// (see sections 13.2.2 and 13.4 on datasheet)
#define PINB (*(volatile unsigned char*) 0x23)
#define DDRB (*(volatile unsigned char*) 0x24)

int main(void) {
    DDRB |= (1 << 5);

    while(1) {
        PINB |= (1 << 5);

        for(volatile long i = 0; i < 500000L; ++i)
            ;
    }

    return 0;
}