In
computingComputing is usually defined as the activity of using and improving computer hardware and software. It is the computer-specific part of information technology...
,
input/output, or
I/O, refers to the communication between an information processing system (such as a
computerA computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...
), and the outside world, possibly a human, or another information processing system.
InputsInformation in its most restricted technical sense is a message or collection of messages that consists of an ordered sequence of symbols, or it is the meaning that can be interpreted from such a message or collection of messages. Information can be recorded or transmitted. It can be recorded as...
are the signals or data received by the system, and
outputOutput is the term denoting either an exit or changes which exit a system and which activate/modify a process. It is an abstract concept, used in the modeling, system design and system exploitation.-In control theory:...
s 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 operationInput/output scheduling is a term used to describe the method computer operating systems decide the order that block I/O operations will be submitted to storage volumes...
. I/O devices are used by a person (or other system) to communicate with a computer. For instance, a
keyboardIn computing, a keyboard is a typewriter-style keyboard, which uses an arrangement of buttons or keys, to act as mechanical levers or electronic switches...
or a mouse may be an input device for a computer, while monitors and
printerIn computing, a printer is a peripheral which produces a text or graphics of documents stored in electronic form, usually on physical print media such as paper or transparencies. Many printers are primarily used as local peripherals, and are attached by a printer cable or, in most new printers, a...
s are considered output devices for a computer. Devices for communication between computers, such as
modemA modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data...
s and
network cardA network interface controller is a computer hardware component that connects a computer to a computer network....
s, typically serve for both input and output.
Note that the designation of a device as either input or output depends on the perspective. Mouse 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. These interactions between computers and humans is studied in a field called
human–computer interactionHuman–computer Interaction is the study, planning, and design of the interaction between people and computers. It is often regarded as the intersection of computer science, behavioral sciences, design and several other fields of study...
.
In computer architecture, the combination of the
CPUThe central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...
and main memory (i.e. memory that the CPU can read and write to directly, with individual instructions) is considered the brain 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
memory-mapped I/OMemory-mapped I/O and port I/O are two complementary methods of performing input/output between the CPU and peripheral devices in a computer...
that is used in low-level
computer programmingComputer programming is the process of designing, writing, testing, debugging, and maintaining the source code of computer programs. This source code is written in one or more programming languages. The purpose of programming is to create a program that performs specific operations or exhibits a...
, such as the implementation of
device driverIn computing, a device driver or software driver is a computer program allowing higher-level computer programs to interact with a hardware device....
s. An I/O algorithm is one designed to exploit locality and perform efficiently when data reside on secondary storage, such as a disk drive.
Interface
An I/O interface is required whenever the I/O device is driven by the processor. The interface must have necessary logic to interpret the device address generated by the processor.
HandshakingIn information technology, telecommunications, and related fields, handshaking is an automated process of negotiation that dynamically sets parameters of a communications channel established between two entities before normal communication over the channel begins...
should be implemented by the interface using appropriate commands (like BUSY, READY, and WAIT), and the processor can communicate with an I/O device through the interface. If different data formats are being exchanged, the interface must be able to convert serial data to parallel form and vice-versa. There must be provision for generating
interruptIn computing, an interrupt is an asynchronous signal indicating the need for attention or a synchronous event in software indicating the need for a change in execution....
s and the corresponding type numbers for further processing by the processor if required.
A computer that uses
memory-mapped I/OMemory-mapped I/O and port I/O are two complementary methods of performing input/output between the CPU and peripheral devices in a computer...
accesses hardware by reading and writing to specific memory locations, using the same assembly language instructions that computer would normally use to access memory.
Higher-level implementation
Higher-level
operating systemAn operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer 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
fileA computer file is a block of arbitrary information, or resource for storing information, which is available to a computer program and is usually based on some kind of durable storage. A file is durable in the sense that it remains available for programs to use after the current program has finished...
s. The
CC is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....
and
C++C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...
programming languages, and operating systems in the
UnixUnix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
family, traditionally abstract files and devices as streams, which can be read or written, or sometimes both. The
C standard libraryThe C Standard Library is the standard library for the programming language C, as specified in the ANSI C standard.. It was developed at the same time as the C POSIX library, which is basically a superset of it...
provides
functions for manipulating streamsThe C programming language provides many standard library functions for file input and output. These functions make up the bulk of the C standard library header...
for input and output.
In the context of the
ALGOL 68ALGOL 68 isan imperative computerprogramming language that was conceived as a successor to theALGOL 60 programming language, designed with the goal of a...
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 errors 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 functions would introduce side-effects to any programming language, but this allows
purely functionalPurely functional is a term in computing used to describe algorithms, data structures or programming languages that exclude destructive modifications...
programming to be practical.
Addressing mode
There are many ways through which data can be read or stored in the memory. Each method is an addressing mode, and has its own advantages and limitations.
There are many type of addressing modes such as direct addressing, indirect addressing, immediate addressing, index addressing, based addressing, based-index addressing, implied addressing, etc.
Direct addressing
In this type of address of the data is a part of the instructions itself. When the processor interprets the instruction, it gets the memory address from where it can be read/written the required information. For example:
MOV register, [address] ; to read
MOV [address], register ; to write
similarly
IN register, [address] ; to read as input
OUT [address], register ; to write as output
Here the
address operand points to a memory location which holds the data and copies it into/from the specified
register. A pair of brackets is a
dereference operatorThe dereference operator or indirection operator, denoted by "*" , is a unary operator found in C-like languages that include pointer variables. It operates on a pointer variable, and returns an l-value equivalent to the value at the pointer address. This is called "dereferencing" the pointer...
.
Indirect addressing
According to the above example, the
address can be stored in another register. Therefore, the instructions will have the register representing the address. So to fetch the data, the instruction must be interpreted appropriate register selected. The value of the register will be used for addressing appropriate memory location and then data will be read/written. This addressing method has an advantage against the direct mode that the register value is changeable so the appropriate memory location can also be dynamically selected.
Port-mapped I/O
Port-mapped I/O usually requires the use of instructions which are specifically designed to perform I/O operations.