at&t inline assembly example

#include <stdio.h>
unsigned short len;
unsigned short width;
// On a 64Bit System (w/ 64bit OS) int is 4Byte, thus long enough
unsigned int area;

int main()
{

  // Figure out the size of a short
  //printf("sizeof(short) == %lu Byte\n", sizeof(short));
  // 2 Byte

  // Figure out the size of a int
  //printf("sizeof(int) == %lu Byte\n", sizeof(int));
  // 4 Byte

  // Figure out the size of a long
  //printf("sizeof(long) == %lu Byte\n", sizeof(long));
  // 8 Byte

/*
  __asm__ __volatile__
    (
     "movw $10000, (width)\n\t"
     "movw $5000, (len)\n\t"
     "movw (width), %%ax\n\t"
     // Multiply. Result goes to DX:AX
     "mulw (len)\n\t"
     // Delete everyting in edx, except for the 16 lowest Bit
     "and $0x0ffff, %%edx\n\t"
     "movl %%edx, (area)\n\t"
     "movw %%ax, %%bx\n\t"
     // Move the upper 16 Bit of the result into eax
     "movl (area), %%eax\n\t"
     // Shift them 16 Bit to the left
     "movl $0x10000, %%ecx\n\t"
     "mull %%ecx\n\t"
     // the result ends up in dx:ax
     // Now add the pieces together
     "add %%bx, %%ax\n\t"
     "mov %%eax, (area)"
     :
     :
     : "%eax", "%ecx", "%edx"
    );
*/
  __asm__ __volatile__
    (
     "movw $10000, (width)\n\t"
     "movw $5000, (len)\n\t"
     "movw (width), %%ax\n\t"
     // Multiply. Result goes to DX:AX
     "mulw (len)\n\t"
     // only copy dx (the upper 16 Bit of the result)
     "movw %%dx, (area)\n\t"
     // Copy the lower 16 Bit of the result to bx
     "movw %%ax, %%bx\n\t"
     // Move the upper 16 Bit of the result into eax
     "movl (area), %%eax\n\t"
     // Now shift them so that they're in the right position
     "sal $16, %%eax\n\t"
     // Now add the pieces together
     "add %%bx, %%ax\n\t"
     "mov %%eax, (area)"
     : /* no output registers */
     : /* no input registers */
     : "%eax", "%ecx"
    );

  printf("width is now: %d \n", width);
  printf("len is now: %d \n", len);
  printf("area is now: %d \n", area);

  return 0;
}
/*
                mov width, 10000
                mov len, 5000
                mov ax, width
                mul len
                // make sure edx is clean, except for the value in dx
                and edx, 0ffffh
                mov area, edx
                mov bx, ax
                mov eax, area
                mov ecx, 10000h
                mul ecx
                add ax, bx
                mov area, eax
*/