INT 14h,  0Fh    Enable or disable flow control

      Entry:  AL         Bit mask describing requested flow control

              DX         Port number

      Exit:   None

    TRANSMIT flow control allows the "other end" to restrain the transmitter
    when you are  over-running it.  RECEIVE flow control tells the FOSSIL to
    attempt to do just that if it is being overwhelmed.

    Two kinds of basic flow control are supported:

                Bit 0 = 1       Xon/Xoff on transmit
                Bit 1 = 1       CTS/RTS (CTS on transmit, RTS on receive)
                Bit 2           Reserved
                Bit 3 = 1       Xon/Xoff on Receive

    Flow control is enabled, or disabled, by setting the appropriate bits in
    AL  for the types of flow control we want to ENABLE (value = 1),  and/or
    DISABLE  (value = 0),  and calling this function.  Bit 2 is reserved for
    DSR/DTR,  but is not currently supported in any implementation.

    Enabling  transmit  Xon/Xoff will cause the FOSSIL  to stop transmitting
    upon receiving an Xoff.  The FOSSIL will resume transmitting when an Xon
    is received.

    Enabling CTS/RTS will cause the FOSSIL to cease transmitting when CTS is
    lowered.  Transmission will resume  when CTS is raised.  The FOSSIL will
    drop RTS when the receive buffer reaches a predetermined percentage full
    The FOSSIL will  raise RTS  when the  receive buffer  empties below  the
    predetermined  percentage full.  The  point(s)  at which  this occurs is
    left to the individual FOSSIL implementor.

    Enabling receive  Xon/Xoff will cause the FOSSIL to send a Xoff when the
    receive buffer reaches a pre-determined percentage full.  An Xon will be
    sent when the receive buffer empties below the pre-determined percentage
    full. The point(s) at which this occurs is left to the individual FOSSIL
    implementor.

    Applications  using this  function  should set all bits  ON  in the high
    nibble of AL as well.  There is a compatible  (but not identical) FOSSIL
    driver implementation that uses the  high nibble as a control mask.   If
    your application sets the high nibble to all ones,  it will always work,
    regardless of the method used by any given driver.