Find the refresh overhead in terms of percentage

Question

All dynamic memories have to be refreshed. A typical DRAM takes 64 ms to refresh. Suppose there are 8 K rows and it takes four clock cycles to access each row. If the clock rate is 133 MHz, find the refresh overhead in terms of percentage (upto 2 decimal places).

Made Easy FLT 6- Practice Test 14

Q 27

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in syllabus?

Answer 1

no of cycles require to refresh= 8k ×4 = 2^15

amount of time to refresh = 2^15 × (1/133×10^6) secs =0.246 ms

%of overhead =( 0.246 /64)*100 =0.38 %

Answer 2

$\text{Refresh overhead} = \frac{\text{time required for refresh in ms}}{\text{refresh interval in ms}}\\ \text{length of refresh cycle} = \frac{4}{133 * 10^6} = 30 ns\\ \text{time required for refresh} = (\text{length of refresh cycle}) * (\text{rows}) = (\text{30 ns}) * (8K) = 0.246ms\\ \text{refresh overhead} = \frac{0.246 ms}{64 ms} = 0.038 = \text{3.8 %} $

ref : https://en.wikipedia.org/wiki/Memory_refresh

Comments

$\frac{0.246}{64} = 0.003843 ms = 0.384 \%$