GATE CSE 2020 | Question: 21

Question

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 $2$0 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______.

Comments

Why sequencial access is not considered here?

Please tell us how to treat this?

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As per GATE official key both $13.3$(Simultaneous access) and $13.5$(Hierarchical access) are answers.

Source: https://web.archive.org/web/20200618164834/http://www.gate.iitd.ac.in/CSKey.php

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Thanks Sagar

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@Arjun Sir, please update the answer here as given in the GATE 2020 official answer key as provided by @Shiva Sagar Rao with 13.3 to 13.3 OR 13.5 to 13.5

It’s also present here link: cs_2020.pdf (iitk.ac.in)

Answer 1

ans is 13.5  consider sequential setup of heirarchy

Explaination : h1*t1 + (1-h1)(t1+t2) = avd access time for the given hierarcy of memeory.

so, 0.94*3 + 0.06(3ns + 20 + 31*5) = 13.5 ns

t2 = first word(20 ns) + 5* remaining words (2pow 5 -1) ; 2 pow 5 = no of words in block.