Q. What is Block size of cache memory?

Block Size: Block size means unit of data (few memory words) exchanged between main and cache memory. As block size increases from very small to larger size then hit ratio (fraction of times which referenced data/instruction is found in cache) will at first increase due to the principle of locality since more and more useful words are brought into cache. But hit ratio will begin to reduce as block size further increases since probability of using newly fetched words becomes less than probability of reusing words which should be moved out of cache to make room for new block. Relying on this fact block size is rightfully chosen to maximise hit ratio.