The final part of the book looks at the increasingly important area of parallel orga- nization. In a parallel organization, multiple processing units cooperate to execute applications. Whereas a superscalar processor exploits opportunities for parallel ex- ecution at the instruction level, a parallel processing organization looks for a grosser level of parallelism, one that enables work to be done in parallel, and cooperatively, by multiple processors. | The preceding sequence explains how data are read from a remote memory using hardware mechanisms that make the transaction transparent to the processor. On top of these mechanisms, some form of cache coherence protocol is needed. Var- ious systems differ on exactly how this is done. We make only a few general remarks here. First, as part of the preceding sequence, node 1’s directory keeps a record that some remote cache has a copy of the line containing location 798. Then, there needs to be a cooperative protocol to take care of modifications. For example, if a modifi- cation is done in a cache, this fact can be broadcast to other nodes. Each node’s di- rectory that receives such a broadcast can then determine if any local cache has that line and, if so, cause it to be purged. If the actual memory location is at the node re- ceiving the broadcast notification, then that node’s directory needs to maintain an entry indicating that that line of memory is invalid and remains so until a write back occurs. If another processor (local or remote) requests the invalid line, then the local directory must force a write back to update memory before providing the data.
