An address in RAM that is associated with a specific device (such as a serial connector, parallel connector, or expansion slot) and allows the microprocessor to communicate with that device.
An I/O address is either a sparse space or dense space address that performs an I/O cycle on a specific bus. See also dense space address and sparse space address.
An address in memory that is associated with a specific device, (serial port, parallel port, expansion slot) and allows the CPU to communicate with that port or expansion slot, thus allowing communication with the device attached.