The operating system

Step 1 Warm-up recall

From M7.L3: what does LOAD 8 then SUB 3 leave in the accumulator?

Your answer

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Step 2 Learning goal

By the end of this lesson you can:

• list the jobs of an operating system (Foundation);
• explain time sharing with an example (Core);
• explain why updates and permissions matter (Core).

Step 3 A familiar example

A building manager does not cook or teach, but makes sure the rooms, power and doors work for everyone who uses the building. An operating system does this for programs.

Step 4 Clear explanation

An operating system (OS) is the program that manages a computer’s resources and runs other programs. Its main jobs are in the table.

A running program is a process. With one processor, the OS creates the feel of many programs running at once by giving each a short turn in rotation, called time sharing.

In the invented example, three programs take 10 ms turns, so each gets a turn every 30 ms; each seems continuous because the turns are so short. The OS also keeps each program inside its own part of memory, so one faulty program cannot easily overwrite another, and it uses permissions to decide who can read or change a file. Software updates to the OS matter because it sits beneath everything else: a flaw in it can affect every program. For the wider picture of how programs are written to run on an OS, see the entry on the programming language.

Step 5 Worked example (4 marks)

Four programs share one processor with 5 ms turns (invented). How often does each get a turn?

1. One full rotation takes 4 × 5 = 20 ms. (1 mark)
2. Each program runs once per rotation. (1 mark)
3. So each gets a turn every 20 ms. (1 mark)
4. Answer: every 20 ms. (1 mark)

Step 6 Learner practice (instant-marked)

Step 7 Recap

• The OS manages the processor, memory, files and devices.
• Time sharing gives each process short turns.
• Permissions and updates protect the system.

Step 8 Exit ticket (ungraded)

Five programs with 4 ms turns: how often does each run?

Your answer

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Step 9 Go further (Extension)

Look at the list of running programs on a device you use and count the processes.