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
}