Operating Systems

What Is an Operating System?

Every time you open an app, save a file, or plug in a USB drive, something is quietly making that possible without you ever thinking about it. That something is the operating system (OS) — the system software that manages a computer's hardware and provides the platform on which every other program runs. Windows, macOS, Linux, Android, and iOS are all operating systems.

Without an OS, application software would need to talk directly to the CPU, memory chips, and disk controller — a different, painstaking job for every single model of hardware. The OS hides that complexity behind a consistent set of services, so a word processor can simply ask to "open this file" or "print this page" without knowing anything about the specific printer or drive attached.

The Core Jobs of an Operating System

An OS juggles several responsibilities at once, all listed under a handful of "managers":

Process management. A process is a program currently running. The OS decides which process gets the CPU and for how long, using a scheduler. On a single-core CPU, only one instruction can execute at any instant — but the scheduler switches between processes so rapidly (many times per second) that several programs appear to run at once. This illusion is called multitasking.

Memory management. RAM is a limited, shared resource. The memory manager allocates a block of RAM to each process when it starts, keeps processes from overwriting each other's memory, and reclaims memory when a program closes. When physical RAM runs out, the OS can use virtual memory — treating part of the storage drive as an overflow area (a page file or swap space) — to keep more processes running than would otherwise fit, though accessing that overflow area is much slower than real RAM.

File management. The OS organises data into files and folders through a file system, tracking where each file lives on the storage device, its name, size, and permissions, and making that structure available to every application.

Device management. Printers, keyboards, webcams, and network cards all speak different technical languages. The OS uses small pieces of software called drivers to translate between each specific device and a standard set of instructions, so applications never need to know which brand of printer is attached.

User interface. The OS also provides the way a person interacts with the machine — a graphical desktop with windows and icons, or a text-based command line — and handles logins, passwords, and permissions to keep accounts separate and secure.

Kernel and Shell

The heart of an operating system is the kernel — the core component that directly manages the CPU, memory, and hardware, running with the highest level of privilege. Surrounding the kernel is the shell, the interface (graphical or text-based) through which users and applications make requests of the kernel. You never touch the kernel directly; every click or command passes through the shell first.

Interrupts

Hardware and software constantly need the CPU's attention at unpredictable moments — a key is pressed, a network packet arrives, a disk finishes reading. Instead of the CPU constantly checking every device in a loop (wasting huge amounts of time), devices send a signal called an interrupt. The CPU pauses its current work, lets the OS handle the interrupt (for example, reading the key that was pressed), and then resumes exactly where it left off. Interrupts are what let a computer feel instantly responsive despite doing one thing at a time on each core.

Types of Operating System

Not every OS is built for the same job:

• Single-user, single-tasking — runs one program for one user at a time (common on very simple embedded devices).
• Single-user, multitasking — the everyday case: Windows, macOS, and Linux desktops run many programs for one user.
• Multi-user — allows several people to use the same machine (often remotely) at once, common on servers.
• Real-time operating system (RTOS) — guarantees a response within a strict, predictable time limit, essential for things like car airbags, medical devices, and industrial control systems, where "usually fast" is not good enough.
• Mobile OS — Android and iOS are optimised for touch input, battery life, and running many apps in a constrained memory footprint.

Worked Example — Why a Frozen App Does Not Freeze the Whole Computer

You have ever had one app "hang" while everything else on your device keeps working. This is process management in action: the OS gives every process its own protected slice of memory and its own scheduled CPU time. When one process gets stuck in a loop, the scheduler simply keeps handing CPU time to the other processes as normal, and the OS can detect the unresponsive one and offer to close it — without needing to restart the entire machine. This isolation between processes is one of the most important reasons modern operating systems are so much more stable than early systems that let one program take over the whole machine.

Key Words

• Operating system (OS) — system software that manages hardware and provides a platform for applications
• Kernel — the core of the OS that directly controls the CPU, memory, and hardware
• Shell — the interface through which users and programs interact with the kernel
• Process — a program currently running on the computer
• Scheduler — the part of the OS that decides which process runs on the CPU and when
• Multitasking — rapidly switching the CPU between processes so they appear to run simultaneously
• Virtual memory — using storage space as overflow RAM when physical memory is full
• Driver — software that lets the OS communicate with a specific hardware device
• Interrupt — a signal that pauses the CPU's current task to handle an urgent event
• Real-time operating system (RTOS) — an OS that guarantees responses within a strict time limit