CSAPP习题3.39 full_umul

// ASM实现全32位无符号乘法

Use the asm statement to implement a function with the following prototype:

void full_umul(unsigned x, unsigned y, unsigned dest[]);

This function should compute the full 64-bit product of its arguments and store the results in the destination array, with dest[0] having the low-order 4 bytes and dest[1] having the high-order 4 bytes

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void full_umul(unsigned x, unsigned y, unsigned dest[])
{
  /*
    movl x %eax
    mull y
    movl %eax dest[0]
    movl %edx dest[1]
   */
  asm("movl %2,%%eax; mull %3; movl %%eax,%0; movl %%edx,%1"
      :"=r" (dest[0]), "=r" (dest[1])
      :"r" (x), "r" (y)
      :"%eax", "%edx"
      );
}

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