This Vivio animation is designed to help you understand the MESI cache coherency protocol.

A multiprocessor system is depicted comprising 3 CPUs with local caches and main memory. For simplicity, main memory contains 4 locations a0, a1, a2 & a3. The caches are direct mapped and contain two sets - addresses a0 & a2 map to set 0 and a1 & a3 to set 1. 

NB: in order to simplify this animation, the size of a cache line and that of a CPU read/write operation are identical. On a write miss, however, the CPU reads memory even though it will completely overwrite it as this behaviour mirrors that of real caches where the size of the cache line will usually be larger than that of a CPU read/write operation.

NB: click on diagram to activate Vivio animation


Each CPU contains buttons which initiate read or write transactions on the specified memory location. A "CPU write" writes an incrementing value (initially 1) to "memory". 

The idea is to press the buttons and see if you can follow the actions and state transitions which occur. It is possible to introduce bugs into the animation by pressing the "bug free" button in the bottom right hand corner. See if you can find out just exactly what the bugs are! 

The direction of the traffic on the address and data busses are indicated by blue & red arrows respectively. The cache lines and memory location involved in the transaction are coloured green. Stale memory locations are coloured gray. 

A cache line can be in one of 4 states. INVALID: cache line NOT present in cache. EXCLUSIVE: cache line present in this cache ONLY and  identical to copy in memory. MODIFIED: cache line present in this cache ONLY and memory copy out of date (stale). SHARED: cache line in this cache and other caches, ALL copies identical to  memory copy. Writes to SHARED cache lines are write through whereas writes to EXCLUSIVE cache lines are write-back. If a cache observes a bus transaction for an address which it contains, it asserts the SHARED bus line. MESI is an invalidate cache coherency protocol.

Here is the state transition diagram for a cache line:

 

Sample sequence to try [from RESET]

1 CPU0: read a0 CPU0 reads a0 from memory [not shared] - state E
2 CPU0: read a0 CPU0 reads  a0 from cache - state E
3 CPU0: write a0 CPU0 updates a0 in cache ONLY - state M
4 CPU0: write a0 CPU0 updates a0 in cache ONLY - state M
5 CPU1: read a0 CPU1 reads a0, CPU0 cache intervenes and supplies data to cache and memory - state S
6 CPU1: write a0 CPU1 updates a0 in cache & memory and invalidates all other caches with address a0 - state E
7 CPU1: write a0 CPU1 updates a0 in cache ONLY - state M
8

CPU0:  write a0

 CPU0 reads a0, CPU1 cache intervenes and supplies data to cache and memory (S), CPU0 then writes to a0 in cache & memory invalidating all other caches with address a0  - state E
9 CPU0:  write a2 CPU0 reads a2 from memory (E) and then writes to a2  - state M
10 CPU0:  write a0 CPU0 flushes a2 to memory, reads a2 from memory (E) and then writes to a0  - state M