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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.
- Spring '14