GATE | GATE CS 2020 | Question 31

A direct mapped cache memory of 1 MB has a block size of 256 bytes. The cache has an access time of 3 ns and a hit rate of 94%. During a cache miss, it takes 20 ns to bring the first word of a block from the main memory, while each subsequent word takes 5 ns. The word size is 64 bits. The average memory access time in ns (round off to 1 decimal place) is ________ .

Note – This question was Numerical Type.
(A) 13.5
(B) 15.5
(C) 23.5
(D) 15.3


Answer: (A)

Explanation: Given,

Word size = 64 bit = 8 byte
And, 
Block size = 256 byte 

Therefore, Number of words per block will be,

= 256 / 8 = 32

According to question, first word takes 20ns and rest (31) each subsequent words take 5ns each to fetch a word from main-memory to cache.

Hence,

Tavg = (0.94 × 3) + (1 – 0.94) [3 + (20 + (31 × 5))]
= 13.5 (in ns) 

Option (A) is correct.

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