Which of the following statements are true?
A. In the write-through policy, only the cache is updated.
B. In the write-back policy, both cache and main memory are updated.
C. Cache coherence problems exist in multiprocessors with private caches because of the need to share writable data.
D. Cache coherence problem can be solved by means of hardware-only scheme.
Choose the correct answer from the options given below:

Correct option is C

The question asks to identify which of the statements regarding cache coherence and write policies in computer architecture are true. Let's break down each statement and its truth value:
Statements:
A. In the write-through policy, only the cache is updated: This statement is incorrect. In the write-through policy, every time data is written to the cache, it is simultaneously written to the main memory as well. This ensures that the main memory always holds the most recent data, but it can result in slower performance due to the constant memory updates.
· Conclusion: This statement is false.
B. In the write-back policy, both cache and main memory are updated: This statement is incorrect. In the write-back policy, the cache is updated immediately when the data is written. However, the main memory is not updated until the data is evicted from the cache. This reduces memory write operations and improves performance but can lead to consistency issues if not handled properly.
· Conclusion: This statement is false.
C. Cache coherence problems exist in multiprocessors with private caches because of the need to share writable data: This statement is correct. In a multiprocessor system, each processor typically has its own private cache. Cache coherence problems arise when multiple processors attempt to modify the same memory location in their respective caches. This leads to inconsistencies in the data, as each processor's cache might have different values for the same memory location.
· Conclusion: This statement is true.
D. Cache coherence problems can be solved by means of hardware-only schemes: This statement is correct. Cache coherence problems can indeed be solved using hardware-only schemes like the MESI protocol (Modified, Exclusive, Shared, Invalid). These hardware protocols ensure that multiple caches in a multiprocessor system maintain a consistent view of memory, avoiding issues such as stale data.
· Conclusion: This statement is true.
Based on the analysis, the correct answer is Option c: C, D Only.
Information Booster:
1. Write-through vs. Write-back:
a. In write-through, updates are immediately reflected in both cache and memory, which ensures memory consistency but reduces performance.
b. In write-back, updates are written only to the cache, and memory is updated later when the cache block is evicted. This improves performance but introduces potential coherence issues.
2. Cache Coherence in Multiprocessors: In multiprocessor systems, where each processor has its own cache, maintaining data consistency across all caches becomes a challenge. Techniques like the MESI protocol and hardware-based coherence mechanisms are used to solve these problems.
Additional Knowledge:
· Cache Coherence Protocols: MESI (Modified, Exclusive, Shared, Invalid) is a widely used hardware protocol to manage cache coherence in multiprocessor systems. It ensures that caches stay synchronized and prevents inconsistencies when multiple processors access the same memory.