# UGC-NET | UGC NET CS 2015 Dec – III | Question 4

A dynamic RAM has refresh cycle of 32 times per msec. Each refresh operation requires 100 nsec and a memory cycle requires 250 nsec. What percentage of memory’s total operating time is required for refreshes?
(A) 0.64
(B) 0.96
(C) 2.00
(D) 0.32

Explanation:
Memory cycle time = 250 nsec
memory is refreshed 32 times per msec
i.e.
Number of refreshes in 1 memory cycle (i.e in 250 nsec) = (32 * 250 * 10-9) / 10-3 = 8 * 10-3.
Time taken for each refresh = 100 nsec
Time taken for 8 * 10-3 refreshes = 8 * 10-3 * 100 * 10-9.
= 8 * 10-10
Percentage of the memory cycle time used for refreshing :
= (Time taken to refresh in 1 memory cycle / Total time) * 100
= (8 * 10-10 / 250 * 10-9) * 100
= 0.032 * 10
= 0.32
So, option (D) is correct.

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