Introduction #

You use an operating system every day even when you are not aware of it.

When you open an application, save a file, connect to the internet, or plug in a USB device, something quietly decides who gets access to hardware, when, and how. That decision is never made by the application itself.

Most beginners hear the term operating system and assume it is just “software that runs programs.”
That explanation is incomplete and it hides the real reason operating systems exist.

An operating system exists because modern computers cannot work safely or reliably without a control layer between software and hardware.

What an Operating System Really Is #

At its core, an operating system (OS) is system software that sits between applications and computer hardware.

Applications do not talk to hardware directly.
They never have and they never should.

Instead, every application communicates through the operating system, and the operating system communicates with the hardware on its behalf.

This separation is intentional. It is what makes modern systems stable, secure, and usable.

Why Applications Cannot Access Hardware Directly #

Consider a simple action: opening a text editor.

For that to happen, the program needs:

• Memory to load itself
• CPU time to execute instructions
• Access to the disk to open or save files
• Input from keyboard and mouse
• Output to the display

None of these resources belong to the application.
They are all shared hardware resources.

If applications were allowed to access hardware directly:

• Two programs could overwrite the same memory
• A bug in one program could crash the entire system
• Any program could read or modify any file
• Malicious software would have full system control

Early computer systems worked like this and they were unstable and unsafe.

The operating system exists to prevent this chaos.

The Operating System as a Controller #

A useful way to think about an operating system is as a controller and coordinator.

The OS decides:

• Which process runs on the CPU, and for how long
• How memory is allocated and protected
• Which files a program can access
• How input and output devices are shared
• How multiple programs run at the same time without interfering

Your system may appear to be doing many things simultaneously.
In reality, the operating system is constantly managing and switching access to hardware resources in a controlled way.

Without this coordination, multitasking would not be possible.

The Operating System as a Service Provider #

Applications do not care how hardware works internally.

A program does not need to know:

• How a hard disk reads sectors
• How network packets are transmitted
• How memory is physically wired
• How a keyboard generates signals

Instead, the operating system exposes services that applications can request.

These requests are made using system calls controlled entry points into the operating system.

Through system calls, an application can:

• Create or terminate processes
• Read from or write to files
• Allocate memory
• Communicate over a network
• Interact with hardware devices

The application asks.
The operating system validates and executes the request safely.

Kernel and User Space: The Core Separation #

Not all software runs with the same level of control.

Modern operating systems enforce two execution levels:

• User space – where applications run
• Kernel space – where the operating system core runs

The kernel is the heart of the operating system.
It has unrestricted access to CPU instructions, memory, and hardware devices.

Applications run in user space with restricted permissions.
When they need access to sensitive resources, they must request it from the kernel.

This separation is critical.

If an application crashes, the operating system continues running.
If an application misbehaves, the OS can stop it without affecting others.

This design is one of the main reasons modern systems are stable and secure.

A Simple Mental Model #

You can visualize a computer system like this:

Applications
↓
Operating System
↓
Hardware

Applications focus on what needs to be done.
The operating system focuses on how it is done safely.
Hardware focuses on executing instructions.

Each layer has a clear responsibility.
None of them should bypass the layer below.

Why This Matters for Linux and System Programming #

Once this mental model is clear, many Linux concepts stop feeling confusing.

Process management, memory allocation, permissions, system calls, scheduling all of these exist because the operating system must control shared hardware resources.

Linux is not “complex by default.”
It is explicit because it exposes operating system behavior instead of hiding it.

Understanding what an operating system is makes learning Linux much easier and more logical.

Summary #

An operating system is not just software that launches programs.

It is:

• A controlled interface between applications and hardware
• A manager of CPU, memory, storage, and devices
• A protection layer that enforces isolation and security
• A service provider that hides hardware complexity

Every time you run a command, open a file, or connect to the internet, the operating system is working in the background quietly ensuring that everything happens correctly, safely, and efficiently.

This understanding is the foundation for everything that follows in operating systems and Linux internals.