mul64fract.c

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    1 #include <u.h>
    2 
    3 /* mul64fract(uvlong*r, uvlong a, uvlong b)
    4  *
    5  * Multiply two 64 numbers and return the middle 64 bits of the 128 bit result.
    6  *
    7  * The assumption is that one of the numbers is a 
    8  * fixed point number with the integer portion in the
    9  * high word and the fraction in the low word.
   10  *
   11  * There should be an assembler version of this routine
   12  * for each architecture.  This one is intended to
   13  * make ports easier.
   14  *
   15  *      ignored         r0 = lo(a0*b0)
   16  *      lsw of result   r1 = hi(a0*b0) +lo(a0*b1) +lo(a1*b0)
   17  *      msw of result   r2 =            hi(a0*b1) +hi(a1*b0) +lo(a1*b1)
   18  *      ignored         r3 = hi(a1*b1)
   19  */
   20 
   21 void
   22 mul64fract(uvlong *r, uvlong a, uvlong b)
   23 {
   24         uvlong bh, bl;
   25         uvlong ah, al;
   26         uvlong res;
   27 
   28         bl = b & 0xffffffffULL;
   29         bh = b >> 32;
   30         al = a & 0xffffffffULL;
   31         ah = a >> 32;
   32 
   33         res = (al*bl)>>32;
   34         res += (al*bh);
   35         res += (ah*bl);
   36         res += (ah*bh)<<32;
   37 
   38         *r = res;
   39 }

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FreeBSD/Linux Kernel Cross Reference sys/port/mul64fract.c

FreeBSD/Linux Kernel Cross Reference
sys/port/mul64fract.c