What is memory mapped I/O Scheme?

In memory mapped I/O scheme we can use only one address space. This particular one address space is allocated to both memory and I/O devices. In total memory address some addresses are assigned to memories and some to I/O devices. But we have to assign the address for I/O devices are different from the addresses which have been assigned to memories. In this scheme remember that I/O device is also treated as a memory location. And one address is assigned to each memory location (unique address) and one address is assigned to each I/O device.