When you open a browser, run a program, save a file, or connect to the internet, your application needs to use the computer’s hardware.

It needs processor time to execute instructions, memory to store data, storage to save files, and network devices to communicate with other computers.

However, applications do not usually control these resources directly. They interact with the operating system, which manages the hardware on their behalf.

An operating system is system software that sits between applications and computer hardware.

Applications never reach down to the hardware on their own. A request for processor time, memory, a file, or a network connection passes through the operating system, which decides how it is carried out and whether it is allowed at all.

Examples of operating systems include Linux, Windows, macOS, Android, and iOS.

Although these systems are designed for different devices, they all solve the same fundamental problem: they make hardware easier and safer for applications to use.

Imagine that you are building a program that saves some text to a file.

Without an operating system, your program would need to understand the exact storage device installed in the computer. It would need to find an unused location, write data using hardware-specific instructions, remember where the data was stored, and prevent other programs from overwriting it.

The program would also need different code for different storage devices.

The same problem would appear whenever the application wanted to use memory, display something on the screen, read keyboard input, or send data over a network.

Building applications this way would be extremely difficult.

The operating system hides these hardware-specific details and gives applications a simpler interface.

Instead of controlling the storage device directly, an application can ask the operating system to create or update a file. The operating system handles the underlying hardware operations.

This makes applications easier to build and allows the same application to work across many computers running the same operating system.

One of the operating system’s main responsibilities is to provide simple abstractions over complex hardware.

An abstraction presents a resource in a form that is easier for applications to understand and use.

For example, storage hardware works with physical blocks and device-specific commands. The operating system presents it to applications using files and directories.

Similarly, the operating system presents:

These abstractions allow developers to focus on application logic instead of hardware implementation details.

A backend application can read a configuration file without knowing which physical disk contains it. A web server can send a response without knowing how the network card transmits individual signals.

The operating system handles those details.

A computer has limited resources.

It may have a small number of processor cores, a fixed amount of memory, limited storage space, and a finite amount of network capacity.

At the same time, many applications may be running.

For example, your computer might be running a browser, a code editor, a music player, a database, and several background services. All of them need access to the same hardware.

The operating system decides how these resources are shared. Every program sends its requests to the same place, and the operating system hands out the limited hardware underneath:

No application talks to another directly. Programs compete for the same cores, the same memory, and the same disk, but they negotiate only with the operating system in the middle.

It determines which program receives processor time, how much memory each program can use, where files are stored, and which application can access a particular device.

Without this coordination, programs could interfere with one another. One application might consume all available memory, overwrite another application’s data, or take permanent control of the processor.

The operating system ensures that resources are shared in a controlled manner.

An operating system performs several important responsibilities.

Managing running programs

The operating system starts, pauses, resumes, and terminates programs. It also coordinates multiple programs that are running at the same time.

The internal representation of a running program is called a process, which we will study in a later chapter.

Managing the processor

Many programs may be ready to run, but the computer has a limited number of processor cores.

The operating system decides which program should run and when it should be paused so another program can use the processor.

This is known as CPU scheduling.

Managing memory

Programs require memory to store instructions and data.

The operating system tracks which parts of memory are being used, provides memory to applications, and prevents one application from freely accessing another application’s memory.

Managing files and storage

The operating system organizes persistent data using files and directories.

It keeps track of where files are stored, who can access them, and how data is read from or written to storage devices.

Managing hardware devices

Devices such as keyboards, displays, storage drives, cameras, and network cards all behave differently.

The operating system communicates with these devices through specialized software called device drivers and exposes simpler interfaces to applications.

Providing protection

The operating system prevents applications from performing unrestricted operations.

It controls access to files, memory, devices, and other programs. This limits the damage that can be caused by bugs or malicious software.

Suppose you open a code editor and run a program while music is playing in the background.

Several things happen at the same time.

The code editor needs memory to store the file you are editing. The compiler needs processor time to build the program. The program may need to read files or connect to the internet. The music player needs regular processor time and access to the audio device.

All of these applications share the same computer.

The operating system coordinates them so that:

The operating system creates the experience that many independent programs are running smoothly, even though they are sharing the same physical resources.

The terms operating system and kernel are related, but they do not mean exactly the same thing.

The kernel is the central and most privileged part of an operating system. It directly manages core resources such as the processor, memory, devices, and running programs.

The complete operating system may also include system libraries, command-line tools, background services, configuration utilities, and a graphical interface.

For example, Linux technically refers to a kernel. A complete Linux distribution combines the Linux kernel with many additional tools and services.

The shell, libraries, and kernel together are what people usually mean by "the operating system." Only the kernel runs with full control of the machine. An application often works through the libraries and tools above it, but it can also ask the kernel directly through a system call.

For now, you can think of the kernel as the core component responsible for controlling the machine, while the operating system is the broader environment built around it.

Backend applications depend heavily on operating-system behavior.

A web server uses operating-system networking interfaces to accept connections. A database uses files and memory to store and process data. A multithreaded application relies on the operating system to schedule its work. Containers depend on operating-system features for isolation and resource control.

Operating-system knowledge also helps explain production problems such as high CPU usage, memory exhaustion, slow file operations, excessive concurrency, and applications running out of available connections.

You do not need to become a kernel developer to benefit from learning operating systems. Understanding how the OS manages resources helps you design more reliable applications and debug them more effectively.

An operating system is the software layer between applications and hardware.

It makes hardware easier to use by providing abstractions such as running programs, memory, files, and network connections.

It also acts as a resource manager by coordinating access to the processor, memory, storage, and hardware devices.

Finally, it protects applications and users by controlling which resources each program is allowed to access.

A useful mental model is:

The operating system makes hardware convenient to use, shares resources between programs, and prevents programs from interfering with one another.

The following chapters will explore these responsibilities in more detail, starting with how programs become running processes.