Which statement about write-back caches is true?

Which statement about write-back caches is true?

• A. They always require more memory bandwidth than write-through.
• B. They never risk data loss on crashes.
• C. Writes are written to memory immediately on every write.
• D. Writes are deferred to memory until the cache line is evicted.

Answer: (D)

Write-back caches defer writes to memory until the cache line is evicted. When the processor updates data, the change is stored in the cache and marked as dirty, not written to main memory immediately. The actual write-back happens when that cache line must be replaced (evicted) to bring in other data, or during a periodic flush. This is why the statement that writes are deferred to memory until eviction is true. It also explains why such caches can reduce memory bandwidth compared to write-through, since many writes are absorbed by the cache first. However, there is a risk of data loss if a crash or power failure occurs before the dirty data is written back. The other statements contradict how write-back caches operate: they don’t write to memory on every write, and they aren’t guaranteed to avoid data loss.

Write-back caches defer writes to memory until the cache line is evicted. When the processor updates data, the change is stored in the cache and marked as dirty, not written to main memory immediately. The actual write-back happens when that cache line must be replaced (evicted) to bring in other data, or during a periodic flush. This is why the statement that writes are deferred to memory until eviction is true. It also explains why such caches can reduce memory bandwidth compared to write-through, since many writes are absorbed by the cache first. However, there is a risk of data loss if a crash or power failure occurs before the dirty data is written back. The other statements contradict how write-back caches operate: they don’t write to memory on every write, and they aren’t guaranteed to avoid data loss.