Memory-mapped I/O:

There is a single address space for memory locations and I/O devices.

The processor treats the status and address register of the I/O modules as memory location.

For example, if the size of address bus of a processor is 16, then there are 2¹⁶ combinations and all together 2¹⁶ address locations can be addressed with these 16 address lines.

Out of these 2¹⁶ address locations, some address locations can be used to address I/O devices and other locations are used to address memory locations.

Since I/O devices are included in the same memory address space, so the status and address registers of I/O modules are treated as memory location by the processor. Therefore, the same machine instructions are used to access both memory and I/O devices.