Subversion Repositories planix.SVN

#include <u.h>

/* mul64fract(uvlong*r, uvlong a, uvlong b)
 *
 * Multiply two 64 numbers and return the middle 64 bits of the 128 bit result.
 *
 * The assumption is that one of the numbers is a 
 * fixed point number with the integer portion in the
 * high word and the fraction in the low word.
 *
 * There should be an assembler version of this routine
 * for each architecture.  This one is intended to
 * make ports easier.
 *
 *      ignored         r0 = lo(a0*b0)
 *      lsw of result   r1 = hi(a0*b0) +lo(a0*b1) +lo(a1*b0)
 *      msw of result   r2 =            hi(a0*b1) +hi(a1*b0) +lo(a1*b1)
 *      ignored         r3 = hi(a1*b1)
 */

void
mul64fract(uvlong *r, uvlong a, uvlong b)
{
        uvlong bh, bl;
        uvlong ah, al;
        uvlong res;

        bl = b & 0xffffffffULL;
        bh = b >> 32;
        al = a & 0xffffffffULL;
        ah = a >> 32;

        res = (al*bl)>>32;
        res += (al*bh);
        res += (ah*bl);
        res += (ah*bh)<<32;

        *r = res;
}