When you open a web browser, play a video, save a document, or run a game, many parts of the computer must work together.
The processor executes instructions. Memory holds active data. Storage keeps files. The keyboard and mouse provide input. The display shows the result. Network hardware exchanges data with other computers.
But an application normally does not control all this hardware directly.
Between the applications and the hardware is one of the most important pieces of software in any computer: the operating system.
Windows, Linux, macOS, Android, and iOS are all operating systems. They look different and serve different kinds of devices, but they perform many of the same fundamental jobs.
An operating system manages hardware resources, provides common services to programs, and creates an organized environment in which applications can run.
To understand modern computers, we need to understand what the operating system actually does.
1. The Operating System Is the Main System Software
A computer contains hardware, but hardware alone is not very useful.
A processor can execute machine instructions, but it does not automatically know how to open a document, display a window, connect to Wi-Fi, or keep several applications running safely.
Those abilities require software.
The operating system is the main layer of system software that coordinates the computer. It starts when the machine boots, remains active while the machine is running, and manages the resources that applications need.
At a simplified level, a computer can be viewed as three layers:
Applications → Operating System → Hardware
Applications include browsers, editors, games, media players, and development tools. Hardware includes the CPU, memory, storage devices, displays, keyboards, network adapters, and many other components.
The operating system sits between these two worlds.
2. Why Applications Do Not Control Hardware Directly
Imagine that every application had to control every model of disk, keyboard, display, and network adapter by itself.
Each developer would need to understand thousands of hardware devices. Programs could interfere with one another, overwrite important data, or take complete control of the processor. A mistake in one application could easily crash the entire machine.
The operating system provides a controlled interface instead.
An application asks the operating system to perform an operation, such as reading a file, allocating memory, displaying graphics, or sending network data. The operating system checks the request, coordinates the required hardware, and returns the result.
This design makes applications easier to write and computers safer to operate.
3. The Kernel Is the Core of the Operating System
At the center of an operating system is the kernel.
The kernel is the privileged part of the system that manages critical resources. It controls access to the CPU, memory, and hardware devices. It also enforces boundaries between running programs.
The kernel is not the entire operating system.
An operating system may also include command-line tools, graphical interfaces, system libraries, background services, installers, and administrative utilities. The kernel is the core that these components depend on.
In Linux, the Linux kernel is combined with many other tools to form a complete operating system. In Windows and macOS, the kernel is also surrounded by large collections of system services and user-interface components.
We will examine the kernel more closely in the next tutorial.
4. The OS Manages the CPU
A computer may appear to run many programs at once. You can listen to music, download a file, edit a document, and receive notifications at the same time.
However, each CPU core can execute only one instruction stream at a particular instant.
The operating system decides which program receives CPU time. It allows one program to run for a short period, then may pause it and give another program a turn. This decision-making process is called scheduling.
Fast switching creates the appearance that many programs are running simultaneously. On a multicore processor, several instruction streams can truly execute at the same time, but the operating system must still organize them.
The OS also prevents one ordinary application from simply taking permanent control of the processor.
5. The OS Manages Memory
Programs need memory for their instructions, variables, stacks, and dynamically allocated data.
The operating system keeps track of which regions of memory are being used and which are available. When a program starts, the OS gives it an address space. When the program needs more memory, the OS may allocate additional pages. When the program ends, those resources can be reclaimed.
Modern operating systems use virtual memory to give each process its own organized view of memory. This helps isolate programs from one another and lets the OS manage physical RAM more flexibly.
If one application tries to access memory that does not belong to it, the hardware and operating system can block the access. This protection is one reason a defective application usually crashes by itself instead of immediately destroying the entire system.
6. The OS Manages Programs as Processes
A program stored on disk is a passive file containing instructions and data. When that program is loaded and begins to run, the operating system represents it as a process.
A process includes more than program code. It has an address space, CPU state, open files, security information, and other resources maintained by the operating system.
The OS creates processes, pauses them, resumes them, and eventually removes them. It also provides ways for processes to communicate while maintaining controlled boundaries between them.
One application may contain multiple threads of execution. Processes and threads are related, but they are not identical. We will cover both in later tutorials.
7. The OS Organizes Files and Storage
Storage devices contain blocks of data. People and applications, however, usually work with named files and folders.
The operating system uses a file system to organize stored information. It keeps track of filenames, directories, sizes, locations, permissions, and other metadata.
When an application opens a file, it does not normally search raw disk sectors by itself. It asks the operating system for access to a path. The OS checks permissions, finds the data through the file system, and coordinates the storage device.
Different operating systems support different file systems, such as NTFS, APFS, ext4, and many others. Although their internal designs differ, they all help turn raw storage into a usable structure.
