timer0 UNO r3 using (ATmega328p)

i need to enable timer0 and when reach to 256 give flag

extern volatile unsigned long timer0_overflow_count;

unsigned long timer0_overflow_mark;
bool timer0_overflow_flag;

void loop( void )
{
  if ( timer0_overflow_count != timer0_overflow_mark )
  {
    timer0_overflow_flag = true;
    timer0_overflow_mark = timer0_overflow_count;
  }
}

timer0_overflow_flag is the "reach to 256 flag";