Cache Coherence

Cache coherence refers to the consistency of data stored in different caches that are part of a multiprocessor system. When multiple processors have their own caches, it is possible for them to have copies of the same data. Maintaining cache coherence ensures that all processors observe a single, consistent view of memory.

Write Policies

Write policies determine how updates to data are handled in the cache. The different write policies are:

- Write-through: In this policy, all write operations are immediately propagated to both the cache and the main memory. This ensures that the data is always consistent between the cache and the main memory. However, it may result in slower write operations due to the need to access both the cache and the main memory.

- Write-back: In this policy, write operations are initially performed only on the cache. The updated data is later written back to the main memory at a later time or when the cache line is evicted. This policy can improve write performance since writes are only performed in the cache. However, it introduces the possibility of data inconsistency between the cache and the main memory until the data is written back.

- Buffered write: This policy is similar to write-back, but it includes an additional buffer between the cache and the main memory. Write operations are initially performed on the cache and then stored in the buffer. The data is eventually written back to the main memory from the buffer. This policy can improve write performance even further by allowing multiple writes to be combined before writing to the main memory.

Not a Write Policy to Avoid Cache Coherence

The correct answer is option 'B' - Write within. However, it should be noted that "Write within" is not a recognized write policy in the context of cache coherence. It seems to be a typo or a misleading option.

To maintain cache coherence, it is important to choose a write policy that ensures the consistency of data between the cache and the main memory. Options 'A' (Write through), 'C' (Write back), and 'D' (Buffered write) are all valid write policies that can be used to maintain cache coherence.