Abstract:

One of the problems a multicomputer has to deal with is the cache coherence problem. To put in simple words, the caches are coherent when each piece of data is up to date in every cache and in the memory of the multicomputer. To ensure that a multicomputer cache is coherent various protocols can be used.

When teaching the cache coherence protocols we looked for simulation tools or animations that could illustrate how these protocols worked. We wanted the students to see how the states in each cache block changed when the different processors accessed different memory positions and which were the transitions between states and which messages where generated in each case. Although we found some multicomputer simulators, they were far to complex and they weren't meant to illustrate how the cache coherence protocols worked. And unfortunately, the animations we found didn't cover all the details that should be covered in our computer architecture course.

We have developed a flash interactive animation that shows how some of the most used cache coherence protocols work. It covers the MSI and MESI snoopy protocols and the MESI directory protocol. For each protocol a sequence of read and write operations is presented that shows all the possible cases that can take place in each protocol. For each case, a full description of what is going on is shown and the student can go forward and backward to see how each step is performed.

Keywords:

Computer architecture, multicomputers, multiprocessors, cache coherence, interactive animation.