Interrupts

This article covers notes on interrupts — events that break the normal pipeline of instructions fed to the CPU.

Say we're on our laptop and want to read a news article on some webpage. The webpage suddenly displays a popup, telling us to disable AdBlock. Annoyed, we click the back button. To do so, we might shift our mouse (or touch our trackpad), to move the cursor. How does the computer know to move the cursor to that position?

The answer is, it doesn't. At least, not on its own. Instead, when we move the cursor with some peripheral (e.g., a mouse, trackpad, or keyboad), the peripheral's controller or adapter sends a sequence of interrupts to the CPU.

We barely notice these interruptions because of how fast the CPU is. Human reaction time is measured in milliseconds — the fastest possible conscious reaction times range from 150 to 300 milliseconds. The CPU, however, operates at the GHz level — we're talking nanoseconds. In 1 millisecond, a tiny fraction of even the fastest reaction times, the CPU has already gone through a million instructions, if not more.

This is even more remarkable when we realize just how much we expect the computer to do: We type an email on a browser window while listening to music on some app or viewing a PDF with some reader. This is all part of the computer's exception control flow.