Why Your PC Freezes: Interrupts, Infinite Loops, and Kernel Deadlocks

Most computer users have experienced a freeze and wonder what the CPU is doing at that moment. While a hardware‑level CPU halt is theoretically possible, it rarely occurs; most freezes are software‑level when the CPU becomes unable to progress.

Consider a classic infinite loop:

void dead_loop() {
    while (1) {
        ...
    }
}

Running this code does not freeze the machine because modern operating systems use timer interrupts to preempt the running thread, allowing other tasks to execute.

Interrupts

Interrupts are one of the most important inventions in computer history. The OS installs a set of interrupt‑handling routines (e.g., the clock interrupt) that periodically regain control of the CPU, schedule other threads, and ensure the system remains responsive.

When a thread enters an infinite loop, its time slice eventually expires; the scheduler forces the CPU to switch to another runnable thread.

Therefore, a simple infinite loop cannot crash the computer; the OS would simply preempt it.

Even on multi‑core CPUs, a single thread stuck in a loop does not freeze the system because other cores can continue to be scheduled.

However, the CPU can become truly unresponsive in two situations:

Interrupts cannot preempt the current code – Interrupts have priorities. If the kernel disables interrupts or a high‑priority interrupt handler enters a dead loop (e.g., due to a spinlock), lower‑priority interrupts cannot break in, leaving the CPU stuck like a “plant”.

Interrupts are handled but no runnable thread exists – A kernel‑level deadlock (for example, two threads waiting on each other or a lock held while processing an interrupt) can leave the scheduler with no eligible thread, causing the whole system to hang.

The first diagram illustrates how the CPU can be trapped when an interrupt handler fails to release a lock.

Tim Chen’s description (shown below) visualizes a kernel deadlock where the system cannot schedule any thread.

In summary, a computer freeze is usually the result of the OS being unable to regain control of the CPU because of interrupt priority issues or kernel‑level deadlocks, not because the CPU itself has stopped executing instructions.