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?

Let us evaluate the effects of changing the cache block size:

• Miss Penalty: Miss penalty is the time taken to bring a block from main memory to the cache on a cache miss. Since smaller blocks contain fewer bytes, transferring a smaller block takes less time (fewer memory transfer cycles), which means a smaller block size incurs a lower cache miss penalty. Hence, (D) is correct.

• Spatial Locality: Larger block sizes generally exploit spatial locality better (up to a certain point) because fetching a larger contiguous chunk brings nearby words that are likely to be accessed soon. A smaller block size reduces spatial locality advantages, so (A) is incorrect.

• Tag Overhead: The number of blocks is given by $\frac{\text{Cache Size}}{\text{Block Size}}$. Smaller block sizes increase the number of blocks/lines in the cache, requiring more tag entries and larger tag widths (since fewer offset bits means more tag bits for a fixed number of sets or lines), thus *increasing* tag overhead. Therefore, (B) is incorrect.

• Hit Time: Larger tag sizes require wider comparators, which can slightly increase or not decrease hit time. More importantly, (D) directly states the fundamental trade-off: smaller block size $\rightarrow$ lower miss penalty.