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I/O Ports and Controllers on IBM Compatibles and PS/2

The following gives the input and output ports used by IBM PC compatible
computers.  Most ports allow access to input and output device
controllers.  A controller may have several internal registers which can
be set to fix the operation of that controller, read its status, or send
data to it.  Other ports access discrete logic latches or registers.
Some ports are write-only or read-only.  It is even possible for a port
to be write-only and read-only by using two different registers at the
same port address.  Port addresses can range from 0 to 0FFFFH (64K
ports). However, PC system boards generally reserve only the first 1024
(0-3FFH). The AT and PS/2 freely employ ports above 3FFH for extended
applications, such as the two port asynchronous adaptors and network
adaptors.

Ports from 0-0FFH have a special significance, since the 80x86 can use
byte rather than word port addresses in the IN and OUT instructions.