Processors are identified with a processor ID number that gives them a unique tag within the system. The state of the processors in your system can be examined by using the psrinfo(1M) command. This command reports whether the processor is online or offline.
When a processor is online, it can dispatch processes and perform normal operating system functions. You can bring a processor online with the psradm(1M) command.
Taking a processor offline removes the processor from operational status. The processor retains its place in the system but does not perform any useful operations while it is idle. Only processors with no bound processes or light-weight processes (LWPs) can be taken offline. Some hardware platforms may require certain processors to be online at all times. A processor can be taken offline with the psradm(1M) command. Once a processor has been taken offline, it remains in that state until it is brought back online using the psradm(1M) command.