Timerinterrupt

Timerinterrupt, commonly referred to as a timer interrupt, is a hardware interrupt generated by a system timer to signal regular time intervals. It is used by the operating system to keep track of elapsed time, measure timeouts, and support time-based scheduling. Timer interrupts are delivered to the CPU via an interrupt controller and invoke a predefined kernel routine.

The hardware timer counts down or up from a programmed value and, when it reaches zero, raises an interrupt request. The interrupt service routine updates the system clock, advances software timers, and, if needed, triggers the scheduler to switch tasks. The frequency of timer interrupts, known as the timer tick, is configurable. Older systems often used a fixed tick rate (for example, around 1000 Hz), while modern kernels frequently operate in a tickless mode, generating interrupts only when necessary for high‑resolution timing.

Common timer sources include the Programmable Interval Timer (PIT, such as the 8253/8254), the Local APIC timer, and the High Precision Event Timer (HPET). In virtualized environments, paravirtualized timers or virtual APIC timers are used to reduce overhead. Some platforms also rely on a power management timer (PM timer) or other architectural timers as a low‑resolution source.

Software components associated with timerinterrupt include the kernel’s timekeeping and scheduling code. Time is maintained through a monotonic clock, with timers and timeouts queued for expiration. In tick-based kernels, timerinterrupts occur at each tick; in tickless kernels, the system programs the timer to fire only when the next timeout is due, improving efficiency and reducing power consumption.