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You can think of I/O addresses like a bunch of small two-way "mailboxes" in the system's memory. Take for example a communications (COM) port that has a modem connected to it. When information is received by the modem, it needs to get this information into the PC. Where does it put the data it pulls off the phone line?
One answer to this problem is to give each device its own small area of memory to work with. This is called memory-mapped I/O. When the modem gets a byte of data it sends it over the COM port, and it shows up in the COM port's designated I/O address space. When the CPU is ready to process the data, it knows where to look to find it. When it later wants to send information over the modem, it uses this address again (or another one near it). This is a very simple way of dealing with the problem of information exchange between devices.