How it works today: timers

Modern machines still need timer interrupts, but they rarely use the PIT as the main timing source.

Modern timer sources

Common facilities include APIC timers, HPET, invariant TSC and TSC deadline timers, paravirtualized clocks in VMs, scheduler timers, and high-resolution timers. The operating system chooses the best source and hides most of that choice from applications.

What changed

Modern timers are more precise, scale across CPUs, and integrate with power management. APIC and MSI/MSI-X replaced much of the older PIC-style routing. Kernels also coalesce timers and use tickless idle to avoid waking CPUs, so real APIs sometimes differ from a simple "interrupt every N milliseconds" model.

What stayed the same

Time still arrives as events, handlers should be small, state should advance predictably, and tests need deterministic time. lcom-ng keeps the PIT-shaped interface because it is small enough to program directly.

Once you understand it, APIs such as timerfd, browser animation callbacks, SDL timers, and fixed timestep loops are easier to reason about. They answer the same question: when should the program advance?