A 32-bit wide main memory unit with a capacity of 1 GB is built using 256M ×…
2018
A 32-bit wide main memory unit with a capacity of 1 GB is built using 256M × 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
Key steps: compute the total refresh time per refresh period and subtract from 100%.
Number of rows in each DRAM chip = 2^14 = 16,384.
Time to refresh all rows once = 16,384 × 50 ns = 819,200 ns = 0.8192 ms.
Refresh period = 2 ms, so fraction of time spent refreshing = 0.8192 / 2 = 0.4096 = 40.96%.
Time available for read/write = 100% − 40.96% = 59.04%, which rounds to 59%.
Answer: 59%