Inline Assembly Code (Cont.)

Code Example—Checking a Prime Number
An integer n is called a prime number if the only positive integers that divide n are 1 and n itself. Integers that are not prime are called composite. Here are the first few prime numbers: 

 
   2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, ...
To check if a number n>2 is prime, repeatedly divide it by numbers m, where 2≤m≤½×n. The number is prime if none of the remainders is zero.

 Prime.cpp  

 
// Prime.cpp——Checking a prime number


 
#include  <iostream>

 
#include  "CheckPrime.h"

 
using namespace  std;


 
int   number;

 
char  result;



 
int  main( ) {

 
  cout << "Enter the number to be checked (>1): ";

 
  cin  >> number;


 
  cout << "The number " << number;

 
  if ( number < 2 )

 
    cout << " must be greater than 1.";

 
  else if ( ( number == 2 ) || ( number == 3 ) )

 
    cout << " is a prime number.";

 
  else {

     

      

 

    

      
      

 

    
    
       
     

      
     

    

 
    if ( result == 't' )

 
      cout << " is a prime number.";

 
    else

 
      cout << " is NOT a prime number.";

 
  }

 
  cout << endl << endl;

 
  return( 0 );

 
}
 CheckPrime.h  

 
// CheckPrime.h——Containing a single function prototype


 
void  CheckPrime( int number, char* result );
 CheckPrime.cpp  

 
// CheckPrime.cpp——Containing an asm block


 
#include  "CheckPrime.h"


 
void  CheckPrime( int number, char* result ) {

 
  __asm {

 
       mov  ecx, number

     

      
       

 
            // (Shift right) ECX = ECX / 2
      
      

 
    
    
       
     

      
     

    

 
   L1: mov  edx, 0

 
       mov  eax, number     // EDX:EAX = Dividend

 
       div  ecx             // ECX     = Divisor

 
       cmp  edx, 0          // EDX     = Remainder

 
       je   L2

 
       dec  ecx

 
       cmp  ecx, 1

 
       jg   L1


 
       mov  esi, result

 
       mov  [esi], 't'

 
       jmp  L3

 
   L2: mov  esi, result

 
       mov  [esi], 'f'

 
   L3:

 
  } // asm

 
}


      “I’m not arguing, I’m just explaining why I’m right.”    
      ― Unknown