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3.3.2 Shared Memory     continued...

Another way of building a shared memory multiprocessor is shown in Figure 8. In these designs, the bus has been replaced by a switch that routes requests from a processor to one of several different memory modules. Even though there are several physical memories, there is one large virtual address space. The advantage of this organization is based on the fact the switch can handle multiple requests in parallel. Each processor can be paired up with a memory, and each can then run at full speed as it accesses the memory it is currently connected to. Contention still occurs, though. If two processors make requests of the same memory module only one will be given access and the other will be blocked. Several machines with this design will be discussed in the survey of MIMD machines following the section on interconnection topology, which introduces concepts that will explain various switch designs.