void setup() {
  volatile char *dirf, *dirk, *dira, *dirc;
  dirf = 0x30;dirk = 0x107;dira = 0x21;dirc = 0x27;
  *dirf = 0x0f;*dirk = 0x00;*dira = 0xff;*dirc = 0x0f;
}

void loop() {
  mul();
}

void mul() {
  volatile char *outf, *outa, *outc;
  volatile char *ink;
  outf = 0x31;outa = 0x22;outc = 0x28;ink = 0x106;
  volatile char numbers[] = {0xC0, 0xF9, 0xA4, 0xB0, 0x99, 0x92, 0x82, 0xF8, 0x80, 0x90};
  volatile long i, j, x, num1 = 0, num2 = 0, num, cs = 0, cs1 = 0, pdt = 0;
  volatile int digit1, digit2, digit3, digit4, mul = 0, equal = 0,dn=0;
  

  while (1) {
    for (i = 0; i < 4; i++) {
      *outf = 1 << i;
      x = *ink;       
      if (x != 0) {//check if anything is pressed
        // Find value entered in keypad
        if (*outf == 1 && x == 1) num = 1;
        else if (*outf == 1 && x == 2) num = 2;
        else if (*outf == 1 && x == 4) num = 3;
        else if (*outf == 2 && x == 1) num = 4;
        else if (*outf == 2 && x == 2) num = 5;
        else if (*outf == 2 && x == 4) num = 6;
        else if (*outf == 4 && x == 1) num = 7;
        else if (*outf == 4 && x == 2) num = 8;
        else if (*outf == 4 && x == 4) num = 9;
        else if (*outf == 8 && x == 2) num = 0;

        //Multiply Button
        if (*outf == 4 && x == 8) {
          cs = num1; 
          mul++;
        }
        //EQUAL Function
        if (*outf == 8 && x == 4) {
          if(dn == 2){
            cs1 = num2 % 10;
          }
          else if(dn == 3){
            cs1 = num2 % 100;
          }
          else if(dn == 4){
            cs1 = num2 % 1000;
          }
          else if(dn == 5){
            cs1 = num2 % 10000;
          }
          
          equal++;
        }

        // FINAL RESULT AND DISPLAY
        if (mul != 0 && equal != 0) {
          pdt = cs*cs1; // Calculate the profuct
          // Display the result on a 7-segment display
          if (pdt >= 1000&&pdt<10000) {
            digit1 = pdt / 1000;
            digit2 = (pdt % 1000) / 100;
            digit3 = (pdt % 100) / 10;
            digit4 = pdt % 10;
            //DISPLAY
            *outc = 0x01;
            *outa = numbers[digit1];
            for (j = 0; j < 200000; j++);
            *outc = 0x02;
            *outa = numbers[digit2];
            for (j = 0; j < 200000; j++);
            *outc = 0x04;
            *outa = numbers[digit3];
            for (j = 0; j < 200000; j++);
            *outc = 0x08;
            *outa = numbers[digit4];
            for (j = 0; j < 200000; j++);
          } 
          else if (pdt>=100 && pdt < 1000) {
            digit1 = pdt / 100;
            digit2 = (pdt % 100) / 10;
            digit3 = pdt % 10;
            //DISPLAY
            *outc = 0x01;
            *outa = numbers[digit1];
            for (j = 0; j < 200000; j++);
            *outc = 0x02;
            *outa = numbers[digit2];
            for (j = 0; j < 200000; j++);
            *outc = 0x04;
            *outa = numbers[digit3];
            for (j = 0; j < 200000; j++);
          }
          else if(pdt>=10&&pdt<100){
            digit1 = pdt/10;
            digit2 = pdt %10;
            *outc = 0x01;
            *outa = numbers[digit1];
            for (j = 0; j < 200000; j++);
            *outc = 0x02;
            *outa = numbers[digit2];
            for (j = 0; j < 200000; j++);
          }
          else if(pdt<10){
            digit1 = pdt;
            *outc = 0x01;
            *outa = numbers[digit1];
            for (j = 0; j < 200000; j++);
          }
          else if(pdt>9999){
          *outc = 0x0F;
          *outa = 0x86;
          for (j = 0; j < 200000; j++);
          }
        //reset
        num1 = 0; num2 = 0;
        mul =0; equal =0;dn=0;
        break;
        
        }


        // Display pressed key digit
        *outc = 0x01;
        *outa = numbers[num];
        for (j = 0; j < 200000; j++);
        // Store and update num1 &num2
        if (mul!=0){
          num2 = (num2 * 10) + num;
          dn++;
          }
        num1 = (num1 * 10) + num;
      }
    }
  }
}