ITPEC FE Morning October 2018 Question 14

Faster transfer when the CPU reads the same data again — the cache keeps a copy of data read from main memory, and later reads are served from it.

Cache memory is a small, fast memory between the CPU and the slower main memory. The first read of a block goes to main memory and a copy is placed in the cache. Programs tend to reuse the same data and nearby data (locality of reference), so later reads usually hit the cache and avoid the slow main-memory access.

Answer (b)

Why not others:

• •(a) — a cache only stores data; it does not process instructions, in parallel or otherwise
• •(c) — reading instructions and data at the same time is a matter of separate paths (Harvard architecture, split caches), and the first read from main memory is no faster; the gain comes on repeated access
• •(d) — decoding and execution take place in the CPU's control unit and ALU, not in the cache

Key rule: A cache speeds up access only when data is reused (a hit); it stores copies and never executes anything, and its benefit depends on the hit ratio.

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