Computer Operating Systems
An operating system (OS) is the software that manages all hardware and software resources on a computer. It acts as the middleman between user-facing applications and the physical hardware beneath them. Without an OS, no app — not even a calculator — could run.
Every computing device you use runs an OS. Windows runs on most PCs. macOS runs on Apple computers. Android runs on most smartphones. iOS runs on iPhones. Linux runs on servers, supercomputers, and many embedded devices.
What the OS Actually Does
┌─────────────────────────────────────┐
│ YOU (the User) │
└──────────────┬──────────────────────┘
│ interacts with
┌──────────────▼──────────────────────┐
│ Applications │
│ (Browser, Word Processor, Games) │
└──────────────┬──────────────────────┘
│ requests services from
┌──────────────▼──────────────────────┐
│ Operating System │
│ (manages resources, safety, I/O) │
└──────────────┬──────────────────────┘
│ controls
┌──────────────▼──────────────────────┐
│ Hardware │
│ (CPU, RAM, Disk, Screen, Network) │
└─────────────────────────────────────┘
Core Functions of an Operating System
1. Process Management
When you open an app, the OS creates a process — a running instance of that program. Multiple processes run at the same time (your browser, music player, and antivirus all run simultaneously). The OS schedules which process uses the CPU at each moment, switching between them so fast that everything appears to run at once.
This technique is called multitasking. On a single-core CPU, true simultaneity is impossible — the OS rapidly switches the CPU between tasks. On a multi-core CPU, true parallelism is possible — each core handles separate processes at the same time.
2. Memory Management
The OS decides which program gets how much RAM and where in RAM it lives. It protects each program's memory space from being overwritten by another program. When you close an app, the OS reclaims that memory for other uses.
3. File System Management
The OS organises data on storage devices using a file system. A file system determines how data is stored, named, found, and deleted on a hard drive or SSD. Common file systems include NTFS (Windows), APFS (macOS), and ext4 (Linux).
4. Device Management
Every hardware component — keyboard, mouse, printer, webcam — communicates with the OS through a device driver. The OS provides a standard interface so applications do not need to know the technical details of each piece of hardware.
5. Security and Access Control
The OS enforces who can access what. User accounts, passwords, file permissions, and process isolation all prevent unauthorized access and stop one program from harming another.
6. User Interface
The OS provides either a Graphical User Interface (GUI) or a Command Line Interface (CLI) for users to interact with the system.
GUI vs CLI
┌──────────────────┬──────────────────────────────┐ │ GUI │ CLI │ ├──────────────────┼──────────────────────────────┤ │ Icons, windows, │ Text commands typed by user │ │ mouse clicks │ │ │ Easy to learn │ Steeper learning curve │ │ Less efficient │ Faster for experts │ │ for repetition │ Powerful for automation │ │ Windows, macOS │ Linux terminal, Command │ │ desktop │ Prompt, PowerShell │ └──────────────────┴──────────────────────────────┘
Most modern OS versions offer both. A Windows user can open a normal desktop (GUI) or open PowerShell (CLI). A Linux server administrator typically works almost entirely in the CLI for speed and control.
Major Operating Systems
Windows
Developed by Microsoft, Windows is the most widely used desktop OS in the world. It runs on computers from many different manufacturers. Windows 11 is the current major version. It is popular in homes, schools, businesses, and government offices.
macOS
Developed by Apple, macOS runs exclusively on Apple's Mac computers. It is known for its polished design, stability, and tight integration with Apple's other products and services. macOS is popular among creative professionals.
Linux
Linux is an open-source OS kernel developed by Linus Torvalds in 1991. Thousands of distributions (versions) of Linux exist — Ubuntu, Fedora, Debian, Kali, and CentOS are popular examples. Linux dominates web servers, cloud infrastructure, supercomputers, and is the foundation of Android.
Android
Android is a mobile OS developed by Google, built on the Linux kernel. It runs on billions of smartphones and tablets from many manufacturers. Its open nature lets manufacturers customise it for their devices.
iOS
iOS is Apple's mobile OS, running exclusively on iPhones. It is known for tight security, a curated app ecosystem, and smooth performance. iPadOS is the closely related variant for iPads.
The Boot Process
When you press the power button, a precise sequence of events runs before you see the desktop:
- Power On: Electricity flows to all components.
- BIOS/UEFI: The firmware stored in ROM runs first. It checks that all hardware is present and working (this check is called POST — Power-On Self-Test).
- Boot Loader: BIOS/UEFI finds and launches the boot loader — a small program that knows how to load the OS.
- OS Kernel Loads: The boot loader copies the OS kernel (the core of the OS) into RAM.
- OS Initialisation: The kernel starts all system services, loads drivers, and prepares the user interface.
- Login Screen: You are prompted to sign in.
Kernel: The Core of the OS
The kernel is the lowest-level part of the OS that communicates directly with hardware. Every file read, network packet, and screen update passes through the kernel. Applications cannot touch hardware directly — they ask the kernel, and the kernel handles it.
This design keeps the system stable. A faulty application cannot crash the hardware because the kernel acts as a gatekeeper.
OS Updates and Why They Matter
Operating systems receive regular updates for three reasons: new features, bug fixes, and security patches. Cybercriminals actively search for vulnerabilities in old OS versions. When a vulnerability is found, the OS developer releases a patch. Running an outdated OS leaves known security holes open — a serious risk for any connected device.