// counting_bits: x86 (32-bit) embedded assembly to count number of set bits

extern "C" int bits_set(int value);

__asm__(
"_bits_set:\n"
"   mov %ecx, %ebx\n"
"   mov $0, %ecx\n"     // ecx: bit counter
"check_zero:\n"
"   cmp $0, %ebx\n"     // done? (that is, no more bits)
"   je done\n"
"   mov %ebx, %edx\n"   // lsb of %ebx is set?
"   and $0x01, %edx\n"
"   jz lsb_is_zero\n"
"   add $1, %ecx\n"     // if so, count it
"lsb_is_zero:\n"
"   shr $1, %ebx\n"     // shift off the bit just counted
"   jmp check_zero\n"   // try next bit
"done:\n"
"   mov %ecx, %eax\n"
"   ret\n"
);

#include <iostream>
using namespace std;

int main() {
  cout << "Enter a number: ";
  int num;
  cin >> num;
  int number_of_bits_set = bits_set(num);
  cout << num << " has " << number_of_bits_set << " bits set" << endl;
  return 0;
}