void timer2_int() using 5 {
   //reload and start it myself
   TR2 = 0; 
   TF2 = 0;
   TH2 = 0x15; TL2 = 0xFF;
   TR2 = 1;

   //Long operation (1000~ mechine cycles) that but will 
   //finish before next overflow will take place
   ...
   .. 
   .
   //end of long operation
}