Computer organisation

Question

If cache access time is $100ns$, main memory access time is $1000ns$ and the hit ratio is $0.9$. Find the average access time and also define hit ratio.

Answer 1

Average time to access = Hit rate x Cache access time + Miss rate x (Cache access time + Memory access time)

Putting values, we get

                                        = 0.9 x 100 + 0.1 (100 + 1000)

                                        = 90 + 110
                                        = 200 ns

Hit ratio is defined as "percentage of memory accesses satisfied by the cache"

Ref: http://homepage.divms.uiowa.edu/~ghosh/4-1-10.pdf

Comments

Hi,

why didn't you use this formula

Average time to access = Hit rate x Cache access time + Miss rate x (Memory access time)

then answer will be 190ns

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Well, there are two types of cache access policies.

• Hierarchal - Here access to cache takes place first and then when it fails, we go to main memory. So, we need to count the time (wasted) in cache access in case of cache miss. Here the formula which I have used is to be used.  
• Simultaneous -  Here access to cache as well as main memory happen simultaneously. So, in case of miss in cache, we do not need to count cache access time again. Here the formula which you have used is to be used.  

Ref: https://gateoverflow.in/108780/simultaneous-vs-hierarchical-memory

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if it is not given in question like it is Hierarchal or Simultaneous access then by default we have to take which access ?

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I am not really sure of that, but you can try this approach - Do both and see which ans is in option. if both are there / or you have to type, go with hierarchal. However, some questions in some exams like that of ISRO use simultaneous  access. May be others can help here