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PDP-10 Archives
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SRI_NIC_PERM_FS_1_19910112
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c/lib5/math/frexp.c
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/*
* FREXP.C - split floating point number
*
* This code conforms with the description of the frexp function
* as defined in Harbison and Steele's "C: A Reference Manual",
* section 11.3.13
*
* For the DECSYSTEM-20 the double precision word format is:
*
* WORD N => SEEEEEEEEMMMMMMMMMMMMMMMMMMMMMMMMMMM
* WORD N+1 => XMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMM
*/
#define EXP_MASK 0377000000000 /* Mask for exponent */
#define MANT_MASK 0400777777777 /* Mask for mantissa */
#define LEXP_MASK 0377 /* Mask for shifted exponent */
#define EXP_SHIFTS 27 /* Shifts for exp in LSBs */
double frexp(x, nptr)
double x;
int *nptr;
{
int *vptr, exp;
if (x != 0) {
vptr = (int *) &x; /* point to first word of double x */
exp = ((*vptr & EXP_MASK) >> EXP_SHIFTS);
if (x < 0) /* get exponent, 1's comp if neg */
exp = ~exp & LEXP_MASK;
*nptr = exp - 128; /* side-effect +/- exponent value */
*vptr &= MANT_MASK; /* flush old exponent */
*vptr |= (((x < 0) ? (~128 & LEXP_MASK) : 128) << EXP_SHIFTS);
return x; /* insert 0 (or 1's c 0) as new exp */
}
else {
*nptr = 0; /* if for 0 arg, side-effect 0 for */
return 0.0; /* exponent and return 0 for mant */
}
}