How does a VxWorks scheduler get executed?

Would like to know how the scheduler gets called so that it can switch tasks. As in even if its preemptive scheduling or round robin scheduling - the scheduler should come in to picture to do any kind of task switching. Supposing a low priority task has an infinite loop - when does the scheduler intervene and switch to a higher priority task?

Query is: 1. Who calls the scheduler? [in VxWorks] 2. If it gets called at regular intervals - how is that mechanism implemented?

Thanks in advance.

--Ashwin