The x-kernel uses a ``thread-per-message'' model of computation, and provides primitives for synchronizing threads. The following operations affect thread scheduling. Of these, only semWait can cause the x-kernel to run a different thread than the current one.
Note that this section does not define any operations for creating or destroying threads. This is because x-kernel threads are created and destroyed implicitly. Threads are created by the device driver (in the case of incoming messages), by the system call interface (in the case of outgoing messages), and by the event library (in the case of an event firing). Threads are destroyed when they return from the outer-most procedure.