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).

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.