Cpu Design Questions
Cache coherence protocols in directory-based cache systems are mechanisms designed to ensure that all copies of a particular memory block in different caches are kept consistent. In directory-based cache systems, each cache has a directory that keeps track of the status and location of each memory block.
When a cache wants to read or modify a memory block, it first checks the directory to determine if any other caches have a copy of that block. If another cache has a copy, the cache coherence protocol ensures that all copies are updated and consistent.
There are different cache coherence protocols, such as the MESI (Modified, Exclusive, Shared, Invalid) protocol. In MESI, each cache block can be in one of four states: Modified, Exclusive, Shared, or Invalid.
- Modified state: The cache block has been modified and is not consistent with other copies in other caches.
- Exclusive state: The cache block is not modified and is exclusive to the cache that holds it.
- Shared state: The cache block is not modified and is shared with other caches.
- Invalid state: The cache block is not valid and cannot be accessed.
When a cache wants to read a memory block, it checks the directory and if the block is in the shared state, it can be read directly. If the block is in the exclusive state, it can also be read directly. However, if the block is in the modified state, it needs to be written back to memory and invalidated in other caches before it can be read.
When a cache wants to modify a memory block, it checks the directory and if the block is in the shared state, it needs to be invalidated in other caches. If the block is in the exclusive state, it can be modified directly. If the block is in the modified state, it can also be modified directly.
Overall, cache coherence protocols in directory-based cache systems ensure that all copies of a memory block are kept consistent by coordinating the actions of different caches and maintaining a coherent view of memory across the system.