INT 21 - DOS 1+ - DIRECT CONSOLE INPUT
        AH = 06h
        DL = FFh
Return: ZF set if no character available
        ZF clear if character available
            AL = character read
Notes:   Break are NOT checked
        if the returned character is 00h, the user pressed a key with an
          extended keycode, which will be returned by the next call of this
          function
        reads from standard input, which is always the keyboard under DOS 1.x,
          but may be redirected under DOS 2+