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GATE and NET Questions on calculation EMAT using TLB

Before seeing all question please follow the previous chapter first:  Effective Access Time using Hit ratio and Miss ratio

Question UGC NET 2017

In a paging system, it takes 30ns to search translation look–aside buffer (TLB) and 90ns to access main memory. If the TLB hit ratio is 70%, the effective memory access time is 

(a) 48ns           (b) 147ns            (c) 120ns            (d) 84ns

Solution:

In question, if the level of paging is not mentioned, we can assume that it is single-level paging.

Effective memory Access Time (EMAT) for single level paging with TLB hit ratio:

Here hit ratio (h) = 70% means here taking 0.7, memory access time (m) = 90ns and TLB access time (t) = 30ns.

So, 

So, option (b) is correct.

Another way by using another formula of EMAT:

Here hit ratio = 70%, so, miss ratio (h) = 30%, memory access time (m) = 90ns and TLB access time (t) = 30ns.

Note: So, we can use any of these two formula answers will be same but keep in your mind one with hit ratio and another with miss ratio.

Question GATE 2008 

In an instruction execution pipeline, the earliest that the data TLB (Translation Lookaside Buffer) can be accessed is

(a) before effective address calculation has started.

(b) during effective address calculation.

(c) after effective address calculation has completed 

(d) after data cache lookup has completed.

Solution:

We know the main disadvantage of paging, it increases memory access times because at least one additional memory access is required due to access the page table.

To reduce it in address translation logical address to a physical address to get the effective address of the required page, we are using TLB (Translation Lookaside Buffer) which keeps most recently and frequently access page entry in it. So, here we do need to access the page table and we can directly translate the address and get the frame number (the effective address) from TLB.

So, during effective address calculation TLB access for required page entry to get corresponding frame number. 

For more details please follow:  Problem of paging  and Implementation of TLB 

So, option (b) is correct.

Question GATE 2008 (IT)

A paging scheme uses a Translation Look-aside Buffer (TLB). A TLB-access takes 10 ns and the main memory access takes 50 ns. What is the effective access time(in ns) if the TLB hit ratio is 90% and there is no page-fault?

A. 54              B. 60               C. 65             D. 75

Solution:

In question, if the level of paging is not mentioned, we can assume that it is single-level paging.

Effective memory Access Time (EMAT) for single level paging with TLB hit ratio:

Here hit ratio (h) = 90% means here taking 0.9, memory access time (m) = 50ns and TLB access time (t) = 10ns.

So, 

So, option (c) is correct.

Note: You can also solve it by another EMAT formula:

and you can see the answer will be the same.