The currently executing thread gives up control by either terminating or executing a semWait operation. In other words, the x-kernel does not preempt threads; threads voluntarily give up control of the processor. However, because each protocol is assumed to be an independent component, protocols are written to assume that control may be given up when a higher or lower level protocol is invoked. Therefore, all protocol-to-protocol operations are considered to have the potential to cause a thread switch, and all data structures must be ``secured'' before calling such operations.