BSUN      trap when BSUN set
          FP_TRP_SNAN      trap when SNAN set
          FP_TRP_OPERR     trap when OPERR set
          FP_TRP_OVFLO     trap when OVFL set
          FP_TRP_UNFLO     trap when UNFL set
          FP_TRP_DIVZERO   trap when DZ set
          FP_TRP_INEX1     trap when INEX1 set
          FP_TRP_INEX2     trap when INEX2 set
          FP_CLR_FPSR      clear FPSR status
          FP_CLR_FPIAR     clear FPIAR status

     For details on the various floating point exception classes,
     see the Motorola MC68881 Floating-Point Coprocessor User's
     Manual (available from Prentice-Hall).

FILES
     /usr/lib/libm.a
                    Mathematical library functions.

     /usr/include/setfp.h
                    Defines to set m68881 to desired state.















Printed 4/6/89                                                  2

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