# Electronics and Communication Engineering - Digital Electronics - Discussion

16.

The access time of a word in 4 MB main memory is 100 ms. The access time of a word in a 32 kb data cache memory is 10 ns. The average data cache bit ratio is 0.95. The efficiency of memory access time is

 [A]. 9.5 ns [B]. 14.5 ns [C]. 20 ns [D]. 95 ns

Answer: Option B

Explanation:

Access time = 0.95 x 10 + 0.05 x 100.

 Daleep said: (Aug 1, 2014) Please mention the formula used above.

 Nithya said: (Apr 22, 2015) Can you please explain clearly?

 Sateesh said: (Apr 24, 2015) There is clearly mentioned that hit ratio is 0.95 and it is always prefer cache memory, so 10*0.95 and 0.05 is failure case it is from main memory. So 100*0.05. So answer is correct.

 Jhklhlkhkgku said: (Jul 16, 2015) What about the 100 ms which has converted in ns?

 Tejas said: (Jul 17, 2015) @ Sateesh. I could not understand, how you calculated?

 Sneha Kulkarni said: (Aug 6, 2015) Ya what about conversion?

 Cr7 said: (Aug 10, 2015) Any alternative?

 Kar said: (Aug 27, 2015) Formula is right, but 100 ms needs to be converted to ns.

 Baishali said: (Aug 27, 2015) Please give a proper explanation for the result.

 Dheeraja said: (Sep 8, 2015) How to taken 0.05 & what formula will be applied?

 Ramesh said: (Jan 26, 2016) If anyone knows clear explanation then please explain.

 Mahesh said: (Mar 10, 2016) Can you please explain clearly?

 Sree said: (Jun 28, 2016) Please mention the formula used here.

 Kokila said: (Jul 19, 2016) I need clear explanation, please elaborate the solution.

 Monica said: (Aug 30, 2016) Please, anyone gives the clear solution for this question with a correct formulas or logic.

 Pradeepa said: (Aug 30, 2016) Hi, I want a clear explanation of the answer. Please provide me.

 Ram said: (Aug 31, 2016) 0.95 * 10ns + 0.05 * 100ms = If you are consider ms and ns, then what will be the answer? Please explain.

 S.Rohini said: (Sep 16, 2016) Please mention the formula and explanation.

 Bikram Kesharee Nayak said: (Dec 21, 2016) Assume that main memory access time is 100 ns instead of ms as given here. When we try to search/ access a bit or word through memory, first the cache memory is searched for access. So it's related to hit ratio directly. The performance of cache memory is measured in terms of hit ratio. If a data item requested by CPU is found in the cache, it is called a 'HIT'.If the requested data item is not found i the cache that means it's in main memory and it is called a 'MISS'. Hit ratio is the ratio of the number of hits divided by total no. of requests. Now given that the memory access time (MAT. cache) for cache is 10 ns and for main memory is 100 ns. So, Average memory access time or efficiency = (Hit ratio * MAT. cache) + (1- hit ratio) * (MAT. main memory). = (0.95* 10 ns) + (0.05* 100 ns), = 9.5 ns + 5 ns, = 14.5 ns. I hope everyone understand this.

 Padmaja said: (Dec 27, 2016) Thank you @Bikram Kesharee Nayak.

 Amrita said: (Feb 5, 2017) Thank you the explanation @Bikram.

 Surya said: (Mar 2, 2017) Thank you all for explaining the solution.

 Rishi Ram Panth said: (Jul 25, 2017) Thank you @Bikram.

 Heena said: (Aug 27, 2017) How do we get 0.05 hit rate from main memory?

 Abarna said: (Oct 13, 2017) @Heena. Hit rate only they said in formula 1-0.95 is 0.05 only, that's applied there.

 Anonym said: (Oct 25, 2017) 1-0.095=0.05 according to the formula

 Ms Pooja Choudhari said: (May 8, 2018) Here, T(avg)=h*Tc+(1-h)*M. where, T(avg)=Average time. h=bit rate,(1-h)=Min Rate, Tc=Time to access information from cache, M=Miss Penalty(Time to main memory).

 Harshini said: (Apr 19, 2019) Thank you @Bikram.

 Vanajavivek said: (Jun 7, 2019) Thank you @Bkram and @Pooja.

 Pujitha said: (Nov 20, 2019) Thank you so much, @Bikram Ji. You can join as a teacher.

 Prajwal said: (Dec 18, 2019) Thank you @Bikram and @Pooja.

 Ali Muhammad said: (Sep 29, 2020) You explained in a good manner. Thank you @ Bikram.

 Tshering Dorji said: (Jan 1, 2021) That means the access time from the main memory is 100ns or 100ms? I think time is 100ns and not 100ms. Please anyone explain it clearly.

 Amit Kumar said: (Mar 27, 2021) @Bikram. Good explanation. Thanks.

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