void init_program(void);
void out_data(int);
void out_con(int);
void delay1(int);
void write_data(char);
void lcd_init(void);
void lcd_control(void);
void write_string(char *ptr);
void switch_in(int);
int switch_out();

//Global Declaration:
char arr[4][4] = { {'1','2','3','+'},{'4','5','6','-'},
                   {'7','8','9','*'},{'D','0','=','/'}};

void setup() 
{
  init_program();
  lcd_init();
  
  volatile long j, value;
  for(int i=0; i<4; i++)
  {
    switch_in(1<<i);           
    value = switch_out();
    if(value != 0)
    {
      write_string(arr[value][1<<i]);
      for(j=0; j<100000; j++);
    }
  }
  
}

void write_string(char *ptr)
{
  while (*ptr != '\0') 
  {
    write_data(*ptr);
    ptr++;
  }
}

void write_data(char wr_data)
{
  out_data(wr_data);
  out_con(0x02);    // RS = 1, E = 0
  delay1(1);
  out_con(0x03);   // RS = 1, E = 1
  delay1(1);
  out_con(0x02);  // RS = 1, E = 0
  delay1(1);
}

void init_program()
{
  volatile char *dir_A = (char *) 0x21; 
  *dir_A = 0xFF; 
  volatile char *dir_C = (char *) 0x27; 
  *dir_C = 0x03;
  volatile char *dir_F = (char *) 0x30; 
  *dir_F = 0x0F; 
  volatile char *dir_K = (char *) 0x107; 
  *dir_K = 0x00; 
}

void out_data(int out_data)
{
  volatile char *out_A = (char *) 0x22;
  *out_A = out_data;
}

void out_con(int con_data)
{
  volatile char *out_C = (char *) 0x28;
  *out_C = con_data;
}

void switch_in(int data)
{
  volatile char *out_F = (char *) 0x28;
  *out_F = data;
}

int switch_out()
{
  volatile char *in_K = (char *) 0x28;
  return *in_K;
}

void delay1(int delay_data)
{
  volatile long j;
  while (delay_data != 0)
  {
    for (j = 0; j < 100000; j++);
    delay_data--;
  }
}

void lcd_control(void)
{
  out_con(0x01);  // RW = 0, E = 0
  delay1(1);
  out_con(0x00); // RW = 0, E = 0
  delay1(1);
}

void lcd_init(void)
{
  out_data(0x38);  // Function Set
  lcd_control();
  out_data(0x0F); // Display On
  lcd_control();
  out_data(0x01);  // Clear Display
  lcd_control();
  out_data(0x06); // Entry Mode Set
  lcd_control();
}

void loop()
{
  // I not to use loop function...
}