GATE2006-62, ISRO2016-50

Question

A CPU generates $32$-bit virtual addresses. The page size is $4$ KB. The processor has a translation look-aside buffer (TLB) which can hold a total of $128$ page table entries and is $4$-way set associative. The minimum size of the TLB tag is:

• A. $\text{11 bits}$
• B. $\text{13 bits}$
• C. $\text{15 bits}$
• D. $\text{20 bits}$

Comments

I am having difficulty in solving virtual memory concepts involving the concept of associative caches. Can someone please provide a link where I can gain more knowledge about it .

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No of entries in TLB= 2^7 4-way set

so each set has 2^7/4= 2^5 sets

2^20 pages of process PTE mapped to 2^5 sets

so each set can accomodate 2^20/2^5= 2^15. so in each set to know which page among 2^15 page table entry preseent we need 15 bits.

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Virtual memory means address of physical memory frame at which desired data is present and this address is stored in TLB for faster access .It can store upto 32 pages means 5 bit will be use to search in TLB and remaining(15bits) will be used as tag bits to check for other information like page is dirty or not, recently referred or not.

Answer 1

Page size of $4KB$. So, offset bits are $12$ bits.

So, remaining bits of virtual address $32 - 12 = 20$ bits will be used for indexing...

Number of sets = 128/4 = 32 (4-way set)  => 5 bits.

So, tag bits $= 20 - 5 = 15$ bits.

So, option (C).

Comments

Can you please tell why you simply subtracted 5 from 20? Is it because Indexing in set associative memory is done via Tag+Set bits?

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"4-way set associative means number of lines is 4 in each set." So my question does in TLB each page table entry correspond to a line?

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yes block size in TLB just correspond to one entry, so 128 entries corresponds to 128 blocks and 32 sets.

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Is the given data is sufficient to find the entry size of the TLB if yes? then how to find it? and is tag bits included in the entry size

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@rajat_mahajan this is the how the virtual address will be broken up into:

4KB page defines the offset = 12 bits
32 sets in the TLB defines the set = 5 bits
Remaining 32 - 17 = 15 bits will be used to define tag bits.

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bro can you explain organization of TLB.

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@shaik mastan @akash.dinkar2 @magma @srestha
can anyone explain how TLB is organized when it is used as Direct mapped and set-assosiative  ?

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Dharmendra Lodhi

here people might not notice u, u can ask as a separate question...