Context Switch
Medium+12 XPWatch the OS save one process's state and load another's — the swap that makes CPU-sharing possible.
No visualization loaded.
Watch
—
Press Run to begin.
To share one CPU among many processes, the OS keeps swapping which one runs. Each swap is a 'context switch'. Press Run to watch it save Process A and load Process B so each can resume exactly where it left off.
▸What's being 'saved' and 'loaded'?
A process's 'context' — the values the CPU was holding for it and a marker of which instruction it was on. Save that into A's storage, load B's back in, and B continues as if it never paused. It's like saving and loading a video game.
▸Why does it have to save A before running B?
Because A and B share the same CPU. If the OS just let B start without saving A's stuff, A's work would be lost — it wouldn't know where it was when its turn comes again. Saving first means A can be resumed perfectly later.
▸Is a context switch 'free'?
No — and that's the catch. During the save-and-load, NO real work gets done; it's pure overhead. So switching too often wastes the CPU on bookkeeping. The OS balances switching often enough to feel responsive, but not so often that it's all overhead.
▸How often does this happen?
Constantly — many times per second. That's how dozens of programs feel like they're running at once on one CPU: the OS switches between them faster than you can notice.