#define DDRB (*(volatile unsigned char *)(0x17 + 0x20))
#define PORTB (*(volatile unsigned char *)(0x18 + 0x20))

int main (void){
   int i;
   DDRB= 0xff;
   while(1){
     PORTB = 0x00;
     for(i=0;i<0xffff;i++);
     PORTB = 0xff;
     for(i=0;i<0xffff;i++);
   }
}