How Does the Operating System Control the Processor?

Every thousand clock ticks there'll be a time interrupt that's always executed. The operating system just automatically retakes control of the system and then decides, do i want to hand back control to the process that was running or am i going to allow a different process to run for a while? So this insures that even if there's no i o, and even the process is not making any system calls, that the operating system still has the ability to usurp control back to itself. And recall also, every time the o s is invoked, the process is returned back to kelnel mode - so that the Operating System has free reign over what it does with your system.