86Box/src/cpu/x87_ops_conv.h

#define BIAS80 16383
#define BIAS64 1023

typedef struct {
    int16_t begin;
    union {
        double   d;
        uint64_t ll;
    } eind;
} x87_conv_t;

static __inline double
x87_from80(x87_conv_t *test)
{
    int64_t exp64;
    int64_t blah;
    int64_t exp64final;
    int64_t mant64;
    int64_t sign;

    exp64      = (((test->begin & 0x7fff) - BIAS80));
    blah       = ((exp64 > 0) ? exp64 : -exp64) & 0x3ff;
    exp64final = ((exp64 > 0) ? blah : -blah) + BIAS64;

    mant64 = (test->eind.ll >> 11) & (0xfffffffffffffLL);
    sign   = (test->begin & 0x8000) ? 1 : 0;

    if ((test->begin & 0x7fff) == 0x7fff)
        exp64final = 0x7ff;
    if ((test->begin & 0x7fff) == 0)
        exp64final = 0;
    if (test->eind.ll & 0x400)
        mant64++;

    test->eind.ll = (sign << 63) | (exp64final << 52) | mant64;

    return test->eind.d;
}

static __inline void
x87_to80(double d, x87_conv_t *test)
{
    int64_t sign80;
    int64_t exp80;
    int64_t exp80final;
    int64_t mant80;
    int64_t mant80final;

    test->eind.d = d;

    sign80      = (test->eind.ll & (0x8000000000000000LL)) ? 1 : 0;
    exp80       = test->eind.ll & (0x7ff0000000000000LL);
    exp80final  = (exp80 >> 52);
    mant80      = test->eind.ll & (0x000fffffffffffffLL);
    mant80final = (mant80 << 11);

    if (exp80final == 0x7ff) /*Infinity / Nan*/
    {
        exp80final = 0x7fff;
        mant80final |= (0x8000000000000000LL);
    } else if (d != 0) { /* Zero is a special case */
        /* Elvira wants the 8 and tcalc doesn't */
        mant80final |= (0x8000000000000000LL);
        /* Ca-cyber doesn't like this when result is zero. */
        exp80final += (BIAS80 - BIAS64);
    }
    test->begin   = (((int16_t) sign80) << 15) | (int16_t) exp80final;
    test->eind.ll = mant80final;
}