to ensure operating system maintains control.•Timer is decremented every clock tick.•When timer reaches the value 0, an interruptoccurs.•Timer commonly used to implement time sharing.•Time also used to compute the current time.•Load-timer is a privileged instruction.
OS KernelReal work is done in the kernelUsersApplicationsOperating System API, AUIOperating System KernelComputer Hardware
Entry Points into KernelSystem CallSignalInterruptTrap
System Calls•User processes must not be given open access to thekernel code•The system call interface layer contains entry pointin the kernel code•Any user or application request that involves accessto any system resource must be handled by thekernel code
Types Of System Calls•Process Control•File Management•Device Management•Information maintenance•Communications