![]() The line may be discarded (changed to the Invalid state) at any time. Shared (S) Indicates that this cache line may be stored in other caches of the machine and is clean - it matches the main memory. Alternatively, it may be changed to the Modified state when writing to it. It may be changed to the Shared state at any time, in response to a read request. Exclusive (E) The cache line is present only in the current cache, but is clean - it matches main memory. The write-back changes the line to the Shared state(S). The cache is required to write the data back to main memory at some time in the future, before permitting any other read of the (no longer valid) main memory state. Modified (M) The cache line is present only in the current cache, and is dirty - it has been modified (M state) from the value in main memory. The letters in the acronym MESI represent four exclusive states that a cache line can be marked with (encoded using two additional bits): This marks a significant improvement in performance. This protocol reduces the number of main memory transactions with respect to the MSI protocol. The Illinois Protocol requires a cache to cache transfer on a miss if the block resides in another cache. There is always a dirty state present in write back caches that indicates that the data in the cache is different from that in main memory. Write back caches can save a lot of bandwidth that is generally wasted on a write through cache. ![]() It is also known as the Illinois protocol (due to its development at the University of Illinois at Urbana-Champaign ). The MESI protocol is an Invalidate-based cache coherence protocol, and is one of the most common protocols that support write-back caches. JSTOR ( May 2023) ( Learn how and when to remove this template message).Unsourced material may be challenged and removed. ![]() Please help improve this article by adding citations to reliable sources. This article needs additional citations for verification. ![]()
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