That is from the time of issuing an io request till

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Unformatted text preview: N a "device busy" indicator so that no further I/O requests for the device will be accepted. It then executes the request and stores the data to be transferred in its local data register. As soon as it cgmpletes the request processing, it sets ON another indicator called "data ready" indicator. After issuing the READ request, the CPU continuously keeps checking the "data ready" indicator of the device and waits until this indicator is set to ON. As soon as the CPU finds this indicator to be ON, it reads the data from the data register of the device controller into its own register. It then sends a data received acknowledgment signal to the device. On receiving this signal, the device controller resets its "device busy" and "data ready" indicators to OFF which indicates that it is now ready to accept new I/O requests. 2. Interrupt. In the busy wait method, we saw that after issuing an I/O request, the CPU remains busy in checking for the completion of the request until the request processing is over. That is, from the time of issuing an I/O request till the completion of the request, the CPU does not do anything else. This is very inefficient use of the CPU because I/O devices are about 1000 times slower than the CPU. The interrupt mechanism has been devised to overcome this problem. In this method, after issuing the READ request to the I/O device, the CPU does not wait for the "data ready" indicator of the device to be set ON. Rather, it suspends the job that made the READ request and takes up another job that is ready for execution and starts executing this job while the device controller is busy processing the READ request. When the device controller completes processing the request and the data to be transferred is ready in its local data register, it sends an interrupt signal to the CPU. When the CPU receives the interrupt signal, it knows that the data it requested is now ready. Hence it suspends the job that it is currently executing, transfers the data from the data register of the device controller to its own register...
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This document was uploaded on 04/07/2014.

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