The Essentials ofComputer(computer) 455

The Essentials ofComputer(computer) 455 - network only the...

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network only the first time x is updated, thus keeping bus traffic light. Write- update keeps all caches current, which reduces latency; however, it increases communications traffic. A second approach to maintaining cache coherency is the use of a write-back protocol that changes only the data in cache at the time of modification. Main memory is not modified until the modified cache block must be replaced and thus written to memory. With this protocol, data values are read in the normal manner, but before data can be written, the processor performing the modification must obtain exclusive rights to the data. It does so by requesting ownership of the data. When ownership is granted, any copies of the data at other processors are invali- dated. If other processors wish to read the data, they must request the value from the processor that owns it. The processor with ownership then relinquishes its rights and forwards the current data value to main memory.
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