Communication between an information processing system and the outside world
In
computing
,
input/output
(
I/O
,
i/o
, or informally
io
or
IO
) is the communication between an information processing system, such as a
computer
, and the outside world, such as another computer system, peripherals, or a human operator.
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 the pieces of
hardware
used by a human (or other system) to communicate with a computer. For instance, a
keyboard
or
computer mouse
is an
input device
for a computer, while
monitors
and
printers
are
output devices
. Devices for communication between computers, such as
modems
and
network cards
, typically perform both input and output operations. Any interaction with the system by an interactor is an
input
and the reaction the system responds is called the output.
The designation of a device as either input or output depends on perspective. Mice and keyboards take physical movements that the human user outputs and convert them into input signals that a computer can understand; the output from these devices is the computer's input. Similarly, printers and monitors take signals that computers output as input, and they convert these signals into a representation that human users can understand. From the human
user
's perspective, the process of reading or seeing these representations is receiving output; this type of interaction between computers and humans is studied in the field of
human?computer interaction
. A further complication is that a device traditionally considered an input device, e.g., card reader, keyboard, may accept control commands to, e.g., select stacker, display keyboard lights, while a device traditionally considered as an output device may provide status data (e.g., low toner, out of paper, paper jam).
In computer architecture, the combination of the
CPU
and
main memory
, to which the CPU can read or write directly using individual
instructions
, is considered the brain of a computer. Any transfer of information to or from the CPU/memory combo, for example by reading data from a
disk drive
, is considered I/O.
[1]
The CPU and its supporting circuitry may provide
memory-mapped I/O
that is used in low-level
computer programming
, such as in the implementation of
device drivers
, or may provide access to
I/O channels
. An
I/O algorithm
is one designed to exploit locality and perform efficiently when exchanging data with a secondary storage device, such as a disk drive.
Interface
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]
An I/O interface is required whenever the I/O device is driven by a processor. Typically a CPU communicates with devices via a
bus
. The interface must have the necessary logic to interpret the device address generated by the processor.
Handshaking
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. Because it would be a waste for a processor to be idle while it waits for data from an input device there must be provision for generating
interrupts
[2]
and the corresponding type numbers for further processing by the processor if required.
[
clarification needed
]
A computer that uses
memory-mapped I/O
accesses hardware by reading and writing to specific memory locations, using the same assembly language instructions that computer would normally use to access memory. An alternative method is via instruction-based I/O which requires that a CPU have specialized instructions for I/O.
[1]
Both input and output devices have a
data processing
rate that can vary greatly.
[2]
With some devices able to exchange data at very high speeds
direct access
to memory (DMA) without the continuous aid of a CPU is required.
[2]
Higher-level implementation
[
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]
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.
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 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
I/O
functions would introduce
side-effects
to any programming language, but this allows
purely functional programming
to be practical.
Channel I/O
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Channel I/O
requires the use of instructions that are specifically designed to perform I/O operations. The I/O instructions address the channel or the channel and device; the channel asynchronously accesses all other required addressing and control information. This is similar to DMA, but more flexible.
Port-mapped I/O
[
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]
Port-mapped I/O
also requires the use of special I/O instructions. Typically one or more ports are assigned to the device, each with a special purpose. The port numbers are in a separate address space from that used by normal instructions.
Direct memory access
[
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]
Direct memory access
(DMA) is a means for devices to transfer large chunks of data to and from memory independently of the CPU.
See also
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References
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External links
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