GATE CS 2024 Set 1 Q43 — Memory Hierarchy: Cache, Main Memory and Secondary Storage

Let us analyze each option step-by-step:

• Option A: Incorrect. In a Write-Back Cache ($\text{WBC}$), when a read miss occurs and the target cache set is full, a block must be evicted using the $\text{LRU}$ policy. If the selected victim block is dirty (i.e., its dirty bit is $1$), it must be written back to the main memory before the new block is loaded. Thus, a read miss in $\text{WBC}$ can evict a dirty block.
• Option B: Correct. In a Write-Through Cache ($\text{WTC}$), every write operation updates both the cache and the main memory simultaneously. Consequently, cache blocks are never "dirty" and there is no dirty bit. When a read miss occurs and a block needs to be evicted, the victim block can simply be overwritten without any write-back operation to the main memory.
• Option C: Correct. In a Write-Back Cache ($\text{WBC}$), a cache block is initially clean (dirty bit $= 0$) when loaded from main memory. If a write hit occurs on this clean block, the data is updated in the cache, and its dirty bit is modified from $0$ to $1$. Thus, a write hit can modify the value of the dirty bit.
• Option D: Incorrect. Since $\text{WTC}$ maintains consistency with main memory on every write, no block in $\text{WTC}$ is ever dirty. Therefore, evicting a victim block in $\text{WTC}$ never requires writing it back to the main memory.

Hence, statements B and C are TRUE.