Why a computer needs an operating system

Imagine a phone with no operating system. Every app would have to know exactly which memory chips are free, how to draw on the screen, and how to talk to the speaker.

Worse, two apps would claim the same memory and crash each other.

The operating system (OS) is the software that sits between the user, the application programs and the hardware. Windows, macOS, Linux, Android and iOS are all operating systems.

Four reasons, each worth a mark:

• It hides the hardware. A program asks to "write a file"; the OS works out which disk and which blocks. Programs do not need to know the details.
• It provides a user interface. A GUI with windows and icons, or a command line, so a human can actually use the machine.
• It manages resources. Memory, processor time, files, printers and other peripherals are shared out fairly and safely.
• It lets several programs run together. Without a manager, two programs would overwrite each other's memory or grab the same device.

Think of a restaurant. Customers (users) order from a menu (the interface). The kitchen (hardware) does the work. The manager (the OS) takes orders, decides which chef cooks what, keeps the storeroom keyed and makes sure two waiters do not deliver to the same table.

Where it stops: a restaurant manager can be replaced mid-service with chaos; an OS is designed so that programs keep running while new ones start.