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  This section discusses the RS-232 asynchronous serial communications port
  services in the ROM BIOS. Before we begin describing the ROM BIOS services
  in detail, you need to know a few important things about the serial
  communications port, particularly the terminology. We assume you have a
  basic understanding of data communications, but if you discover that you
  don't understand the following information, turn to one of the many
  specialty books on communications for some background information.

  Many words are used to describe the RS-232 data path in and out of the
  computer. One of the most common is port. However, this use of the word
  port is completely different from our previous use of the word. Throughout
  most of this book, we have used port to refer to the addressable paths
  used by the 8088 microprocessor to talk to other parts of the computer
  within the confines of the computer's circuitry. All references to port
  numbers, the BASIC statements INP and OUT, and the assembly-language
  operations IN and OUT refer to these addressable ports. The RS-232
  asynchronous serial communications port differs because it is a
  general-purpose I/O path, which can be used to interconnect many kinds of
  information-processing equipment outside the computer. Typically, the
  serial ports are used for telecommunications (meaning a telephone
  connection through a modem) or to send data to a serial-type printer.

  Four serial communications services are common to all IBM models. These
  services are invoked with interrupt 14H (decimal 20), selected through
  register AH, and numbered 00H through 03H. (See Figure 12-1.) The PS/2
  ROM BIOS contains two additional services that provide extended support
  for the more capable PS/2 serial port.

  The original design of the IBM personal computers allowed up to seven
  serial ports to be added, although a computer rarely uses more than one or
  two. The PS/2 ROM BIOS explicitly supports only four serial ports. No
  matter how many serial ports exist, the serial port number is specified in
  the DX register for all ROM BIOS serial communications services. The first
  serial port is indicated by 00H in DX.

  Service            Description
  --------------------------------------------------------------------------
  00H                Initialize Serial Port.
  01H                Send Out One Character.
  02H                Receive One Character.
  03H                Get Serial Port Status.
  04H                Initialize Extended Serial Port.
  05H                Control Extended Communication Port.
  --------------------------------------------------------------------------

  Figure 12-1.  The RS-232 serial port services available through interrupt
  14H (decimal 20).