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.
• The OS spools print jobs into a queue, so you can keep working while it prints.
• Backing store: the OS tracks where each file is stored and organises files into folders.

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.
• 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: anti-virus, backup, compression, disk defragmenters and many more on your list.

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

• Backup software copies files to another device or the cloud, so they can be restored if lost or damaged.
• Compression software makes files smaller, so they use less storage space and transfer faster.
• An archiver bundles files into one compressed file; disk compression compresses a whole drive.
• Encryption software scrambles data so it can't be read without the key, e.g. on a lost laptop.
• Encryption tools are cryptographic utilities.

Protecting and repairing files

• Anti-virus software scans files and memory for malware, comparing them with known malware signatures and watching for suspicious behaviour. It quarantines or deletes what it finds.
• Data recovery software gets back files that were deleted or are on a damaged disk.

More utilities on your list

You tap a key mid-game. How does the CPU notice straight away?

A signal that asks for attention

• An interrupt is a signal to the processor that something needs attention.
• Hardware interrupts come from devices, e.g. pressing a key, moving the mouse, or a printer running out of paper.
• Software interrupts come from programs, e.g. a division by zero, or two processes trying to access the same memory location.
• With interrupts, the CPU doesn't waste time repeatedly checking each device.

Handling an interrupt

• The CPU checks for interrupts at the end of each fetch–decode–execute cycle.
• If one is waiting with a higher priority than the current task, the CPU saves its current state: the contents of its registers.
• It runs the interrupt service routine (ISR): code that deals with that interrupt, e.g. reading which key was pressed.
• Then it restores the saved registers and carries on exactly where it stopped.

Priorities and results

• The operating system manages interrupts: it gives each type a priority and runs the right ISR.
• Higher-priority interrupts are handled first; others wait their turn. A power failure comes before a mouse movement.
• The result depends on the interrupt: a letter appears on screen, a "paper out" message is shown, or a program that divided by zero stops with an error.

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 backing store, the interface 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.
• Backup, compression, encryption and anti-malware tools each protect or shrink your data in a different way.
• An interrupt is a signal to the CPU that something needs attention; it can come from hardware or software.
• The CPU checks for interrupts after each fetch–decode–execute cycle, saves its registers, runs the interrupt service routine, then restores its registers and carries on.
• The operating system manages interrupts by priority, so urgent ones are handled first.