AMD 250 用户手册

25112

Rev. 3.06

September 2005

The following examples illustrate the derivation of algorithm, multiplier and shift factor for signed 
and unsigned integer division.

The utility 

udiv.exe

 was compiled from the code shown in this section. The utilities provided in this 

document are for reference only and are not supported by AMD.

The following code derives the multiplier value used when performing integer division by constants. 
The code works for unsigned integer division and for odd divisors between 1 and 2

31

– 1, inclusive. 

For divisors of the form d = d' * 2n, the multiplier is the same as for d' and the shift factor is s + n.

/* This program determines the algorithm (a), multiplier (m), and

    shift factor (s) to be used to accomplish *unsigned* division by

    a constant divisor. Compile with MSVC.

*/

#include <stdio.h>

typedef unsigned __int64 U64;

typedef unsigned long    U32;

U32 log2(U32 i)

{

   U32 t = 0;

   i = i >> 1;

   while (i) {

      i = i >> 1;

      t++;

   }

   return(t);

}

U32 res1, res2;

U32 d, l, s, m, a, r, n, t;

U64 m_low, m_high, j, k;

int main (void)

{

   fprintf(stderr, "\n");

   fprintf(stderr, "Unsigned division by constant\n");

   fprintf(stderr, "=============================\n\n");