Direct console I/O character (INT 21h, 06h)

        Reads or writes a character from/to the CON-device. This
        function does not check for Ctrl-Break, and if a character is
        read, it isn't echoed to the console.

        Output:
          Entry:
            AH = 06h
            DL = character <> FFh
        Input:
           Entry:
              AH = 06h
              DL = 0FFh
           Return:
              ZF set   = no character
              ZF clear = character recieved
              AL = character