Memory-mapped I/O (MMIO) and port-mapped I/O (PMIO) (which is also called isolated I/O) are two complementary methods of performing input/output between the CPU and peripheral devices in a computer. An alternative approach is using dedicated I/O processors—commonly known as channels on mainframe computers—that execute their own instructions.

Memory-Mapped I/O:- Memory-mapped I/O (not to be confused with memory-mapped file I/O) uses the same address bus to address both memory and I/O devices – the memory and registers of the I/O devices are mapped to (associated with) 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. Each I/O device monitors the CPU’s address bus and responds to any CPU access of an address assigned to that device, connecting the data bus to the desired device’s hardware register. To accommodate the I/O devices, areas of the addresses used by the CPU must be reserved for I/O and must not be available for normal physical memory. The reservation might be temporary—the Commodore 64 could bank switch between its I/O devices and regular memory—or permanent.

Port-Mapped I/O:- Port-mapped I/O often uses a special class of CPU instructions specifically for performing I/O. This is found on Intel microprocessors, with the IN and OUT instructions. These instructions can read and write one to four bytes (outb, outw, outl) to an I/O device. I/O devices have a separate address space from general memory, either accomplished by an extra “I/O” pin on the CPU’s physical interface, or an entire bus dedicated to I/O. Because the address space for I/O is isolated from that for main memory, this is sometimes referred to as isolated I/O.