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In computing, input/output, or I/O, refers to the communication between an information processing system (such as a computer), and the outside world – possibly a human, or another information processing system. Inputs are the signals or data received by the system, and outputs are the signals or data sent from it. The term can also be used as part of an action; to "perform I/O" is to perform an input or output operation. I/O devices are used by a person (or other system) to communicate with a computer. For instance, keyboards and mouses are considered input devices of a computer, while monitors and printers are considered output devices of a computer. Devices for communication between computers, such as modems and network cards, typically serve for both input and output.

Note that the designation of a device as either input or output depends on the perspective. Mouses and keyboards take as input physical movement that the human user outputs and convert it into signals that a computer can understand. The output from these devices is input for the computer. Similarly, printers and monitors take as input signals that a computer outputs. They then convert these signals into representations that human users can see or read. (For a human user the process of reading or seeing these representations is receiving input.)

In computer architecture, the combination of the CPU and main memory (i.e. memory that the CPU can read and write to directly, with individual instructions) is considered the heart of a computer, and from that point of view any transfer of information from or to that combination, for example to or from a disk drive, is considered I/O. The CPU and its supporting circuitry provide I/O methods that are used in low-level computer programming in the implementation of device drivers.

Higher-level operating system and programming facilities employ separate, more abstract I/O concepts and primitives. For example, most operating systems provide application programs with the concept of files. The C and C++ programming languages, and operating systems in the Unix family, traditionally abstract files and devices as streams, which can be read or written, or sometimes both. The C standard library provides functions for manipulating streams for input and output.

• Transput - In the context of the ALGOL 68 programming language, the input and output facilities are collectively referred to as transput. The ALGOL 68 transput library recognizes the following standard files/devices: stand in, stand out, stand error and stand back.

An alternative to special primitive functions is the I/O monad, which permits programs to just describe I/O, and the actions are carried out outside the program. This is notable because the I/O functions would introduce side-effects to any programming language, but now purely functional programming is practical.

• C file input/output

• I/O Meter
• Din rail I/O modules