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The main tasks of a computer are the management of operations of input / output and processing.
The calculation operations are made on data collected by the devices of the I / O, thus making the system I / O important to the performance of the entire system.
There are devices I / O very different in speed, functionality and this makes the subsystem I / O very complex.

Despite the diversity of devices I / O, the architecture of connection with the computer is quite simple: a device communicates with the computer by means of signals, which pass through a cable and, through a door, they arrive at the computer. If the cable is shared among multiple devices, it is said BUS. Each BUS, as well as from the wire, is also defined by a communication protocol, which defines what the rules that allow multiple devices to use it without conflict.

There are several types of bus, for both speed and for protocols.An example is the PCI bus, used in most computers to connect devices together fast (video memory, SCSI controller for the disks), which is also connected, via an interface, an expansion bus that allows access to the PCI bus also to slower devices (mouse, keyboard, serial port.

A controller is a set of electronic components, which allows operation of a communication protocol. A SCSI controller is often implemented through an adapter, or through a processing unit and the execution of microcopie, allows to process complex messages SCSI protocol.
The CPU and the devices do not communicate directly because the messages go directly to the controller, who then manages them. The communication between the devices / CPU and controller takes place through the use of registers, where they are stored the information regarding the type of communication and data to be transferred.
And 'possible to access the I / O through the appropriate system calls or through standard system calls such as read and write.

The ports I / O are generally constituted by 4 registers of communication:
-Status, which contains the port status
-Data-in, which contains the information received
-Data-out register, which contains the information to be sent
-Control, which contains data for the port configuration.

Calls to I / O
It 'important to decide whether the system calls for I / O are blocking or not. In the case of blocking call, when a process makes a request for an I / O, it is stopped and put in the queue of waiting processes.
A 'alternative to non-blocking calls are asynchronous calls, for which at the time of a request for I / O execution process continues; is terminated at the time when the command execution of I / O, is sent to a sign the application itself.

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