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_bios_serialcom
===============

Syntax
------

     #include <bios.h>
     
     unsigned _bios_serialcom(unsigned cmd, unsingned serialport, unsigned data);

Description
-----------

The _bios_serialcom routine uses INT 0x14 to provide serial
communications services. The SERIALPORT argument is set to 0 for COM1,
to 1 for COM2, and so on. The CMD argument can be set to one of the
following manifest constants:

`_COM_INIT'
     Initialize com port (DATA is the settings)

`_COM_RECEIVE'
     Read a byte from port

`_COM_SEND'
     Write a byte to port

`_COM_STATUS'
     Get the port status

The DATA argument is ignored if CMD 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         7 bits/character
     _COM_CHR8         8 bits/character
     _COM_STOP1        1 stop bit
     _COM_STOP2        2 stop bits
     _COM_NOPARITY     no parity
     _COM_EVENPARITY   even parity
     _COM_ODDPARITY    odd parity
     _COM_110          110 baud
     _COM_150          150 baud
     _COM_300          300 baud
     _COM_600          600 baud
     _COM_1200         1200 baud
     _COM_2400         2400 baud
     _COM_4800         4800 baud
     _COM_9600         9600 baud

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
CMD 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

Example
-------

     /* 9600 baud, no parity, one stop, 8 bits */
     _bios_serialcom(_COM_INIT, 0, _COM_9600|_COM_NOPARITY|_COM_STOP1|_COM_CHR8);
     for(i=0; buf[i]; i++)
       _bios_serialcom(_COM_SEND, 0, buf[i]);