What is an operating system?¶

Exercise OS census PDF

Right now your PC has a browser open, maybe a terminal, maybe music. Your phone has thirty apps. But the CPU has only a few cores — a few chefs. So who decides what each chef cooks this millisecond? Who stops Spotify from writing over your document in RAM?

Nobody in Unit 1 did that job. In The boot process the relay handed the baton to the kernel and we stopped there. This unit lives there. In the next five weeks you will install three operating systems with your own hands. First you need to know what that thing you are installing actually is.

The four elements of a computer system¶

Before we meet the manager, let's look at the whole system from the outside. Any computer system, from a phone to a data centre, is made of the same functional elements:

Through all of these layers flows the fourth element: data — a photo, a saved game, your contact list.

flowchart TB
    U["<b>Users</b><br>you, the teacher"] --> A["<b>Application software</b><br>Firefox, WhatsApp, a game"]
    A -->|"asks for services"| S["<b>System software</b><br>the operating system + drivers"]
    S -->|"the only one that<br>talks to the hardware"| H["<b>Hardware</b><br>CPU, RAM, SSD, keyboard"]
    D(["<b>Data</b><br>flows through every layer"]) -.- A
    D -.- S
    D -.- H

Two items always divide the class:

• A driver is system software. It serves the operating system, not you. You never "open" a printer driver like an app.
• Firmware (Unit 1's card by the door) is also system software — it just lives in a chip on the motherboard instead of on the disk.

What is an operating system?¶

An operating system (OS) is the software that manages the hardware and gives services to all the other programs. It is the only program that talks to the hardware directly. Everybody else has to ask.

Remember the kitchen from Unit 1: the CPU is the chef, RAM is the countertop, storage is the pantry, firmware is the card by the door. Now the kitchen gets its last member.

One chef, many orders: processes and the scheduler¶

Here is the problem the manager solves first. Your PC has a few cores, but hundreds of programs want to run. How?

In class you play it out: one chef (the only one with a pen) and three programs, each a sheet with a job — write the numbers 1 to 40, write the alphabet twice, draw 30 squares.

• Round 1, one after the other. The chef finishes the first sheet, then the second, then the third. The third program waits a long time before it even starts.
• Round 2, with a manager. Every 10 seconds the manager says "switch". The chef drops the sheet and continues the next one exactly where it stopped.

In round 2 the third program starts much sooner. But round 1 finishes everything a little earlier: switching costs time. And the chef knows where to continue because the progress is written on the sheet.

Now the real names:

flowchart LR
    P1["<b>Process A</b><br>music"] --> CORE["<b>One core</b><br>the chef"]
    P2["<b>Process B</b><br>browser"] --> CORE
    P3["<b>Process C</b><br>terminal"] --> CORE
    SCHED["<b>Scheduler</b><br>the manager:<br>'switch!' every few ms"] -->|"saves the state,<br>loads the next one"| CORE

Because each time slice lasts milliseconds, you never notice the switching. It feels simultaneous.

The five jobs of an operating system¶

Processes are only the first job. The manager has five:

You talk to the manager through an interface: windows and a mouse, or a terminal where you type commands. The next page looks at both.

A little history: from waiting lists to Linux¶

In the early 1950s there was no manager at all. A programmer wrote their name on a waiting list, like at a crowded restaurant. When their turn came, they stood at the machine with a deck of punched cards. Between two programs, a computer that cost millions sat doing nothing.

• 1956 — Robert L. Patrick (General Motors Research) and Owen Mock (North American Aviation) write GM-NAA I/O for the IBM 704. When one job ends, it starts the next one by itself. It is often called the first operating system. Its whole job was job number one in our table.
• 1969 — Ken Thompson and Dennis Ritchie at Bell Labs start Unix on a spare PDP-7. Many users, many programs, files in folders: the shape of every OS you use today.
• 25 August 1991 — Linus Torvalds, a 21-year-old student in Helsinki, announces a free kernel: "just a hobby, won't be big and professional". It is now the kernel of the classroom PCs and of every Android phone.

Catch the manager at work¶

Time to see each job with your own eyes. Open a terminal with Ctrl+Alt+T. Commands are case-sensitive: Cat does not exist, cat does. The Exercise sheet has space for every result.

Which OS am I running?¶

cat /etc/os-release
uname -sr

The first command shows lines like NAME="Ubuntu" and VERSION=.... The second prints Linux and a kernel version number. Two different names: Linux is the kernel (the core of the OS); Ubuntu is the whole operating system built around it. More on that on the next page.

Job 1 · Processes¶

ps -e | wc -l

This counts the running processes (one line each, plus one title line). On a desktop you get a few hundred. You did not open that many: the OS started them. Now open the Calculator from the applications menu and look for it:

ps -e | grep -i calc

The first number on the line is its PID (process ID), a number the OS gives every process. Type kill followed by that number (the number, not the word "PID"):

kill 4321

The Calculator window closes without you touching it. The manager took the order off the list.

Job 2 · Memory¶

free -h

Look at the row Mem:. The total column is the RAM installed (a bit less than on the box — the two rulers from Unit 1, plus what the hardware reserves). Open Firefox with five tabs and run it again: available goes down.

Job 3 · Files¶

echo hello > note.txt
ls -l note.txt

The output shows the owner (your user), the size and the date. The size is 6 bytes, not 5: h-e-l-l-o plus the invisible end-of-line character (text as bytes, from Unit 1). You wrote the content; the OS wrote the label.

Job 4 · Devices¶

lsusb

One line per USB device. Unplug your mouse and run it again: one line is gone. Plug it back in: it is back in under a second, because a driver inside the kernel recognised it. To count the kernel modules loaded right now (most of them drivers):

lsmod | wc -l

Job 5 · Users¶

whoami
id
cat /etc/shadow

whoami prints your user name. id adds your number (uid) and your groups. The last one answers Permission denied. That file holds the scrambled passwords of every user, and the manager lets only the administrator read it.

Nobody in the room wrote the code that said no. The file is there, the disk works, the command is correct. The operating system decided you may not. That is job number five.

The OS census: how many systems do you own?¶

Your PC is not the only device with a manager. Make a list of every device you own or use at home — phone, console, TV, watch, tablet, router — and the operating system each one runs. For the phone and the PC, read the exact version from the device:

Every update has a number. Even if your phone "just updates", Settings → About shows which version it is on.

Now group the class's list by kernel. Different brand names often hide the same core:

Count the Linux kernels: the phone in your pocket (if it is Android), the TV at home, the router that gives you Wi-Fi, and the PC in front of you. Most people who say "I have never used Linux" use it every day.

Key takeaways¶

• A computer system has four functional elements: hardware, software (system and application), data and users.
• An operating system manages the hardware and serves the other programs; it is the only one that talks to the hardware directly. It is software: you can replace it.
• A running program is a process. The scheduler gives each one a time slice of a few milliseconds, so one core seems to do many things at once.
• The five jobs of an OS: processes, memory, files, devices (through drivers) and users (accounts and permissions).
• Linux is a kernel; Ubuntu and Android are different systems built around it.
• Permission denied is the OS deciding, not a broken command.

Quick check¶

Next: open the manager's office and see how it is built inside — OS architecture.