Input-Output Processors

The input/output processor or I/O processor is a processor that is separate from the main processor or CPU and is designed to handle only input/output processes for a device or the computer. The I/O processor is capable of performing actions without interruption or intervention from the CPU. The CPU only needs to initiate the I/O processor by telling it what activity to perform. Once the necessary actions are performed, the I/O processor then provides the results to the CPU. Doing these actions allow the I/O processor to act as a Bus to the CPU, like a CPU Bus, carrying out activities by directly interacting with memory and other devices in the computer.

The concept of I/O processor is an extension of the concept of DMA. The I/O processor can execute specialised I/O program residing in the memory without intervention of the CPU. Thus, CPU only needs to specify a sequence of I/O activity to I/O processor. The I/O processor then 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 processor can have its own memory, enabling a large set of I/O devices to be controlled without much involvement from the CPU. Thus, an I/O processor has the additional ability to execute I/O instructions which provide it a complete control on I/O operations. Thus, I/O processors are much more powerful than DMA which provides only a limited control of I/O device. For example, if an I/O device is busy then DMA will only interrupt the CPU and will inform the CPU again when the device is free while I/O device and once it has found to be free go a head with I/O and when I/O finishes, communicate it to the CPU.

In computer systems which have I/O processors the CPU normally do not execute I/O data transfer instructions. I/O instructions are stored in memory and are executed by I/O processors. The I/O processor can be provided with the direct access to the memory and can control the system bus. An I/O processor can execute a sequence of data transfer instructions involving different memory regions and different devices without intervention of the CPU.

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