x86 Assembly programming - Introduction to the x86 instruction set

1:  //******************************************  
2:  //Note st0 refers to the top of  
3:  //the floating point stack.  
4:  //1. fldpi loads st0 with PI.  
5:  //2. flds or fldl loads st0 with the radius provided by the user  
6:  //flds is used for float and fldl is used if the radius is  
7:  //declared as double. This is required by the gnu assembler.  
8:  //3. (first fmul) multiply st0 with st0  this computes r*r  
9:  //4. (second fmul) multiply st0 with st1 this computes pi * r * r  
10:  //Then save the top of floating point stack(st0) into the variable area.  
11:  //In inline assembly, the "=t" means top of floating point stack and in   
12:  //this program it is mapped to area.  
13:  //******************************************  
14:  #include <stdio.h>  
15:  int main(int argc, char* argv[]){  
16:   double radius, area;  
17:   printf("Enter the radius\n");  
18:   scanf("%lf", &radius);  
19:   asm volatile(  
20:    "fldpi\n"   
21:    "fldl %1\n"   
22:    "fmul %%st(0),%%st(0)\n" //compute r ^ 2  
23:    "fmul %%st(1), %%st(0)\n" //compute pi * r^2  
24:    :"=t"(area)  
25:    :"m"(radius)  
26:    :  
27:   );  
28:   printf("The area is %lf\n",area);  
29:   return 0;  
30:  }  

 $ gcc -g -Fstabs area.c

 $ ./a.out 
Enter the radius
7.0
The area is 153.938040