Process States

A process doesn't simply run from start to finish — it moves through a fixed set of states as the operating system juggles it against every other process. The OS records the current state, plus everything needed to resume, in the process's PCB (Process Control Block). On a single CPU core, only one process is actually Running at any instant; the rest are waiting their turn or blocked on slow work.

The five states

1. 1New — the process is being created and is not yet admitted to memory.
2. 2Ready — loaded and able to run, sitting in the ready queue, waiting only for the CPU.
3. 3Running — currently executing on the CPU. Only one process per core is here at a time.
4. 4Waiting (Blocked) — paused for an event such as a disk read or user input; it can't use the CPU until that event completes.
5. 5Terminated — finished or killed; its resources are being reclaimed.

The transitions

1. 1Admit: New -> Ready, once the long-term scheduler lets it into memory.
2. 2Dispatch: Ready -> Running, when the short-term scheduler, via the dispatcher, hands it the CPU.
3. 3Preempt / timeout: Running -> Ready, when a timer interrupt ends its time slice — it rejoins the queue, it is *not* blocked.
4. 4Block: Running -> Waiting, when it requests I/O or waits for an event.
5. 5Wake: Waiting -> Ready, when the awaited I/O or event completes — note it returns to *Ready*, not straight to Running.
6. 6Exit: Running -> Terminated, when it finishes or is killed.