Timers:

A System needs to keep account of time for various purpose. For e.g. to carry out rescheduling, waiting for some specified amount of time or for knowing the wall time.

The support for managing time is provided by means of two hardware devices: the system timer (responsible for timer interrupts and updates jiffies) and the real-time clock.

RTC: RTC or Real time clock is generally used for providing the wall time. It is on (non-volatile) even when the system is off, through the CMOS battery prsent on the system board. Kernel reads the RTC at booting time to keep track of wall time.

Sytem Timer: System Timer is required to generate interrupts at periodically. Generally provided through via clocks osciallating at programmable frequency. In x86, it is provided through PIT (programmable interrupt timer). Kernel can program the PIT to define the system timer interrupt (0) frequency Hz.