Cache Memory
General Concept
CPU Core Caching
Cache Lines
Cache Memory
Associative Memory
Direct-Mapped Memory
Set Associative Memory
Cache Read/Write Policies
MESI protocol: (Modified, Exclusive, Shared, Invalid)
- Invalid lines are cache lines that are either not present in the cache, or whose contents are known to be stale. For the purposes of caching, these are ignored. Once a cache line is invalidated, it’s as if it wasn’t in the cache in the first place.
- Shared lines are clean copies of the contents of main memory. Cache lines in the shared state can be used to serve reads but they can’t be written to. Multiple caches are allowed to have a copy of the same memory location in “shared” state at the same time, hence the name.
- Exclusive lines are also clean copies of the contents of main memory, just like the S state. The difference is that when one core holds a line in E state, no other core may hold it at the same time, hence “exclusive”. That is, the same line must be in the I state in the caches of all other cores.
- Modified lines are dirty; they have been locally modified. If a line is in the M state, it must be in the I state for all other cores, same as E. In addition, modified cache lines need to be written back to memory when they get evicted or invalidated – same as the regular dirty state in a write-back cache.
Principle Of Locality
Summary
