a1_1.c

#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 = 0x0; /* LED state: 0/4 = on/off */

int main() {
  *PBDIR = 0xFC; // set port b directions
  *CTCON = 0x02; // turn countdown off initially and disable timer interrupt
  *CTSTAT = 0x0; // clear "reached 0" bit
  *CNTM = 100000000; // set countdown timer to 1 second
  *IVECT = (unsigned int *) &intserv; // set interrupt vector
  asm(“MoveControl PSR,#0x40”); // enable interrupts
  *PBOUT = 0x04; //set display to 0 and turn off LED (0)
  *PCONT = 0x40; // SET ENBIN=1 - enable interrupts when PBIN is ready for writing
  *CTCON = 0x01; // start countdown timer

  while(1) {
    while((*CTSTAT & 0x1) != 0x1); // "reached 0" bit not set
    if(led == 0x0) { // LED is ON. Turn it OFF
      led = 0x4;
    }
    else {
      led 0x0;
    }
    *PBOUT = ((digit << 4) | led); // set/unset LED bit in PBOUT
    *CTSTAT = 0x0; // clear "reached 0" bit
    *CNTM = 100000000; // set countdown timer to 1 second
  }
}

interrupt void intserv() {
  if((*PBIN & 0x1) == 0) { // INC pressed
    digit = (digit + 1) % 10;
  }
  else if ((*PBIN & 0x2) == 0){ // DEC pressed
    digit = (digit - 1) % 10;
  }
  *PBOUT = ((digit << 4) | led); // set display digit bits
}