For simplicity, take the shortest interval at which rows in the same bank can be activated to be tRC = 50 ns. With a refresh period of 64 ms, the maximum number of times a neighbouring row can be activated between one refresh of a given row and the next is 64 ms ÷ 50 ns. What is this maximum number when the refresh rate is doubled?

Answer: A. about 640,000

Doubling the refresh rate makes the period 64 ms ÷ 2 = 32 ms. The maximum number of activations in that time is 32 ms ÷ 50 ns = 640,000, half of the original 64 ms ÷ 50 ns = 1,280,000. Research has reported DRAM that flips after only a few thousand activations, so even halving the number leaves plenty for an attack, and adding refreshes alone cannot prevent it. Moreover, banks cannot be used during refresh and power rises, so the burden on performance and power consumption becomes large.

• A. Correct. 32 ms ÷ 50 ns = 640,000.
• B. This is the value before the refresh rate is changed (64 ms ÷ 50 ns).
• C. This is the value of the reverse calculation, doubling the refresh period (128 ms).