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In computer science, channel I/O is a generic term that refers to an advanced, high-performance input/output architecture that is implemented in various forms on a number of computer architectures, especially on mainframe computers. more...

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Description

Thanks allot from sanjay

The reference implementation of channel I/O is that of the IBM System/360 family of mainframe computers and its successors, but similar implementations have been adopted by other mainframe vendors, such as Bull (General Electric/Honeywell) and Unisys.

Computer systems that use channel I/O have special hardware components that handle all input/output operations in their entirety independently of the systems' CPU(s). The CPU of a system that uses channel I/O typically has only one machine instruction in its repertoire for input and output; this instruction is used to pass input/output commands to the specialized I/O hardware in the form of channel programs. I/O thereafter proceeds without intervention from the CPU until an event requiring notification of the operating system occurs, at which point the I/O hardware signals an interrupt to the CPU.

A channel is an independent hardware component that coordinates all I/O to a set of controllers or devices. It is not merely a medium of communication, despite the name; it is an intelligent hardware device that handles all details of I/O after being given a list of I/O operations to carry out (the channel program).

Each channel may support one or more controllers and/or devices. Channel programs contain lists of commands to the channel itself and to various controllers and devices to which it is connected. Once the operating system has prepared a complete list of I/O commands, it executes a single I/O machine instruction to initiate the channel program; the channel thereafter assumes control of the I/O operations until they are completed.

It is possible to develop very complex channel programs, initiating many different I/O operations on many different I/O devices simultaneously. This flexibility frees the CPU from the overhead of starting, monitoring, and managing individual I/O operations. The specialized channel hardware, in turn, is dedicated to I/O and can carry it out more efficiently than the CPU (and entirely in parallel with the CPU). Channel I/O is not unlike the Direct Memory Access (DMA) of microcomputers, only more complex and advanced. Most mainframe operating systems do not fully exploit all the features of channel I/O.

On large mainframe computer systems, CPUs are only one of several powerful hardware components that work in parallel. Special input/output controllers (the exact names of which vary from one manufacturer to another) handle I/O exclusively, and these in turn are connected to hardware channels that also are dedicated to input and output. There may be several CPUs and several I/O processors. The overall architecture optimizes input/output performance without degrading pure CPU performance. Since most real-world applications of mainframe systems are heavily I/O-intensive business applications, this architecture helps provide the very high levels of throughput that distinguish mainframes from other types of computer.

Read more at Wikipedia.org