Hardware Clocks and Timers
Architectures provide two hardware devices to help with time keeping: the system timer, which we have been discussing, and the real-time clock. The actual behavior and implementation of these devices varies between different machines, but the general purpose and design is about the same for each.
The real-time clock (RTC) provides a nonvolatile device for storing the system time. The RTC continues to keep track of time even when the system is off by way of a small battery typically included on the system board. On the PC architecture, the RTC and the CMOS are integrated and a single battery keeps the RTC running and the BIOS settings preserved.
On boot, the kernel reads the RTC and uses it to initialize the wall time, which is stored in the xtime variable. The kernel does not typically read the value again; however, some supported architectures, such as x86, periodically save the current wall time back to the RTC. Nonetheless, the real time clock's primary importance is only during boot, when the xtime variable is initialized.
The system timer serves a much more important (and frequent) role in the kernel's timekeeping. The idea behind the system timer, regardless of architecture, is the sameto provide a mechanism for driving an interrupt at a periodic rate. Some architectures implement this via an electronic clock that oscillates at a programmable frequency. Other systems provide a decrementer: A counter is set to some initial value and decrements at a fixed rate until the counter reaches zero. When the counter reaches zero, an interrupt is triggered. In any case, the effect is the same.
On x86, the primary system timer is the programmable interrupt timer (PIT). The PIT exists on all PC machines and has been driving interrupts since the days of DOS. The kernel programs the PIT on boot to drive the system timer interrupt (interrupt zero) at HZ frequency. It is a simple device with limited functionality, but it gets the job done. Other x86 time sources include the local APIC timer and the processor's time stamp counter (TSC).