Computer Science › Computer Architecture

Interrupt

A signal that makes the CPU pause and handle an event.

Also known as: interrupt, interrupt handler, hardware interrupt

An interrupt is a signal that tells the CPU to stop what it’s doing and handle an event now. Instead of the CPU endlessly asking “has the disk finished? has a key been pressed?”, devices and timers can raise their hand and let the processor work until it’s needed. Handling it means jumping to an interrupt handler (a small routine in the OS), doing the minimum, and returning to whatever was running.

Interrupts come in a few flavours:

• Hardware interrupts — a device (network card, disk, timer) signals the CPU.
• Software interrupts / traps — the running program raises one deliberately, which is how a system call enters the kernel.
• Exceptions/faults — the CPU itself detects something (a page fault, a divide by zero).

running code ──interrupt──▶ save context → run handler → restore context → resume

The pattern — pause, save state, handle, resume — is the same machinery as a context switch, and the boundary crossed is the user/kernel one. Interrupts are also how the OS gets control back from a runaway program: a periodic timer interrupt lets the scheduler preempt it.

The classic mistakes:

• Thinking the CPU polls everything. Polling wastes cycles; interrupts exist to avoid it. (High-throughput networking may deliberately switch back to polling in user space — epoll busy loops — to cut interrupt overhead, a flag that the trade-off runs both ways.)
• Doing too much in a handler. Handlers run in a privileged, delicate context; long work there delays everything. Real handlers defer heavy work to be done later.
• Assuming interrupts are free. Each one has overhead: saving state, cache effects, the handler itself. A flood of interrupts (say, one per packet) can dominate CPU time, which is why interfaces batch them.
• Forgetting the privilege angle. Interrupts are how control transfers into the kernel; they’re part of why user code can’t do everything directly.
• Confusing them with signals. An OS signal is a process-level notification; an interrupt is a CPU-level one. Related in spirit, different in layer.

Interrupts are a foundational OS mechanism: they make the system reactive rather than busy-waiting, and they’re the mechanism that hands the CPU control from programs to the kernel. See how a CPU executes for where they fit in the cycle.