GETFCR(3)

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 control 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 identify 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 precision), 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 architectures. By default, the initial state is equivalent to

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

The default state of the floating point unit is fixed for a given architecture but is undefined across Plan 9: the default is to provide what the hardware does most efficiently. Use these routines if you need guaranteed behavior. Also, gradual 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):

    ulong fcr;
    fcr = getfcr();
    fcr |= FPOVFL;
    fcr &= ~FPPMASK;
    fcr |= FPPDBL;
    setfcr(fcr);

SOURCE
/usr/local/plan9/src/libc/$objtype/getfcr.s