In this method, an I/O device is treated as a memory location. The microprocessor uses 16- bit address to identify and I/O device. Thus the memory map is shared between memory and I/O devices.
Memory-mapped I/O uses the same to address both memory and I/O devices. The memory
and registers of the I/O devices are mapped to address values. So when an address is accessed by the CPU, it may refer to a portion of physical RAM, but it can also refer to memory of the I/O device. Thus, the CPU instructions used to access the memory can also be used for accessing devices.