GETFCR(2)                                               GETFCR(2)

     NAME
          getfcr, setfcr, getfsr, setfsr - control floating point

     SYNOPSIS
          #include <u.h>
          #include <libc.h>

          ulong getfcr(void)

          void setfcr(ulong fcr)

          ulong getfsr(void)

          void setfsr(ulong fsr)

     DESCRIPTION
          These routines provide a fairly portable interface to con-
          trol the rounding and exception characteristics of IEEE 754
          floating point units.  In effect, they define a pair of
          pseudo-registers, the floating point control register, fcr,
          which affects rounding, precision, and exceptions, and the
          floating point status register, fsr, which holds the accrued
          exception bits.  Each register has a get routine to retrieve
          its value, a set routine to modify it, and macros that iden-
          tify its contents.

          The fcr contains bits that, when set, halt execution upon
          exceptions: FPINEX (enable inexact exceptions), FPOVFL
          (enable overflow exceptions), FPUNFL (enable underflow
          exceptions), FPZDIV (enable zero divide exceptions), and
          FPINVAL (enable invalid operation exceptions).  Rounding is
          controlled by installing in fcr, under mask FPRMASK, one of
          the values FPRNR (round to nearest), FPRZ (round towards
          zero), FPRPINF (round towards positive infinity), and
          FPRNINF (round towards negative infinity).  Precision is
          controlled by installing in fcr, under mask FPPMASK, one of
          the values FPPEXT (extended precision), FPPSGL (single pre-
          cision), and FPPDBL (double precision).

          The fsr holds the accrued exception bits FPAINEX, FPAOVFL,
          FPAUNFL, FPAZDIV, and FPAINVAL, corresponding to the fsr
          bits without the A in the name.

          Not all machines support all modes.  If the corresponding
          mask is zero, the machine does not support the rounding or
          precision modes.  On some machines it is not possible to
          clear selective accrued exception bits; a setfsr clears them
          all.  The exception bits defined here work on all architec-
          tures.  Where possible, the initial state is equivalent to

     GETFCR(2)                                               GETFCR(2)

               setfcr(FPPDBL|FPRNR|FPINVAL|FPZDIV|FPOVFL);

          However, this may vary between architectures: the default is
          to provide what the hardware does most efficiently.  Use
          these routines if you need guaranteed behavior.  Also, grad-
          ual underflow is not available on some machines.

     EXAMPLE
          To enable overflow traps and make sure registers are rounded
          to double precision (for example on the MC68020, where the
          internal registers are 80 bits long):

               setfcr((getfcr() & ~FPPMASK) | FPPDBL | FPOVFL);

     SOURCE
          /sys/src/libc/$objtype/getfcr.s