GATE CS 2022 Set 1 — Question 24
Go beyond PYQs with Success TrackerAI-powered personalised practice and doubt support. Unlimited practice on eligible plans; AI usage limits apply.MSQ+1 / -0MediumCache Replacement PoliciesMemory Hierarchy & CacheComputer Organization & ArchitectureWrite-Through vs Write-Back
Computer Organization & Architecture → Memory Hierarchy & Cache → Cache Replacement Policies
Last updated
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?
Correct answer
(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.; (D) A read miss in WB will never lead to eviction of a dirty block from WB.
Solution
The question asks for the FALSE statements.(A) Each cache block in WB and WT has a dirty bit.
A dirty bit is required for a write-back (WB) cache to track whether a block has been modified. This is necessary to know if the block needs to be written back to main memory upon eviction. However, in a write-through (WT) cache, every write is immediately propagated to main memory, so the cache and memory are always consistent. Thus, a WT cache does not need a dirty bit. Since the statement claims that WT also has a dirty bit, the statement is FALSE.(B) Every write hit in WB leads to a data transfer from cache to main memory.
This describes a write-through policy. In a write-back (WB) cache, on a write hit, only the cache block is updated, and its dirty bit is set. The data is written to main memory only when the dirty block is evicted. Therefore, the statement is FALSE.(C) Eviction of a block from WT will not lead to data transfer from cache to main memory.
In a write-through (WT) cache, main memory is updated on every write operation. This ensures that the main memory always holds the most recent version of the data. Consequently, when a block is evicted from the cache, there is no need to write it back to memory. The statement is TRUE.(D) A read miss in WB will never lead to eviction of a dirty block from WB.
A read miss requires fetching a new block from memory. If the target cache set is full, a block must be evicted based on the LRU policy. The block chosen for eviction could be clean or dirty. If the LRU block happens to be dirty, it must be evicted (and written back to memory) to make space for the new block. The statement claims this will never happen, which is incorrect. Therefore, the statement is FALSE.The false statements are A, B, and D.
A dirty bit is required for a write-back (WB) cache to track whether a block has been modified. This is necessary to know if the block needs to be written back to main memory upon eviction. However, in a write-through (WT) cache, every write is immediately propagated to main memory, so the cache and memory are always consistent. Thus, a WT cache does not need a dirty bit. Since the statement claims that WT also has a dirty bit, the statement is FALSE.(B) Every write hit in WB leads to a data transfer from cache to main memory.
This describes a write-through policy. In a write-back (WB) cache, on a write hit, only the cache block is updated, and its dirty bit is set. The data is written to main memory only when the dirty block is evicted. Therefore, the statement is FALSE.(C) Eviction of a block from WT will not lead to data transfer from cache to main memory.
In a write-through (WT) cache, main memory is updated on every write operation. This ensures that the main memory always holds the most recent version of the data. Consequently, when a block is evicted from the cache, there is no need to write it back to memory. The statement is TRUE.(D) A read miss in WB will never lead to eviction of a dirty block from WB.
A read miss requires fetching a new block from memory. If the target cache set is full, a block must be evicted based on the LRU policy. The block chosen for eviction could be clean or dirty. If the LRU block happens to be dirty, it must be evicted (and written back to memory) to make space for the new block. The statement claims this will never happen, which is incorrect. Therefore, the statement is FALSE.The false statements are A, B, and D.
Continue learning with Success Tracker
A step still unclear? Work through it with support
Use Success Tracker to ask about the reasoning, then try another GATE CS question to check your understanding.
AI-powered practice· Unlimited practice on eligible plans
- PYQs with solutions
- Attempt available previous-year questions, then compare your reasoning with the worked solution. Coverage varies by stream.
- Practice that adapts
- Choose a topic, work on weaker areas and bookmark questions to revisit. Your attempts feed your progress tracking.
- AI doubt support
- Ask follow-up questions about a step or concept while practising, instead of stopping at the final answer.
Unlimited practice is available on eligible plans. Free practice and AI usage have limits; check the current plan allowances before choosing.
This page stays readable without an account. AI responses can be wrong; check them against the solution and source material.
More questions on Memory Hierarchy & Cache
2026 Set 1 Q14Match each addressing mode in List I with a data element or an element of a data structure (in…2026 Set 1 Q15Consider a processor P whose instruction set architecture is the load-store architecture. The…2026 Set 1 Q16Which one of the following dependencies among the register operands of different instructions can…2026 Set 2 Q18Consider the following two statements about interrupt handling mechanisms in a CPU. S1: In…2026 Set 1 Q38The size of the physical address space of a processor is bytes. The capacity of a cache…