System Sorter

The layer you never see

Without an operating system a computer is a box that runs one program you have loaded by hand. The OS is what turns it into something a person can use. It sits between you and the hardware and does **five jobs** at once: giving you something to interact with, sharing out memory and processor time, talking to devices, keeping users apart, and organising files. The exam will ask you to describe those roles. Naming all five is worth about one mark. Saying what each one actually DOES, with an example, is worth the rest, and that is what this module drills.

Five roles, five examples

Learn each one attached to something you have seen happen.

Match each role to what you notice

Which role is at work?

A school computer stops one pupil from opening another pupil's coursework folder. Which OS role is doing that, and how?

• User management, by giving each account its own permissions so files belong to the person who made them
• File management, because it is about files
• Memory management, because it controls access
• The user interface, because the pupil clicked on it

Where virtual memory actually happens

Topic 1.2.1 introduced **virtual memory**: when RAM fills up, part of secondary storage is used as though it were RAM. What that topic did not say is WHO does it. **The operating system does.** Memory management is the OS deciding which program gets which part of RAM, stopping one program from writing over another's data, and moving data out to virtual memory and back again when RAM runs short. **Multitasking** is the same role seen from the other side. The processor works on one thing at a time, and the OS switches between programs so quickly that they all appear to run at once. The appearance of several programs running together is a scheduling decision, not a hardware fact.

True about multitasking

Select the TWO statements that are true.

• The operating system switches the processor between programs quickly enough that they appear to run at the same time
• The OS also keeps each program's memory separate, so one cannot overwrite another's data
• A single processor genuinely runs several programs at exactly the same instant
• Multitasking makes each individual program run faster than it would alone

The five roles in a paragraph

The operating system gives you something to interact with through the _____, and shares out RAM and processor time through _____. When a new printer is plugged in it loads a _____. Keeping separate accounts and permissions apart is the job of _____, while organising folders and tracking where data sits on the disk is _____.

Two answers to the same question

**"Describe three roles of an operating system. (6 marks)"** **A weak answer:** "The operating system does user interface, memory management and file management." Three roles named, and that is all. \"Describe\" has not been attempted, so it is worth about half of one of the six marks per role. **A full answer:** "The user interface gives the user a way to work the machine, such as the windows and icons you click rather than typed hardware instructions. Memory management shares RAM and processor time between programs, which is why a browser and a game can both stay open. File management organises storage into folders and keeps track of where each file physically sits on the disk." Same three roles. Each one has what it does and something concrete attached, and that is the whole difference between the two answers.

Explain what a driver is

Build an accurate one-sentence explanation of a device driver.

Something is not working

Three complaints arrive from a school IT room. Work out which operating system role each one points at.

• A pupil says everything slows to a crawl once they open a fourth application, though nothing crashes.
• A newly delivered scanner is plugged in and nothing happens at all.
• A teacher can see a folder exists but cannot open it, and gets a message saying access is denied.

Your turn: describe the system

Explain what an operating system does, for someone who has only ever used a computer without thinking about it.

• Describe three of the five roles, each with a concrete example of it working
• Explain what multitasking really is, and why "at the same time" is not quite accurate
• Explain the difference between file management and user management
• Say what happens, and which role is responsible, when RAM runs out