Three things at once

Your phone plays music. A chat message arrives. A game is open too.

One small processor does all of this. Who decides what runs and when? The operating system is the main program that runs the computer. Different jobs need different kinds of it.

Key words

Word	What it means	Example
Operating system	The main program. It runs the computer and shares its parts between other programs.	Windows, Linux, Android
Batch	Jobs are collected and run later, one after another. Nobody waits for the result.	Printing 10000 bills (papers that say how much to pay) at night
Scheduling	Choosing which job runs next, and when.	Run the jobs in a set order
Multitasking	Running many programs at the same time.	Music and chat together
Time-slicing	Giving each program a very short turn on the processor, again and again.	10 milliseconds each (1 millisecond is one thousandth of a second)
Interrupt	A signal that says "stop, I need attention now".	A key press
Real-time	The answer must come within a fixed, short time.	A car airbag
Network	Computers joined together so they can share things.	College Wi-Fi

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Five types at a glance

Type	Main idea	Key features	Example use
Batch	Run big jobs later, with no user	No user waiting, very many jobs, scheduling	Paying wages, printing bills
Multitasking	Many programs seem to run together	Time-slicing, interrupts	A laptop or phone
Real-time	Always answer in time	Monitoring and control, transaction processing (payments and bookings)	Airbags, robots, card payments
Network	Look after a whole network	Resource sharing (many people use one printer or files), user management, communication	A college server
Mobile	Work well on small devices	Small processor, long battery life	Android, iOS

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Batch and multitasking

A batch system has no user waiting. The jobs wait in a line (a queue). Scheduling decides the order. A company pays all its workers at night. Nobody types, and the machine stays busy.

Multitasking is different. You are there, and you want many things now. The processor gives each program a short turn. This is time-slicing. It happens so fast that all the programs seem to run together.

Try it: time-slicing

Code: python

# The processor gives each task one short slice of time, in turn, again and again.
tasks = ["music", "chat", "game"]
for round_number in range(1, 4):
    for name in tasks:
        print("Round", round_number, "->", name, "gets a slice")

# Add a new task, such as "photos", and run it again.

What is an interrupt?

data-callouts.note.label

You press a key while the processor is busy. The keyboard sends an interrupt. The processor pauses, does what the key asks, then goes back to its work. Without interrupts the computer would not notice you.

Extra: Meet a real operating system

Command

uname -sr

A command for the lesson’s lab terminal. It is shown here and not run.

Extra: See many tasks at once

Command

ps -e | head -8

A command for the lesson’s lab terminal. It is shown here and not run.

Extra: Count the tasks

Command

ps -e | wc -l

A command for the lesson’s lab terminal. It is shown here and not run.

Extra: What you saw

data-callouts.note.label

The first command named the operating system, Linux. The second listed tasks that run now. The third counted them. A small server already runs many tasks, and each one gets short turns on the processor.

Real-time operating systems

A real-time operating system must answer inside a fixed time. A late answer is a wrong answer.

It is used for monitoring and control: an airbag, a factory robot, a heart monitor. It is also used for transaction processing: a card payment or a plane seat booking must finish at once and without mistakes. A transaction is one payment or one booking.