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PDP-10 Archives
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SRI_NIC_PERM_FS_1_19910112
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c/lib5/math/ldexp.c
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/*
* LDEXP.C - scale exponent of double floating number
*
* This code conforms with the description of the ldexp function
* as defined in Harbison & Steele's "C: A Reference Manual",
* section 11.3.15
*
* For the DECSYSTEM-20 the double precision word format is:
*
* WORD N => SEEEEEEEEMMMMMMMMMMMMMMMMMMMMMMMMMMM
* WORD N+1 => XMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMM
*
* Note that on the DECSYSTEM-20, the exponent for negative
* numbers is encoded as 1's complement.
*
*/
#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 ldexp(value, scale)
double value;
int scale;
{
int *vpntr, exp;
vpntr = (int *) &value;
exp = (*vpntr & EXP_MASK) >> EXP_SHIFTS; /* get old exponent */
if (value < 0.0)
exp = ~exp & LEXP_MASK; /* 1's c if neg # */
exp += scale; /* scale it now */
if (value < 0.0)
exp = ~exp & LEXP_MASK;
*vpntr = (*vpntr & MANT_MASK) | ((exp << EXP_SHIFTS) & EXP_MASK);
return value;
}