Source : Free On-Line Dictionary of Computing

memory mapped I/O
     
         The use of the same instructions and {bus} to
        communicate with both {main memory} and {input/output}
        devices.  This is in contrast to processors that have a
        separate I/O {bus} and special instructions to access it.
     
        The I/O devices are addressed at certain reserved address
        ranges on the main memory bus.  These addresses cannot
        therefore be used for {RAM}.  {Motorola} and {Mostec}
        architectures, among others, use memory mapped I/O.
     
        {Video cards} and other cards with on-board memory might be
        accessed in this way though the term applies not just to
        devices containing memory but to any device connected to the
        memory bus.  Accessing the devices usually consists of reading
        and writing certain built-in {registers} though sometimes the
        mere presence of a particular address can trigger the device.
     
        (1997-04-14)