GATE CS 2022 Q.24: Memory Hierarchy: Cache, Main Memory and Secondary Storage

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

Each cache block in $W_B$ and $W_T$ has a dirty bit.

Every write hit in $W_B$ leads to a data transfer from cache to main memory.

Eviction of a block from $W_T$ will not lead to data transfer from cache to main memory.

A read miss in $W_B$ will never lead to eviction of a dirty block from $W_B$ .

Official answer and solution

Official answer: A, B and D
From: Official GATE 2022 answer key

Solution

Identify the false statements regarding write-through ( $W_T$ ) and write-back ( $W_B$ ) cache policies.

Let us check each statement to determine which are FALSE:

(A) FALSE: A dirty bit indicates that a cache block has been modified relative to main memory. In a write-through cache ( $W_T$ ), memory is updated synchronously on every write, so blocks in $W_T$ are never dirty, and $W_T$ does not require dirty bits.
(B) FALSE: In a write-back cache ( $W_B$ ), a write hit only updates the cache block and sets its dirty bit to 1; data is not written to main memory until that block is evicted.
(C) TRUE: Since $W_T$ always maintains consistency with main memory, evicting a block from $W_T$ never requires writing data back to main memory.
(D) FALSE: A read miss in $W_B$ requires loading a new block into a cache set. If all lines in the set are occupied, the LRU replacement policy may select a dirty block for eviction, which must be written back to main memory.

Since the question asks for FALSE statements, the correct options are (A), (B), and (D).

Final answer

Options (A), (B), (D)

Common trap

Selecting the TRUE statement (C) instead of the FALSE statements requested by the question.

The answer is the one in the official GATE answer key. The solution is this site's own explanation.

Source

Paper: GATE 2022 Computer Science and Information Technology (CS)
Question: 24 of 65
Section: Computer Science and Information Technology

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