I/O Address
                -----------


What is an I/O Address?
-----------------------

Your computer is composed of several sub-systems:
 - Central Processing Unit (CPU), such as a '486.
 - Memory.
 - Input/Output (I/O) devices such as a keyboard, printer
   port, and video display.

The CPU does not have separate wires going to each of the
I/O devices.  Instead they are all connected to a common set
of connections called a "bus."  Memory and I/O devices respond
only when their particular address is on the bus.

The original IBM PC had the ability to use 1024 different
I/O addresses.  Very simple devices might use only a single
address, more complicated devices require several adjacent
addresses.  I/O addresses are usually written as hexadecimal
(base 16) numbers in the range of 000H through 3FFH.  Often
an H is written at the end of a number to indicate hexadecimal.

Addresses 000H through 1FFH are normally used for devices
on the mother board such as timers, interrupt controllers,
the real-time clock, math coprocessor, and CMOS memory 
holding configuration information.

Addresses in the range of 200H through 3FFH are used for 
peripheral devices.  These have traditionally used plug in
cards but many newer computers have standard devices built
in to the mother board.

Most of the standard devices, such as printer ports, serial
(COM) ports, video display, and the floppy disk controller
use addresses in the 300H to 3FFH range.  The 2xx range was
mostly unused in the original PC and that is why most add-on
devices, such as sound cards, CD-ROM drives, and Pasokon TV
use addresses in the 2xx range.

Clearly the hardware and software configuration must agree.
If a serial port is listening to address 3F8H, the software
must write to that address, to use the serial port.

Two different devices may not share the same address.  It would
cause great confusion if two different devices responded to
the same address and tried to put their data on the bus at
the same time.


I/O Address Selection
---------------------

Standard I/O addresses for serial ports are shown below:

                Serial Port     I/O Address
                -----------     -----------
                COM1            3f8
                COM2            2f8
                COM3            3e8
                COM4            2e8

Normally you won't have to worry about this.  All you need to
do is:
        - Go to "Setup" menu.
        - Pick "SSTV Interface".
        - Choose the desired serial port.

The program displays the standard I/O addresses and the actual
addresses obtained from the BIOS.  

If you have a situation where you have more than 4 serial ports
or the BIOS is confused, it will be necessary to manually edit
the configuration file.  See CONFIG.TXT for an explanation of
the COM_PORT command.