GATE | GATE CS 2020 | Question 13

Consider the following statements.

  • I. Daisy chaining is used to assign priorities in attending interrupts.
  • II. When a device raises a vectored interrupt, the CPU does polling to identify the source of interrupt.
  • III. In polling,the CPU periodically checks the status bits to know if any device needs its attention.
  • IV. During DMA, both the CPU and DMA controller can be bus masters at the same time.

Which of the above statements is/are TRUE ?
(A) Ⅰ and Ⅱ only
(B) Ⅰ and Ⅳ only
(C) Ⅰ and Ⅲ only
(D) Ⅲ only

Answer: (C)

Explanation: I. True. The daisy-chaining method involves connecting all the devices that can request an interrupt in a serial manner. This configuration is governed by the priority of the devices. The device with the highest priority is placed first followed by the second highest priority device and so on.

II. False. Vectored interrupt doesn’t involve polling but non-vectored interrupt involves polling.
Vectored interrupts are achieved by assigning each interrupting device a unique code, typically four to eight bits in length. When a device interrupts, it sends its unique code over the data bus to the processor, telling the processor which interrupt service routine to execute.

III. True. Polling is not a hardware mechanism, its a protocol in which CPU steadily checks whether the device needs attention.

IV. False. During DMA only one of the CPU or DMA can be bus master at a time.
CPU release bus only after getting request from DMA and get after DMA release the BUS.

Option (C) is correct.

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