ITPEC FE Morning April 2023 Question 13
Which of the following is the technique that minimizes the number of write operations to the main memory by updating the data only in the cache memory and updating into the main memory later?
Explanation
Write back — writes go to the cache only, and main memory is updated later, when the modified block leaves the cache.
With write back, a write changes the cache block and marks it as modified (dirty). The block is copied to main memory only when it is replaced, so many writes to the same block cost a single memory write. The price is that cache and main memory differ for a while, which multiprocessor systems must handle.
Answer (b)
Why not others:
- •(a) Overlay — loads parts of a program in turn into the same memory area when the whole program does not fit; it has nothing to do with cache writes
- •(c) Write protected — forbids writing to a disk or memory area; it is not a cache write policy
- •(d) Write through — writes to the cache and main memory together on every write: simple and always consistent, but it does not reduce memory writes
Key rule: Write through updates main memory on every write; write back delays the update until the block is evicted, trading immediate consistency for fewer memory writes.
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