🖥️ How Does Your Computer Decide What to Do First?

OS Process Scheduling Simulator

Your computer runs lots of programs at once — but the CPU can only do one thing at a time. How does it decide what goes first?

CS GCSE §1.7 CS A-Level Unit 4

🤔 What is process scheduling?

A process is just a program that's running — Spotify, Chrome, a game. Your CPU can only work on one process at a time, switching between them so fast it looks like they're all running simultaneously.

A scheduling algorithm is the rule the operating system uses to decide which process gets the CPU next.

🍕 Real-world analogy: Imagine a pizza shop with one oven. Five orders come in at different times for different sizes of pizza. How do you decide which one to cook first?

First Come First Served — cook them in the order they arrived
Shortest Job First — cook the smallest pizza first (quickest to finish)
Round Robin — give each pizza 2 minutes in the oven, then rotate
Priority — VIP customers go first

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Choose a scenario

Pick a real-world situation to see scheduling in action, or build your own below.

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Your Phone

Spotify, Instagram, WhatsApp and a game all competing for your phone's CPU

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School Printer Queue

5 students send print jobs — some big, some small. Who prints first?

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Gaming PC

Fortnite needs lots of CPU, while Discord and Chrome run in the background

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Build Your Own

Create your own processes and experiment

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Setup — choose the scheduling rule

Algorithm:

FCFS = processes run in the order they arrive. Just like a queue in a shop — first person in line gets served first. Simple and fair, but if one process takes ages, everyone behind it has to wait (this is called the convoy effect).

or use a scenario above

	Process	Arrival Time	Burst Time	Priority

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Results — What happened

Gantt Chart (a timeline showing which process used the CPU and when)

📝 Key terms for your exam:

Turnaround Time = Finish Time − Arrival Time (how long from submitting the job to getting the result)
Waiting Time = Turnaround Time − Burst Time (how long the process sat doing nothing, waiting for the CPU)
Gantt Chart = a bar chart showing which process had the CPU at each point in time

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Scheduling algorithms explained

Algorithm	How it works	Real-world example	Problem
FCFS	Run processes in arrival order	Queue at a shop	Convoy effect — slow job blocks everyone
SJF	Shortest job runs first	"10 items or fewer" express checkout	Starvation — long jobs may never run
Round Robin	Each process gets a fixed time slice, then goes to the back of the queue	Taking turns on a games console	Context switching overhead
Priority	Most important process runs first	A&E triage — most urgent patient first	Starvation — low priority may wait forever

Process states:

🆕 New — just created ⏳ Ready — waiting for CPU 🏃 Running — using CPU now ⏸️ Waiting — needs I/O ✅ Terminated — done

🧠 What next? Try running the same processes with different algorithms. Which one gives the lowest average waiting time? Is the "best" algorithm always the same?