Program the PIT for Kernel Preemption

Programmable Interval Timer

The Intel 8254 PIT channel 0 fires IRQ0 at a programmable rate via divisor latch. Kernels use it for time slices, jiffies, and sleep accounting.

Divisor Math

Output frequency = 1193182 / divisor

• Divisor 1193 ≈ 1 kHz (1 ms ticks)
• Write mode 3 square wave to channel 0

For example, divisor 119318 yields roughly 10 Hz timer interrupts for coarse scheduling demos.

Why This Matters in Production

Programming the 8253/8254 PIT at 100Hz is literally how the Linux kernel's early boot timer worked before the APIC timer and HPET took over, and it remains the simplest way any teaching kernel like xv6 implements preemptive multitasking. Get the frequency divisor calculation wrong and your entire scheduler's sense of elapsed time drifts, silently breaking any timeout logic built on top of it.

Debugging Checklist

Before you call the implementation done, walk failure modes on purpose. Test empty structures, single-element edge cases, maximum concurrency, and errno paths that must not crash the program. OS code usually fails in production when happy-path tests pass but invariants break under contention or memory pressure.

Implementation Notes

Keep structures small and name fields after kernel counterparts when possible. That lets you read man pages and kernel source side by side while you work. Print observable events during development; remove noisy logs once tests pass reliably.

For This Exercise

You will program PIT registers and increment a tick counter in the IRQ0 handler. This exercise requires converting ticks to milliseconds in a kernel_sleep helper.