text: #define VIDEO_ADDRESS 0xb8000
  #define MAX_ROWS 25
  #define MAX_COLS 80
  
  // default color scheme
  #define WHITE_ON_BLACK 0x0f
  
  //Screen device I/O ports
  #define REG_SCREEN_CTRL 0x3D4
  #define REG_SCREEN_DATA 0x3D5
  
  /* publicly exposed functions */
  void print(char *s);
  void print_at(char *s, int col, int row);
  