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
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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
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