Step 02

Process Life Cycle & States

A process does not run from start to finish in one go. It moves between five states. Play the animation, then click any state to study it in detail.

The five states

State	Description
New (Start)	The process is actively being created by the OS.
Ready	The process is prepared to run and is waiting for CPU allocation.
Running	The CPU is actively executing the process instructions.
Waiting (Blocked)	The process is paused until a resource becomes available (e.g., waiting for input or file access).
Terminated (Exit)	Execution is completed or stopped, and the process is removed from memory.

The five-state model: a process goes through five different states during its lifetime, and each arrow is labelled with the event that causes that transition.

Ready queue — list of all processes waiting for the CPU

Dispatch — the act of giving the CPU to a chosen ready process

Preemption — forcibly taking the CPU away from a running process

Time quantum — fixed slice of CPU time given in Round Robin

Real-world analogy: The doctor's clinic

Registering at the desk is New; sitting in the waiting hall is Ready; being examined by the doctor is Running; being sent for a blood test is Waiting; walking out with the prescription is Terminated. After the blood test you return to the waiting hall — not directly to the doctor. That is exactly Waiting → Ready.

Experiment: Animated state transition diagram

Speed

Step 1 / 7

P1 enters New

Event responsible: User launches the program (fork/exec system call).

Process P1 is created — the OS builds its PCB.

Events responsible for each transition
Transition	Triggering event	Who causes it
New → Ready	Admitted — memory allocated and PCB built	Long-term scheduler
Ready → Running	Dispatch — CPU allocated to the process	Short-term scheduler / dispatcher
Running → Waiting	I/O request or wait for a resource / event	The process itself (system call)
Waiting → Ready	I/O complete or resource available	Hardware interrupt handled by the OS
Running → Ready	Time quantum expired or higher-priority arrival	Timer interrupt (preemption by OS)
Running → Terminated	exit() called, or killed / fatal error	The process itself or the OS

Running state

Definition: The process's instructions are actually being executed by the CPU.

What happens: Only one process per CPU core can be in this state at a time.

Example: The media player is decoding and playing your song right now.

Typical OS action: Runs it until it finishes, blocks for I/O, or its time quantum expires.

Transition condition: Running → Waiting (I/O request), Running → Ready (time quantum expired / preemption), Running → Terminated (finished).

All valid transitions (and one common trap)

New → Ready — admitted by the long-term scheduler.
Ready → Running — dispatched by the short-term scheduler.
Running → Waiting — the process requests I/O or waits for an event.
Waiting → Ready — the I/O or event has completed.
Running → Ready — preempted (time quantum over or a higher-priority process arrived).
Running → Terminated — execution completed or the process was killed.

Common exam trap: Waiting → Running is not a valid transition. A blocked process must always pass through Ready first, because the CPU is given only by the scheduler.

Quick quiz

1. A running process requests data from a hard disk. Which transition occurs?

2. How many processes can be in the Running state on a single-core CPU?

3. Which transition is IMPOSSIBLE?

Practice problem

Process P executes 4 ms of CPU work, then reads a file (6 ms), then executes 3 ms more and finishes. With a Round Robin quantum of 3 ms, list the sequence of states P passes through.

Next: Process Control Block