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

Let us evaluate each statement using standard cache memory principles.

(A) False.
In a write-through (WT) cache, every write operation updates both the cache and the main memory immediately, so a dirty bit is not required. However, in a write-back (WB) cache, updates are made only to the cache and written back to main memory later upon eviction, which requires a dirty bit to track modifications. Hence, the statement is false.

(B) False.
In a write-back cache, a write hit updates only the cache block and sets the dirty bit. The data is written to main memory only when the block is evicted, not immediately. Therefore, the statement is false.

(C) True.
In a write-through cache, every write operation updates main memory immediately. Since blocks are always consistent with memory, evicting a block does not require any additional data transfer. Thus, this statement is true.

(D) False.
In a write-back cache, a read miss may cause a dirty block to be evicted. If that happens, the dirty block must be written back to main memory before replacement. Hence, it is incorrect to say that a read miss cannot cause a write-back. The statement is false.

Final Answer:
The correct option is (C).