A thread in a process can migrate from processor to processor, with each migration reloading the processor cache. Under heavy system loads, specifying which processor should run a specific thread can improve performance by reducing the number of times the processor cache is reloaded. The association between a processor and a thread is called the processor affinity.
Each processor is represented as a bit. Bit 0 is processor one, bit 1 is processor two, and so forth. If you set a bit to the value 1, the corresponding processor is selected for thread assignment. When you set the value to zero, the operating system's scheduling algorithms set the thread's affinity. When the value is set to any nonzero value, the value is interpreted as a bitmask that specifies those processors eligible for selection.
The following table shows a selection of values for an eight-processor system.