Characteristics of input- output channels, Computer Engineering

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Q. Characteristics of input- output channels?

The I/O channel represents an extension of DMA concept. An I/O channel has ability to execute I/O instructions that gives complete control over I/O operation. With such devices the CPU doesn't execute I/O instructions. These kinds of instructions are stored in main memory to be executed by a special-purpose processor in I/O channel itself. So CPU initiates an I/O transfer by instructing I/O channel to execute a program in memory. Two kinds of I/O channels are commonly used that can be seen in Figure (a and b).

2437_Characteristics of input- output channels.png

Figure: I/O Channel Structures

A selector channel controls multiple high-speed devices and at any one time is dedicated to transfer of data with one of those devices. Every device is handled by a controller or I/O interface. So I/O channel serves in place of CPU in controlling these I/O controllers.

A multiplexer channel can handle I/O with many devices at the same time. If devices are slow then byte multiplexer is used. Let's explain this with illustration. If we have 3 slow devices that need to send individual bytes as:

X1 X2 X3 X4 X5 ......

Y1 Y2 Y3 Y4 Y5......

Z1 Z2 Z3 Z4 Z5......

Then on a byte multiplexer channel they can transmit bytes as X1 Y1 Z1 X2 Y2 Z2 X3 Y3 Z3...... For high-speed devices blocks of data from various devices are interleaved. These devices are known as block multiplexer.

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