In designing a computer’s cache system, the cache block (or cache line) size is an important parameter. Which one of the following statements is correct in this context?
(A) A smaller block size implies better spatial locality
(B) A smaller block size implies a smaller cache tag and hence lower cache tag overhead
(C) A smaller block size implies a larger cache tag and hence lower cache hit time
(D) A smaller block size incurs a lower cache miss penalty


Answer: (D)

Explanation: Block : The memory is divided into equal size segments. Each segment is called a block. Data in cache is retrieved in form of blocks. The idea is to use Spatial Locality (Once a location is retrieved, it is highly probable that the nearby locations would be retrieved in near future).

TAG bits : Each cache block is given a set of TAG bits to identify which main memory block is present in that cache block.

Option A : If the block size is small, there would be less number of near-by address for future references by CPU to be present into that block. Hence this is not better spatial locality.

Option B : If the block size is smaller, no of blocks would be more in cache, hence more cache tag bits would be needed, not less.

Option C : Cache tag bits are more ( because more no of blocks due to smaller block size ), but more cache tag bits can\’t lower the hit time ( even it will increase ).

Option D : If there is a miss at cache memory ( i.e. the needed block by the CPU is not present in the cache memory ), then that block has to be moved from next lower level of memory ( lets say main memory ) in the memory hierarchy, and if the block size is lower, then it takes less time to be placed into cache memory, hence less miss penalty.

Hence option D.


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