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Sixth-Semester-BE-Degree-Examination-JuneJuly-2013-Operations-Research-Question-paper-->View question


Asked On2017-06-07 08:24:32 by:Aparna-Dasgupta

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The daisy-chaining method of establishing priority consists of a serial connection of all devices that request an interrupt. The device with the highest priority is placed in the first position, followed by lower-priority devices upto the device with the lowest priority, which is placed last in the chain. The following figure demonstrate the method of connection between three devices and the CPU.

The hardware priority function can be established by either a serial or a parallel connection of interrupt lines. The serial connection is called daisy chaining method.
In daisy chaining method all the devices are connected in serial. The device with the highest priority is placed in the first position, followed by lower priority devices.


The interrupt request line is common to all devices and forms a wired logic connection. If any device has its interrupt signal in the low-level state, the interrupt line goes to the low-level state and enables the interrupt input in the CPU. The interrupt line stays in the high-level state and no interrupts are recognized by the CPU, only when no interrupts are pending. And is equivalent to a negative logic or operation. The CPU responds to an interrupt request by enabling the interrupt acknowledge line. 
This signal is received by device 1 at its PI(priority in) input. The acknowledge signal passes on the next device through the PO(Priority Out) Output only if device 1 is not requesting an interrupt. It blocks the acknowledgement signal from the next device by placing a 0 in the PO output, if device I has a pending interrupt. It then proceeds to insert its own interrupt vector address (VAD) into the data bus for the CPU to use, during the interrupt cycle.

A device with a 0 in its PI input generate a O in its PO output to inform the next-lower-priority device that the acknowledge signal has been blocked. A device that is requesting an interrupt and has a 1 in its PI input will intercept the acknowledge signal by placing a O in its PO output. It transmits the acknowledge signal to the next device by placing in 1 in its PC Output, if the device does not have pending interrupts. 
Thus the device with PI = 1 and PO = 0 is the one with the highest priority that is requesting an interrupt, and this device places its vector address (VAD) on the data bus. The daisy chain arrangement gives the highest priority to the device that receives the interrupt acknowledge signal from the CPU. The farther the device is from the first position; the lower is its priority.
Image result for diagram of daisy chaining technique.


Answerd on:2017-06-26 Answerd By:13priya

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