Memory Hierarchy Q.61

61A dynamic RAM has a memory cycle time of 64 nsec. It has to be refreshed 100 times per nsec and each refresh takes 100 nsec. What percentage of the memory cycle time is used for refreshing?

Option : C
Explanation : Memory cycle time = 64 ns
Memory is refreshed 100 times per msec.
Number of refreshes in 1 memory cycle (i.e in 64 ns)
= (100 * 64 * 10 – 9)/10 – 3 = 64 * 10 – 4.
Time taken for each refresh = 100 ns
Time taken for 64 * 10 – 4 refreshes
= 64 * 10 – 4 * 100 * 10-9 sec
= 64 * 10 – 11 sec.
Percentage of the memory cycle time used for refreshing :
= (Time taken to refresh in 1 memory cycle/ Total time) * 100
= (64 * 10 – 11/64 * 10 – 9) * 100
= 1 %

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