UGC-NET | UGC NET CS 2015 Dec – III | Question 5

A DMA controller transfers 32-bit words to memory using cycle stealing. The words are assembled from a device that transmits characters at a rate of 4800 characters per second. The CPU is fetching and executing instructions at an average rate of one million instructions per second. By how much will the CPU be slowed down because of the DMA transfer?
(A) 0.6%
(B) 0.12%
(C) 1.2%
(D) 2.5%


Answer: (B)

Explanation: DMA controller transfers 32 bit(4 byte) words to memory(cycle stealing mode).
Device transmits 4800 character per second (1 character = i byte)
So, for 1 byte it will take 1 / 4800 sec.
Since the controller transfers 4 byte in cycle stealing mode, it will take 4 * (1 / 4800) = 1 / 1200 sec.
i.e. 1200 character will be transfered in cycle stealing mode and it is given that CPU is fetching and executing instructions at an average rate of one million instructions per second.
slow down or cycle wasted % in DMA transfer = ( 1200 / 1000000) * 100
= 0.12 %
So, option (B) is correct.

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