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GATE | GATE-CS-2016 (Set 2) | Question 42

  • Difficulty Level : Easy
  • Last Updated : 29 Sep, 2021

The width of the physical address on a machine is 40 bits. The width of the tag field in a 512 KB 8-way set associative cache is ____________ bits
(A) 24
(B) 20
(C) 30
(D) 40


Answer: (A)

Explanation: An easy approach would be we know that physical address is 40 bytes

We know cache size = no.of.sets*
                     lines-per-set*
                     block-size

Let us assume no of sets = 2^x 
And block size= 2^y

So applying it in formula.
2^19 = 2^x + 8 + 2^y;
So x+y = 16

Now we know that to address block size and 
set number we need 16 bits so remaining bits
must be for tag 
i.e., 40 - 16 = 24
The answer is 24 bits 

If question extends and asks as what is the size of comparator, we need then it is 24 bit comparator.

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The above explanation is contributed by Sumanth Sunny



Alternate Explanation:

Physical Address Bits = T(Tag Bits) + S(Set Bits) + O(Offset Bits) = 40 bits    (given)
Set = 8    (given)
Size of cache = 512 KB     (given)

Size of lines = 512 / 8 = 64 KB 
So, O = 64/8 = 8 bits

Now, S + O = 8 + 8 = 16 bits
Hence, T = 40 - 16 = 24 bits 

physical-address-structure

This explanation is contributed by Mohit Gupta.

Refer the following links for more understanding in the above topic:

Cache Memory
Cache Organization | Introduction



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