int main (void) {
    void delayMs(int n);
    PORTD->PCR[0] = 0x100;      /* make PTD0 pin as GPIO */
    PORTD->PCR[1] = 0x100;      /* make PTD1 pin as GPIO */
    PORTD->PCR[2] = 0x100;      /* make PTD2 pin as GPIO */
    PORTD->PCR[3] = 0x100;      /* make PTD3 pin as GPIO */
    PORTD->PCR[7] = 0x103;      /* make PTD7 pin as GPIO and enable pullup */
    PTD->PDDR |= 0x0F;          /* make PTD0-3 as output pin */
    PTD->PDDR &= ~0x80;         /* make PTD7 as input pin */
    
    if((PTD->PDIR & 0x80) == 0)
    {   /* PTD7 == 0 */
        PTD->PDOR &= ~0x0F;     /* open all switches */
        delayMs(100);           /* wait 0.1 second */
        PTD->PDOR |= 0x09;      /* close SW1 & SW4 */

        while((PTD->PDIR & 0x80) == 0) ;    /*PTD7 == 0 */
    }
    else
    {   /* PTD7 == 1 */
        PTD->PDOR &= ~0x0F;     /* open all switches */
        delayMs(100);           /* wait 0.1 second */
        PTD->PDOR |= 0x06;      /* close SW2 & SW3 */
        while((PTD->PDIR & 0x80) != 0) ;    /*PTD7 == 0 */
    }
}