GATE CS 2021 Set 2 — Question 37
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Computer Organization & Architecture → Memory Hierarchy & Cache → Write-Through vs Write-Back
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Question
Assume a two-level inclusive cache hierarchy, L1 and L2, where L2 is the larger of the two. Consider the following statements.
: Read misses in a write through L1 cache do not result in writebacks of dirty lines to the L2.
: Write allocate policy must be used in conjunction with write through caches and no-write allocate policy is used with writeback caches.Which of the following statements is correct?
: Read misses in a write through L1 cache do not result in writebacks of dirty lines to the L2.
: Write allocate policy must be used in conjunction with write through caches and no-write allocate policy is used with writeback caches.Which of the following statements is correct?
Correct answer
(A) S₁ is true and S₂ is false
Solution
Statement is true: In a write-through L1 cache, every write operation updates both the L1 and the L2 (or memory) simultaneously. Therefore, the L1 cache never contains "dirty" data (data that is different from the next level). When a read miss occurs and a line needs to be replaced in L1, there is no need to write back the replaced line to L2 because it is already consistent. Statement is false: While write-through caches are commonly paired with no-write-allocate (write-around) and write-back caches with write-allocate, these are not mandatory requirements. It is possible to design a system with write-through and write-allocate, or write-back with no-write-allocate. The word "must" makes the statement incorrect.
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