Question

Let WB and WT be two set associative cache organizations that use LRU algorithm for cache block replacement. WB is a write back cache and WT is a write through cache. Which of the following statements is/are FALSE?

• A. Each cache block in WB and WT has a dirty bit.
• B. Every write hit in WB leads to a data transfer from cache to main memory.
• C. Eviction of a block from WT will not lead to data transfer from cache to main memory.
• D. A read miss in WB will never lead to eviction of a dirty block from WB.

Hint:

In write-through caches, all writes go to memory immediately, while in write-back caches, writes are stored in the cache until eviction.

Answer 1

The Correct Option is A, B, D

(A) False. In a write-through cache (WT), there is no need for a dirty bit since all writes go immediately to the main memory. However, in a write-back cache (WB), a dirty bit is used to track whether a block has been modified. (B) False. In a write-back cache, a write hit does not immediately lead to data transfer to the main memory; instead, it marks the block as dirty and updates it only when it is evicted. (C) True. In a write-through cache, every write causes an immediate update to the main memory, but evicting a block does not require a transfer if the block is not dirty. (D) False. In a write-back cache, a read miss can result in the eviction of a dirty block, which will cause a write-back to main memory.