Two private caches make nearby reads cheap, but they also create several places where the same address can live. This topic explains what must agree, what may differ, and how a home node coordinates permission. Start with one location X inside a 64-byte line and follow its value rather than memorising protocol initials.
Lesson | After it you can... |
|---|---|
Explain why a local store can leave memory behind. | |
Watch two copies drift apart, then separate harmless old bits from a usable stale copy. | |
Check permissions and values independently. | |
Match six messages to the exact permission handoff. |
Suggested order: read top to bottom. The first trace uses MESI's clean exclusive state; the final trace deliberately uses MSI's shared read permission. Each names its protocol so the two choices are never mixed. Keep the Course Model beside the snapshots.
No. A write-back cache may hold the only current value with exclusive permission. You must check which copies are usable, who may write, and how later requests obtain that value.