// GPIO AVR addresses
volatile uint8_t* portb = 0x25;
volatile uint8_t* ddrb = 0x24;
volatile uint8_t* pinb = 0x23;

int main(void){
  *ddrb |= (1 << 5); // Setting pin 13 as output
  for(;;){
    // Toggle pin logic
    if ((*portb >> 5) & 1){ // Reading pin 13
      *portb &= ~(1 << 5);  // Clearing pin 13 if its state is true
    } else {
      *portb |= (1 << 5); // Setting pin 13 if its state is false
    }
    
    // A very, very imprecise delay
    for (volatile uint64_t timer = 0 ; timer < 80000 ; timer++);
  }
}