Operating Systems

What Is an Operating System?

An operating system (OS) is the software layer that manages computer hardware and provides services to applications. It’s the intermediary between you (or your software) and the hardware.

An OS has two main parts:

• Kernel - the core: manages hardware, processes, memory, and I/O
• Userspace - the environment where applications and user interactions happen

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The Kernel

The kernel is the heart of the OS. It runs with full hardware access and handles:

Responsibility	Description
Process management	Creates, schedules, and terminates processes
Memory management	Allocates RAM to processes, manages virtual memory
I/O management	Coordinates all input/output between software and devices
File system	Manages how data is stored and retrieved from storage

Kernel Mode vs User Mode

• Kernel mode: Direct hardware access - only the kernel runs here
• User mode: Restricted access - all applications run here and make system calls to request kernel services

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Processes

A program is an application stored on disk (e.g., Chrome).
A process is a running instance of that program - it has allocated memory and CPU time.

• You can have multiple processes from the same program (e.g., many browser tabs)
• The kernel uses time slices to rapidly switch the CPU between processes - creating the illusion of parallelism on a single core

Process States

NEW → READY → RUNNING → (WAITING / TERMINATED)

The kernel’s scheduler decides which process runs next based on priority and fairness algorithms.

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Memory Management

RAM & Virtual Memory

Processes need RAM to run. When physical RAM fills up, the OS uses virtual memory - a combination of RAM and disk space:

1. Least-recently-used memory pages are written to a swap partition/file on disk
2. When needed again, they’re loaded back into RAM

This allows the OS to run more processes than physical RAM alone could support - at the cost of speed (disk is much slower than RAM).

Tip

On Linux, use free -h to see RAM and swap usage. On Windows, check Task Manager → Performance → Memory.

Block Storage

Files are stored on disk in data blocks - fixed-size chunks. This allows:

• Faster random access (no need to read a file sequentially from start to end)
• Efficient space reuse when files are deleted

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File Systems

The file system defines how data is organized on storage.

OS	Default File System
Windows	NTFS
macOS	APFS
Linux	ext4 (most common)

Key concepts:

• File extension: Indicates file type (.exe, .pdf, .txt) - used by the OS to determine how to open it
• Metadata: Information about the file - name, size, timestamps, permissions
• File managers: Finder (macOS), File Explorer (Windows), Nautilus (Ubuntu)

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I/O Management

I/O (Input/Output) covers everything that communicates with the CPU:

• Input devices: Keyboard, mouse, microphone, webcam
• Output devices: Monitor, speakers, printer
• Both: Hard drives, USB drives, SSDs, network cards

The kernel uses device drivers - software that knows how to talk to specific hardware - to abstract hardware differences from applications.

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Shells

A shell is a program that takes text commands and sends them to the OS to execute. Two major types:

CLI (Command-Line Interface)

Text-based. Powerful, scriptable, efficient for automation.

• Linux: bash, zsh, fish
• Windows: cmd.exe, PowerShell

GUI (Graphical User Interface)

Visual windows, icons, and menus. More approachable for general users.

• Windows Explorer, macOS Finder, GNOME

Note

Remote access typically uses CLI (SSH) because GUIs are bandwidth-heavy. IT professionals should be comfortable in both environments.

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System Logs

Logs are files that record system events - like a diary for the OS.

OS	Log location
Linux/macOS	/var/log/
Windows	Event Viewer (eventvwr)

Common logs:

• System log: hardware, driver events
• Application log: app crashes and errors
• Security log: login attempts, access events

Logs are your best friend when troubleshooting - always check them before guessing.

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Major OS Types

OS	Primary Use
Windows	Enterprise desktops, consumer PCs
macOS	Apple hardware, creative/dev professionals
Linux	Servers, cloud, developers, embedded systems
Chrome OS	Chromebooks - cloud-centric, lightweight, secure
Android / iOS	Mobile - based on Linux and Darwin respectively

Tip

The OS an organization uses is heavily shaped by the software and systems it needs to run. As IT support, you’ll encounter all of the above.

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Virtual Machines

A virtual machine (VM) is a software emulation of a physical computer. It runs a complete OS inside another OS.

Use cases:

• Testing software in isolation
• Running multiple OS environments on one machine
• Creating identical, reproducible system images for deployment