#define PIND  *((volatile char*)(0x29)) /* Input Pins, Port D */
#define DDRD  *((volatile char*)(0x2A)) /* Data Direction Register, Port D */
#define PORTD *((volatile char*)(0x2B)) /* Data register, Port D */ 

#define DDRB  *((volatile char*)(0x24)) /* Data Direction Register, Port B */
#define PORTB *((volatile char*)(0x25)) /* Data register, Port B */

void leds_on(char val) {
    PORTB = val;
}

char buttons_chk() {
    return PIND;
}

int main(void) {
    DDRB  = 0xFF; // Set PORTB to output
    PORTB = 0x00; // Turn off LEDs
    DDRD  = 0x00; // Set PORTD to input
    PORTD = 0x00; // Turn off pull-up resistor

    while (1) 
        leds_on(buttons_chk());

    return 0;
}