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if there is a miss in cache and hit in mm what is access time 60ns or 60+20=80ns.

how is the statement " reference started again" related to the  meaning of the question?

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Since nothing is mentioned, I would assume Hierarchical access.

Tavg = H1T1 + (1-H1)H2(T1+T2) + (1-H1)(1-H2)(T1+T2+T3)

Since there are 3 levels. 

This formula can be interpreted as

[(Hit in Level 1) * (Time to access in level 1)] + [(Miss in Level 1) *  (Hit in Level 2) * (Time to access Level 1 memory + Time to access Level 2 memory)] + [(Miss in Level 1) * (Miss in Level 2) * (Time to access Level 1 memory + Time to access Level 2 memory + Time to access Level 3 memory)]

In the last term, H3 is assumed to be 1 as the word has to be there in the last level memory.

Thus in this case,

H1 = 0.9

H2 = 0.6

T1 = 20

T2 = 60

T3 = 12 * 106 ns

So, Tavg = 480,026 ns

edited by

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