startup.c

    0,                  /*!%5 RCC                          */                                           

    exti0_irq_handler,  /*!%6 EXTI Line0                   */                       

    0,                  /*!%7 EXTI Line1                   */

/* exti0 irq handler */

void exti0_irq_handler(void) {

    /* if button pressed */

    if (BUT_PORT->IDR & BUT1) {

        /* enable leds */

        LED_PORT->ODR |= ALL_LEDS;

    }

    /* if button not pressed */

    else {

        /* disable leds */

        LED_PORT->ODR &= ~ALL_LEDS;

    }

}