Statement Analysis: Cache Policies & Coherence

Let's analyze each statement regarding cache memory policies and cache coherence.

Statement A: Write-Through Policy

The write-through policy is a method for handling write operations in a cache system. In this policy, when a write operation occurs, the data is written to both the cache and the main memory simultaneously. The statement claims that only the cache is updated, which contradicts the fundamental principle of the write-through policy. Therefore, statement A is false.

• Write-Through Mechanism: Write Cache → Write Main Memory.
• Result: Both cache and main memory contain the updated data immediately.

Statement B: Write-Back Policy

The write-back policy (also known as write-deferred) handles write operations differently. When a write occurs, the data is written only to the cache line, and that cache line is marked as 'dirty' (modified). The updated data is written back to the main memory only when the modified cache line is about to be replaced or flushed. The statement asserts that both cache and main memory are updated, implying an immediate update to main memory, which is characteristic of write-through, not write-back. Therefore, statement B is false.

• Write-Back Mechanism: Write Cache (Mark as Dirty) → Update Main Memory (Later, on eviction).
• Result: Cache has the updated data; Main memory may have stale data until eviction.

Statement C: Cache Coherence in Multiprocessors

In a multiprocessor system where each processor has its own private cache, multiple copies of the same main memory block can exist across different caches. When these processors need to share writable data, a problem arises. If one processor modifies the data in its private cache, other processors might continue to use the older, stale version of the data from their caches, leading to inconsistencies. This issue is known as the cache coherence problem. Statement C correctly identifies this scenario. Therefore, statement C is true.

Statement D: Hardware Solutions for Cache Coherence

The cache coherence problem requires mechanisms to ensure all caches have a consistent view of memory. Various techniques exist to address this. Hardware-only schemes are a common approach. Protocols like the MESI (Modified, Exclusive, Shared, Invalid) protocol, snooping protocols (where caches monitor bus transactions), and directory-based protocols are implemented using hardware logic to detect potential coherence violations and enforce consistency. While software techniques can also play a role, hardware solutions are effective and widely used. Therefore, statement D is true.

Conclusion

Based on the analysis:

• Statement A is false.
• Statement B is false.
• Statement C is true.
• Statement D is true.

The true statements are C and D.