INT 14H: Serial Port I/O

                AH  BIOS Service
                ▀▀▀ ▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀
                00H Initialize communications port

                01H send a character

                02H receive a character

                03H Query communications port status

 These functions provide access to the four RS-232 ports supported by BIOS.

    Note: BIOS provides only the most rudimentary of unbuffered serial I/O
          (DOS is just as bad) and can initialize only up to 9600 baud.
          For performance reasons, most communications programs perform
          direct I/O port access.

 The base address of each of up to four serial ports is stored starting at
 0040:0000.  If the POST does not detect a serial port, it writes 0000H
 into the corresponding BIOS Data Area variable.  Also, POST quits checking
 as soon as it hits an unused port (it will never find COM3 if there is no

See Also: Asynchronous Adapter Ports
          Character I/O Functions
          ROM-BIOS Functions
          DOS Fn 4403H (IOCTL write control data)
          Cables and Pin Outs