States
Every cache line is marked with one of the four following states (coded in two additional bits):
- Modified
- The cache line is present only in the current cache, and is dirty; it has been modified from the value in main memory. 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is required to write the data back to main memory at some time in the future, before permitting any other read of the (no longer valid) main memory state. The write-back changes the line to the Exclusive state.
- Exclusive
- The cache line is present only in the current cache, but is clean; it matches main memory. It may be changed to the Shared state at any time, in response to a read request. Alternatively, it may be changed to the Modified state when writing to it.
- Shared
- Indicates that this cache line may be stored in other caches of the machine and is "clean" ; it matches the main memory. The line may be discarded (changed to the Invalid state) at any time.
- Invalid
- Indicates that this cache line is invalid (unused).
For any given pair of caches, the permitted states of a given cache line are as follows:
| M | E | S | I | |
|---|---|---|---|---|
| M | N | N | N | Y |
| E | N | N | N | Y |
| S | N | N | Y | Y |
| I | Y | Y | Y | Y |
Read more about this topic: MESI Protocol
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