ANS :81.92 microsecond
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now, at a time one refresh operation take one row which means
in this we have 2^12 rows
so 1 refresh operation = 1 row ->20ns
so 2^12 rows=20ns * 2 ^12 =81.96micro second
extra info:
why refresh needed in dram ??
dram stores the capacitor to store the charge as data as there is capacitor so possibilty of lossing the charges will occur hence to avoid data loss we need refresh