8. Device Drivers Connect the OS to Hardware
Computers can use many kinds of hardware, including graphics processors, sound devices, printers, cameras, storage controllers, and network adapters.
The operating system communicates with these devices through specialized software called device drivers.
A driver understands the registers, commands, interrupts, and data formats of a particular device or family of devices. It translates general operating-system requests into operations that the hardware understands.
For example, an application may ask to send data over a network connection. The networking system inside the OS processes that request, and a driver communicates with the network adapter.
This separation means an application usually does not need separate hardware-control code for every network card on the market.
9. The OS Provides Common Services to Applications
Applications repeatedly need many of the same capabilities: opening files, allocating memory, creating processes, communicating over networks, reading input, and displaying output.
The operating system provides standard ways to request these services.
At the lowest software boundary, applications request kernel services through system calls. Most application developers do not issue raw system calls directly. They often use programming-language libraries or operating-system APIs, which eventually reach the kernel when privileged work is required.
For example, a C program may call a library function to open a file. That library function ultimately asks the operating system to perform the operation.
System calls will be an important topic later in this series.
10. The OS Protects the Computer
Resource management is also a security problem.
The operating system controls which users and programs may access files, memory, devices, and network services. It can assign permissions, isolate processes, authenticate users, and restrict sensitive operations.
Modern processors support at least two broad privilege levels: a restricted level for ordinary applications and a privileged level for the kernel. These are commonly described as user mode and kernel mode.
An ordinary application cannot freely execute privileged instructions or directly modify critical kernel memory. It must request services through controlled interfaces.
This design does not eliminate every security vulnerability, but it creates essential boundaries that limit what programs are allowed to do.
11. The OS Provides a User Interface
The operating system also gives people ways to interact with the computer.
A graphical user interface provides windows, icons, menus, pointers, and touch controls. A command-line interface allows users to enter textual commands. Many operating systems support both.
The user interface is highly visible, so people often think it is the operating system itself. But it is only one part of the complete system.
The deeper work of CPU scheduling, memory protection, file management, and device control continues even when no graphical desktop is present. This is why servers and embedded systems can run an operating system without using a conventional desktop interface.
12. What Happens When You Start an Application?
Suppose you click a browser icon.
The user interface recognizes the action and asks the operating system to start the application. The OS locates the executable file on storage, creates a new process, establishes its virtual address space, and loads the required code and data.
The operating system creates an initial thread and allows the scheduler to assign it CPU time. As the browser runs, it requests more memory, opens configuration files, communicates with the display system, accepts keyboard and mouse input, and sends data through the network.
Many hardware components are involved, but the application reaches them through services managed by the operating system.
What looks like one simple click begins a coordinated sequence across software and hardware.
13. Different Devices Need Different Operating Systems
Desktop computers, servers, smartphones, cars, routers, industrial controllers, and tiny embedded devices do not have identical requirements.
A desktop operating system emphasizes interactive applications and broad hardware support. A server operating system may prioritize reliability, networking, security, and the ability to run many services. A mobile operating system must carefully manage battery power, sensors, wireless connections, and application permissions.
Some embedded systems use a real-time operating system, or RTOS, designed to respond within predictable timing limits. Others may run without a full operating system at all.
The implementation changes with the device, but the central idea remains the same: the operating system organizes resources and provides a controlled environment for software.
14. The Operating System Is Both Manager and Abstraction
The operating system performs two closely related roles.
First, it is a resource manager. It decides how CPU time, memory, storage, and devices are shared.
Second, it provides abstractions. Instead of forcing every program to work with raw hardware details, it presents useful concepts such as processes, virtual memory, files, sockets, and windows.
A file is easier to use than a collection of raw storage blocks. A process is easier to manage than an unstructured stream of CPU instructions. Virtual memory is easier and safer for programs than direct access to arbitrary physical RAM.
These abstractions are among the operating system’s most important contributions.
Conclusion
An operating system is the main system software that connects applications to computer hardware.
It manages CPU time, allocates and protects memory, organizes programs as processes, controls files and storage, communicates with devices through drivers, and provides common services to applications.
It also creates security boundaries and gives users ways to interact with the machine.
The operating system is therefore much more than a desktop, a set of icons, or a startup screen.
It is both a manager and an abstraction layer.
Applications see processes, files, virtual addresses, and standard interfaces. Underneath, the operating system coordinates processors, memory chips, storage devices, and many other hardware components.
Understanding this layer creates the foundation for studying kernels, user mode and kernel mode, system calls, processes, threads, scheduling, interrupts, device drivers, and file systems.
In the next tutorial, we will look inside the most important part of the operating system: the kernel.