Computer · Chapter 03

🖥️ Operating System

OS functions, types, process management, Linux and Windows.

🖥️ The Master Software

An Operating System (OS) is system software that manages hardware resources and provides an interface between user and hardware. Without OS, a computer cannot function.

Functions of OS:
• Process Management — creates, schedules, terminates processes. CPU scheduling algorithms: FCFS, Round Robin, Priority, SJF.
• Memory Management — allocates/deallocates RAM. Virtual memory extends RAM using disk. Paging and segmentation.
• File Management — organizes files in directories/folders. Handles read/write/delete operations.
• Device Management — manages I/O devices via drivers.
• Security — user authentication, access control, firewall.

Types of OS: Batch OS, Time-Sharing OS, Real-Time OS (RTOS — for critical systems like aircraft), Network OS, Distributed OS, Mobile OS.

💻 Popular operating systems

Windows (Microsoft) — most popular desktop. Windows 11 latest. GUI-based. NTFS file system.
macOS (Apple) — Unix-based. HFS+/APFS file system. Only for Apple hardware.
Linux — open source. Distributions: Ubuntu, Fedora, Debian, Kali. Ext4 file system. Powers 96% of web servers, Android is Linux-based.
Android (Google) — Linux-based mobile OS. Most popular mobile OS. 72% market share.
iOS (Apple) — Unix-based mobile OS. Only for iPhones/iPads.
Chrome OS — Google. Cloud-centric. Used in Chromebooks.

📋 Key OS concepts — SSC/CCC exam

GUI (Graphical User Interface) — icons, windows, menus. Easy to use. Windows, macOS.
CLI (Command Line Interface) — text commands. Linux terminal, Windows CMD. Powerful but technical.
Kernel — core of OS. Types: Monolithic (Linux), Microkernel (Windows NT), Hybrid.
BIOS/UEFI — firmware that starts computer before OS loads. POST (Power-On Self Test).
Booting — starting the computer. Cold boot = full start. Warm boot = restart.
Multitasking — multiple tasks running simultaneously (appears so). Multiprocessing — multiple CPUs.
Deadlock — two processes waiting for each other indefinitely.

🎬

OS Functions — Click to Explore

Animation
OPERATING SYSTEM LAYERS — CLICK EACH LAYER HARDWARE CPU · RAM · Hard Disk · Input/Output Devices · Network Cards KERNEL (Core of OS) Process Management · Memory Management · Device Drivers · System Calls Directly interacts with hardware SYSTEM PROGRAMS File Manager · Task Manager · Compiler · Assembler · Linker · Loader Shell (CLI) and Window Manager (GUI) APPLICATION PROGRAMS MS Word · Chrome · VLC · Photoshop · Games · WhatsApp User interacts here — these use OS services via APIs USER You — interacting via GUI (icons, menus) or CLI (commands) User sees only the application layer — OS works behind the scenes CLICK A LAYER TO UNDERSTAND ITS ROLE OS acts as intermediary between hardware and applications — users never interact with hardware directly.

OS is the invisible foundation — every click, file save, or print goes through OS before reaching hardware.

💻

OS Concepts Explorer

Interactive
ProcessA program in execution — has its own memory space
ThreadLightweight process — shares memory with parent process
Process statesNew → Ready → Running → Waiting → Terminated
PCBProcess Control Block — stores process info (PID, state, registers)
Task ManagerCtrl+Shift+Esc — see all running processes in Windows
Practice (SSC/CCC): What is virtual memory? How does it help?
Virtual Memory — a technique where OS uses part of the hard disk as an extension of RAM.

How it works:
• When RAM is full, OS moves some inactive data from RAM to a special area on disk called Swap space (Windows: page file = pagefile.sys)
• This frees up RAM for active programs
• When needed, data is swapped back from disk to RAM

Benefits:
• Programs can run even when total memory needed exceeds physical RAM
• Allows multitasking with many applications
• Each process gets its own virtual address space — security and isolation

Drawback:
• Disk is much slower than RAM — heavy swapping = system slowdown (called "thrashing")

Paging vs Segmentation:
• Paging: divide memory into fixed-size pages (e.g., 4KB). Internal fragmentation can occur.
• Segmentation: divide by logical sections (code, data, stack). External fragmentation can occur.
• Most modern OS use paging with segmentation combined.
Practice (CCC): What are the differences between GUI and CLI? Give examples.
GUI (Graphical User Interface):
• Uses visual elements: icons, windows, menus, buttons
• Operated by mouse clicks and touch
• User-friendly — no need to memorize commands
• Examples: Windows 11, macOS, Android, iOS
• More system resources required (graphics)
• Slower for repetitive tasks

CLI (Command Line Interface):
• Text-based — user types commands
• Requires memorizing commands
• Faster for experts — powerful scripting/automation
• Less system resources
• Examples: Windows Command Prompt (cmd.exe), PowerShell, Linux Terminal (bash)

Common CLI commands:
Windows CMD: dir (list files), cd (change directory), cls (clear screen), del (delete)
Linux/Mac: ls (list), cd (change dir), clear, rm (remove), pwd (print working directory)

System administrators prefer CLI — can manage thousands of servers with scripts. Regular users prefer GUI — intuitive and visual.
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