GATE | Sudo GATE 2020 Mock III (24 January 2019) | Question 54

How many tag bits are required for a system that has a Main Memory with 16 MB of addressable locations and a 64 KB direct mapped cache with 16 bytes per block.
(A) 12
(B) 13
(C) 14
(D) 15


Answer: (C)

Explanation:

Cache memory size = 256KB,  
Main memory size = 4GB = 2^{32} byte
Block size = 64B

Number of blocks 
= Number of lines 
= 256 KB / 64 B =
 2^{18} / 2^{6} 
=2^{12} blocks/cache

And, 12 bits are needed for index/line 
Word offset = log(64 B) = 6 bits

Addressing main memory requires = log (2^{32}) = 32 bits
Tag bits required = 32-(12+6) = 14 bits 

Option (C) is correct.

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