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

<architecture> The use of the same instructions and bus to communicate with both main memory and input/output devices. This is in contrast to processors that have a separate I/O bus and special instructions to access it. The I/O devices are addressed at certain reserved address ranges on the main memory bus. These addresses cannot therefore be used for RAM. Motorola and Mostec architectures, among others, use memory mapped I/O. Video cards and other cards with on-board memory might be accessed in this way though the term applies not just to devices containing memory but to any device connected to the memory bus. Accessing the devices usually consists of reading and writing certain built-in registers though sometimes the mere presence of a particular address can trigger the device. (1997-04-14)

A memory-mapped file is a segment of virtual memory that has been assigned a direct byte-for-byte correlation with some portion of a file or file-like resource. This resource is typically a file that is physically present on disk, but can also be a device, shared memory object, or other resource that the operating system can reference through a file descriptor.