GATE 2018 CS – Question 33
A 32-bit wide main memory unit with a capacity of 1 GB is built using 256M $\times$ 4-bit DRAM chips. The number of rows of memory cells in the DRAM chip is $2^{14}$. The time taken to perform one refresh operation is 50 nanoseconds. The refresh period is 2 milliseconds. The percentage (rounded to the closest integer) of the time available for performing the memory read/write operations in the main memory unit is ________.
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Correct answer: 59
Explanation
Refreshing all $2^{14}=16384$ rows takes $16384\times50\text{ ns}=819.2\ \mu$s per 2 ms period, which is $40.96\%$ of the time. The remaining $59.04\%\approx59\%$ is available for reads and writes.