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Chapter 1. Hardware, Processes, and Thre... > The Characteristics of Multiprocesso...

The Characteristics of Multiprocessor Systems

Although processors with multiple cores are now prevalent, it is also becoming more common to encounter systems with multiple processors. As soon as there are multiple processors in a system, accessing memory becomes more complex. Not only can data be held in memory, but it can also be held in the caches of one of the other processors. For code to execute correctly, there should be only a single up-to-date version of each item of data; this feature is called cache coherence.

The common approach to providing cache coherence is called snooping. Each processor broadcasts the address that it wants to either read or write. The other processors watch for these broadcasts. When they see that the address of data they hold can take one of two actions, they can return the data if the other processor wants to read the data and they have the most recent copy. If the other processor wants to store a new value for the data, they can invalidate their copy.


  

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