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     The _bios_serialcom routine uses INT 0x14 to provide serial
     communications services. The <serial_port> argument is set to 0
     for COM1, to 1 for COM2, and so on. The <service> argument can be
     set to one of the following manifest constants:
 
     _COM_INIT     _COM_RECEIVE     _COM_SEND     _COM_STATUS
 
     Because of the overhead associated with servicing computer
     interrupts, the _bios_serialcom routine may not be able to
     establish reliable communications in excess of 1,200 baud
     (_COM_1200). Faster data communication rates are possible with
     direct programming of serial-port controllers.
 
     Note that this function works only with IBM(R) personal computers
     and true compatibles.
 
     The <data> argument is ignored if <service> is set to _COM_RECEIVE
     or _COM_STATUS. The <data> argument for _COM_INIT is created by
     combining one or more of the following constants (with the OR
     operator):
 
     _COM_CHR7         _COM_CHR8
 
     _COM_STOP1        _COM_STOP2
 
     _COM_NOPARITY     _COM_EVENPARITY     _COM_ODDPARITY
 
     _COM_110          _COM_600            _COM_4800
     _COM_150          _COM_1200           _COM_9600
     _COM_300          _COM_2400
 
     The default value of <data> is 1 stop bit, no parity, and 110 baud.
 
     Return Value
 
     The function returns a 16-bit integer whose high-order byte
     contains status bits. The meaning of the low-order byte varies,
     depending on the <service> value. The high-order bits are as
     follows:
 
     Bit     Meaning If Set
 
     15      Timed out
     14      Transmission-shift register empty
     13      Transmission-hold register empty
     12      Break detected
     11      Framing error
     10      Parity error
      9      Overrun error
      8      Data ready
 
     When service is _COM_SEND, bit 15 is set if data cannot be sent.
 
     When service is _COM_RECEIVE, the byte read is returned in the
     low-order bits if the call is successful. If an error occurs, any
     of the bits 9, 10, 11, or 15 is set.
 
     When <service> is _COM_INIT or _COM_STATUS, the low-order bits are
     defined as follows:
 
     Bit     Meaning If Set
 
     7       Receive-line signal detected
     6       Ring indicator
     5       Data-set ready
     4       Clear to send
     3       Change in receive-line signal detected
     2       Trailing-edge ring indicator
     1       Change in data-set ready status
     0       Change in clear-to-send status
                                    -♦-