With this section, serial communications are controlled in Clipper.  The
 functions are low level and use the Interrupt Driven Service task of the
 serial ports.

    These functions manage 4 serial ports, from COM1 to COM4 on AT systems
 and 2 ports, COM1 and COM2 on XT systems.

    The system is compatible with all kinds of current UART's existing on
 the market for PC-XT-AT.  Access to the system is via 8259A, therefore,
 the BIOS routines are taking charge of the UART'S compatibility.

    The system has internal and external buffers available (the same kind that
 the BIOS has available for the keyboard). Thanks to them, we have available
 in memory the incoming and outgoing data for use when we are ready to receive
 or send it. The incoming buffer has been established at 16384 bytes and
 the outgoing buffer at 4096 bytes.