volatile unsigned char intr_flag = 0; // To track the occurence of ext interrupt
void ext_intr_init(){
  volatile unsigned char *INT_EICRA = 0x69, *INT_EICRB = 0x6A, *INT_EIMSK= 0x3D, *INT_SREG = 0x5F;
  *INT_EICRA = 0x03; //INT0 as rising edge
  *INT_EICRB = 0x00; //not using any other ext int
  *INT_EIMSK = 0x01; //External Interrupt Request INT0;
  *INT_SREG = 0x80; //ENabling I flag

}
void setup() {
  // put your setup code here, to run once:
  ext_intr_init();
  volatile unsigned char* ddr_f = 0x30;
  *ddr_f = 0x01; //0th bit as output
}

ISR(INT0_vect){
  intr_flag = 1;
}
void loop() {

  volatile unsigned char* out_f = 0x31;
  if(intr_flag){
    //handling debouncing logic by giving some delay
    delay(300);
    *out_f ^= 1; // Toggle LED on pin PF0 
    intr_flag = 0; // resetting the interrupt flag
  } 
}