a1_2.c

#define PAIN (volatile unsigned char *) 0xFFFFFFF0
#define PAOUT (volatile unsigned char *) 0xFFFFFFF1
#define PADIR (volatile unsigned char *) 0xFFFFFFF2
#define PBIN (volatile unsigned char *) 0xFFFFFFF3
#define PBOUT (volatile unsigned char *) 0xFFFFFFF4
#define PBDIR (volatile unsigned char *) 0xFFFFFFF5
#define CNTM (volatile unsigned int *) 0xFFFFFFD0
#define CTCON (volatile unsigned char *) 0xFFFFFFD8
#define CTSTAT (volatile unsigned char *) 0xFFFFFFD9
#define PCONT (volatile unsigned char *) 0xFFFFFFF7
#define IVECT (volatile unsigned int *) (0x20)

interrupt void intserv();
unsigned char digit = 0; /* Digit to be displayed */
unsigned char led = 0x1; /* LED state: 0/1 = on/off */

int main() {
  *PADIR = 0x0F; // set port A directions
  *PBDIR = 0xFF; // set port B directions (all output)
  *PCONT = 0x10; // enable interrupts when PAIN has input to be read
  asm(“MoveControl PSR,#0x40”); // enable interrupts
  *CTCON = 0x02; // turn countdown off initially and disable timer interrupt
  *CNTM = 100000000; // set countdown timer to 1 second
  *CTSTAT = 0x0; // clear "reached 0" bit
  *CTCON = 0x01; // turn countdown on, but keep timer interrupts disabled
  *IVECT = (unsigned int *) &intserv; // set interrupt vector
  *PAOUT = 0x0;

  while(1) {
    while(*CTSTAT == 0x0); // "reached 0" bit not set
    if(led == 0x0) { // LED is ON. Turn it OFF
      led = 0x1;
    }
    else {
      led 0x0;
    }
    *PBOUT = led; // set/unset LED bit in PBOUT
    *CTSTAT = 0x0; // clear "reached 0" bit
    *CNTM = 100000000; // set countdown timer to 1 second
  }
}

// triggered when the value of PAIN has changed
interrupt void intserv() {
  // button was released. it was pressed down prior to this
  if((*PAIN & 0x80) != 0) {
    digit = (digit + 1) % 10; // increment digit with rollover
    *PAOUT = digit; // set display digit bits
  }
}