Ans. Need of Input/Output Processor:
1) Increased Efficiency - CPU need not wait for I/o operations to complete.
2) Direct control of CPU on I/o device.
3) Direct access of I/o module to the memory.
4) CPU is separated from the details of external I/o interfaces.
5) Use of programmed I/o - In a programmed I/o method, the responsibility of CPU is to constantly check the status of the I/o device to check whether it has become free in case output is desired, or its has finished inputting the current series of data in case input is going on.
For the above reasons, we need the I/o processor.
The I/O processor can execute specialized I/o program residing in the memory without intervention of the CPU. CPU only needs to specify a sequence of I/o activity to I/o processor. Then, I/o processor executes the necessary I/o instructions which are required for the task, and interrupts the CPU only after the entire sequence of I/o activity as specified by CPU have been completed. An advanced I/o activity as specified by CPU has been completed. An advanced I/o processor can have its own memory. An I/o processor has the additional ability to execute I/o instructions, which provide it’s a complete control of I/O operations. The architecture of I/O processor is shown below.
For high-speed devices a selector channel is used. This channel can transfer data from one high-speed device at a time. I/o modules can handle each of these high-speed devices. Therefore, I/o processors have taking the place of CPU in controlling various I/o modules.
The communication between I/o processor and CPU can be achieved by writing messages in the memory area shared by both the processors. CPU instructs I/o processor to execute an I/o program in the memory. Then the program will specify the device or devices and the area of the memory where the I/o data is stored, or to be stored.
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