The Operating System: Functions, Types and User Interfaces

Hello students, welcome to Kwickprep. You play music, chat on WhatsApp and download a file, all at the same time. Who makes sure these apps do not fight over the phone? Today we will learn what an operating system does, its types, CLI versus GUI, and interrupts.

An operating system, or OS, is the most important system software. It manages all the hardware and software of the computer. It sits as a middle layer between the user, the apps and the hardware, so apps do not need to control hardware directly. It is the first major program loaded from storage into RAM when the computer boots. Windows, macOS and Linux run on computers, while Android and iOS run on phones.

This diagram shows the layers from top to bottom. The user works with application software, like a browser. The application asks the operating system for what it needs, such as saving a file. The operating system controls the hardware to do the actual work. Then the result comes back up to the user.

The OS does four main management jobs, starting with process management. A process is a program that is running right now. The CPU can do only a little at one instant, so the OS decides which process gets the CPU next. It switches between processes so fast that they seem to run together, and this is multitasking. It also starts, pauses and ends processes, like when you force close a frozen app.

The second job is memory management. The OS gives each running process its own part of RAM. It stops one app from reading or damaging another app's memory, which keeps your data safe. When you close an app, the OS frees its memory for others. When RAM runs out, it uses virtual memory on storage.

The third job is file management. The OS organises files into folders, which are also called directories. It lets you create, open, copy, move, rename and delete files. It controls permissions, so only allowed users can read or change a file. It also keeps an index of exactly where each file is stored on the disk.

The fourth job is device management. The OS controls input and output devices like the keyboard, screen and printer. It uses device drivers to talk to each device in its own language. When several documents are sent to one printer, the OS lines them up in a queue. When you plug in a pen drive, the OS detects it and makes it ready, which is called plug and play.

The OS also does a few more jobs. It provides security through passwords, user accounts and updates. It gives a user interface so you can give commands. And it detects errors and reports them, such as a disk that is full.

Operating systems come in several types. A single user OS allows one user at a time, though it can still multitask, like Windows 11 on a laptop. A multi user OS lets many users share one computer at the same time from different terminals, like a Linux server for a bank. A real time OS must respond within a fixed, tiny time, as in car anti lock brakes, a pacemaker, or a missile system. A mobile OS is designed for touch, battery saving and apps, like Android and iOS. Some courses also mention a batch OS, which runs similar jobs together in groups without user interaction.

Let us pause and predict. A car airbag must open within a few milliseconds of a crash. A laptop may take a second to open an app. Which one needs a real time OS? The airbag, because even a small delay could cost a life.

Every OS gives a user interface, and the two main kinds are CLI and GUI. In a command line interface, you type commands, while in a graphical user interface you click icons, menus and windows, or touch them. A GUI is easy for beginners, but a CLI needs you to remember commands. A CLI uses very little memory and CPU, while a GUI needs more. For experts, a CLI is faster and can automate many tasks at once. Windows Command Prompt and the Linux terminal are CLIs, while the Windows desktop and Android home screen are GUIs.

Here is a small taste of a command line on Windows. The command mkdir project makes a new folder named project. The command dir lists what is inside the current folder. In a GUI, you would right click and choose New Folder instead.

Some courses, including Cambridge IGCSE, also teach interrupts. An interrupt is a signal sent to the CPU to say that something needs attention now. A hardware interrupt comes from a device, like a key being pressed or a printer running out of paper. A software interrupt comes from a program, like an attempt to divide by zero. The CPU pauses its current task, runs a small program to handle it, and then carries on.

Here is how the CPU handles an interrupt, step by step. At the end of each instruction, the CPU checks whether any interrupt is waiting. If nothing is waiting, it simply keeps running its task. If one is waiting, it first saves the state of the current task. Then it runs the interrupt service routine, or ISR, which is the handler for that interrupt. Next it restores the saved state. Finally, it resumes the task exactly where it stopped.

Let us revise what we learned today. The OS is the manager between the user, the apps and the hardware. It manages processes, memory, files and devices. Its types include single user, multi user, real time and mobile. A CLI uses typed commands, while a GUI uses icons and touch. An interrupt is a signal that makes the CPU pause and handle something urgent. Open Task Manager on a computer and see process management live.