Answer :
Final answer:
In a system with 2 private L1 caches and a shared L2 cache, the MSI protocol ensures cache coherence by allowing each processor to have its own copy of data in the L1 cache while the shared L2 cache manages coherence. So the correct answer is option B and C.
Explanation:
In a scenario with 2 private L1 caches and a shared L2 cache, the MSI protocol will function as follows: Each processor will maintain its own copy of the data in its L1 cache, while the L2 cache will manage the coherence between the L1 caches. The MSI protocol is a basic cache-coherence protocol that stands for Modified, Shared, and Invalid. It operates in a system with multiple cache levels to ensure that multiple processors have a consistent view of the memory.
When one of the L1 caches wishes to write to a data block, it must first gain exclusive access by sending a message through the L2 cache, which then ensures that no other L1 cache has a valid copy of that block. If a processor reads a data block that another processor wishes to write, the L2 cache mediates to ensure that the read occurs before the write or that the write invalidates the other processor's read copy, ensuring proper coherence.
In this way, the L2 cache is responsible for acting as an intermediary to manage coherence, rather than the L1 caches communicating directly with one another. Therefore, communication between L1 caches is indirect and facilitated by the shared L2 cache using the MSI protocol.
