One CPU core, three apps at once. How is that possible?

The manager between apps and hardware

• The operating system (OS) is system software that manages the hardware and provides a platform for applications to run on.
• Apps don't control the hardware themselves: they ask the OS.
• The OS loads apps into memory, starts them and closes them.

Sharing the processor and memory

• Each running program is a process. One core runs one instruction at a time.
• The OS schedules processes: each gets a short time slice on the CPU, then the OS switches to the next.
• Switching is so fast that programs seem to run at once: multitasking.
• Memory management: the OS gives each program its own area of RAM, keeps programs apart, and frees the area when a program closes.

Devices and files

• The OS manages peripherals (input, output and storage devices) and the data passing between them and the processor.
• A device driver lets the OS communicate with one particular device. It translates OS commands into ones that device understands.
• File management: the OS organises files into folders and lets you name, save, move, copy and delete them.

Interface, users and security

• The user interface lets you control the computer, e.g. a graphical user interface (GUI) with windows, icons, menus and a pointer, or a command-line interface where you type commands.
• User management: the OS creates user accounts, checks logins and gives each account access rights to particular files and programs.
• Security: the OS asks for a password or other login, blocks unauthorised access to files and memory, and installs security updates.

Why could shuffling files around make an old hard drive faster?

Housekeeping software

• Utility software is system software that helps to maintain, protect or configure a computer.
• It carries out housekeeping tasks that the operating system may not do, or may not do well enough, on its own.
• Some utilities come with the OS; others are installed separately.
• Examples: encryption, defragmentation and data compression software.

Fragmented files

• A magnetic hard disk stores files in blocks. The read/write head moves to each block to read it.
• As files are saved and deleted, free space becomes scattered gaps, so new files get split up: they are fragmented.
• Reading a fragmented file needs many slow head movements.
• A disk defragmenter moves each file's pieces next to each other (contiguous) and groups the free space together, so files load faster.

Backup, compression and encryption

• Compression software makes files smaller, so they use less storage space and transfer faster.
• Encryption software scrambles data so it can't be read without the key, e.g. on a lost laptop.

Big ideas

• The OS is system software that manages the hardware and gives applications a platform to run on.
• It shares the processor between processes (multitasking) and shares out memory, and manages peripherals using device drivers.
• It also manages files, the user interface, user accounts and security.
• Utility software is system software that does housekeeping tasks to maintain, protect or configure the computer.
• A defragmenter makes files contiguous on a magnetic hard disk so they load faster; it isn't needed on an SSD.
• Compression shrinks files; encryption keeps them unreadable without the key.