Operating System & Interfaces

What is an Operating System?

An Operating System (OS) is system software that acts as an intermediary between the user and the computer hardware. Without an OS, you would have to write binary instructions for every task — even moving the mouse cursor. The OS manages all hardware resources, runs your applications, and provides a user-friendly interface.

Definition: An Operating System is a collection of programs that manages hardware resources, provides services to application programs, and acts as an interface between the user and the computer.

Functions of an Operating System

flowchart TD
OS["Operating System"]
OS --> PM["Process Management"]
OS --> MM["Memory Management"]
OS --> FM["File Management"]
OS --> DM["Device Management"]
OS --> SEC["Security"]
OS --> UI["User Interface"]

Types of Operating Systems

This is one of the most frequently asked topics. Know each type with its key characteristic.

Real-time OS — Two Types

Popular Operating Systems

Unix (1969) is the “grandfather” of modern OS. Linux, macOS, and Android all have Unix roots. Linux distributions include Ubuntu, Fedora, CentOS, Debian, and Red Hat.

Kernel — The Core of an OS

The Kernel is the core component of an operating system. It is the first program loaded into memory when a computer starts and remains in memory throughout the session. The kernel directly communicates with the hardware.

flowchart TD
A["User Applications<br/>(Word, Chrome, Excel)"]
B["OS Services<br/>(File Mgmt, GUI, Security)"]
C["KERNEL"]
D["HARDWARE<br/>(CPU, RAM, Disk)"]
A --> B --> C --> D

Types of Kernel

CLI vs GUI

Common CLI Commands

Process Management

A process is a program in execution. When you open Chrome, the OS creates a process for it. Each process has its own memory space, registers, and state. The OS manages thousands of processes simultaneously.

Process States

stateDiagram-v2
[*] --> New
New --> Ready : Admitted
Ready --> Running : Dispatch
Running --> Ready : Preempted
Running --> Waiting : I/O request
Waiting --> Ready : I/O complete
Running --> Terminated : Exit
Terminated --> [*]

Context Switching — When the CPU switches from one process to another, it saves the state of the current process and loads the state of the next one. This happens so fast that it appears as if multiple programs run simultaneously.

Memory Management

Virtual Memory makes it possible to run programs larger than your physical RAM. Windows uses a page file; Linux uses a swap partition.

flowchart LR
A["Program<br/>(Large)"] --> B["Virtual Memory<br/>(Pages)"]
B --> C["RAM<br/>(Frames)"]
B --> D["Disk<br/>(Swap/Page File)"]

File Management

The OS manages how data is stored, organized, and accessed on storage devices.

Device Drivers

A device driver is a specialized program that allows the OS to communicate with a hardware device. Without the correct driver, the OS cannot “talk to” the hardware.

• Printer driver — tells OS how to send print commands
• Graphics driver — enables display of images and video (NVIDIA, AMD drivers)
• Audio driver — enables sound output
• Network driver — enables Wi-Fi and Ethernet connectivity
• Plug and Play (PnP) — OS automatically detects and installs drivers for new devices
• Updated drivers fix bugs and improve performance

When you plug in a new device and Windows says “Installing device driver…” — it is loading the translator between OS and hardware.

Booting Process

Booting is the process of starting a computer and loading the OS into memory.

flowchart LR
A["Power On"] --> B["BIOS/UEFI"]
B --> C["POST<br/>(Self Test)"]
C --> D["Boot Loader"]
D --> E["OS Loads<br/>into RAM"]
E --> F["Login Screen"]

• Cold Boot = starting from power off (full POST runs)
• Warm Boot = restart without power off (Ctrl+Alt+Del)

BIOS vs UEFI

Deadlock

A deadlock occurs when two or more processes are stuck forever, each waiting for a resource held by the other. Neither can proceed — like two trains on a single track facing each other.

Four Necessary Conditions for Deadlock (all must hold simultaneously)

To prevent deadlock, break any one of these four conditions.

flowchart LR
P1["Process A<br/>holds Resource 1"] -->|waits for| R2["Resource 2"]
P2["Process B<br/>holds Resource 2"] -->|waits for| R1["Resource 1"]

Spooling & Thrashing

Spooling (Simultaneous Peripheral Operations On-Line) is a technique where data is temporarily held in a buffer (usually on disk) so that a slower device like a printer can access it at its own pace. When you print multiple documents, they are placed in a print queue — the OS sends them to the printer one by one while you continue working.

Thrashing occurs when the OS spends more time swapping pages between RAM and disk than actually executing processes. This happens when too many programs are loaded and physical memory is insufficient, causing constant page faults. The system becomes extremely slow despite high CPU activity.

Exam Tip: Spooling = queue management for slow devices. Thrashing = excessive paging that kills performance.

Multiprogramming vs Multitasking vs Multiprocessing vs Multithreading

DOS Commands (Frequently Asked)

MS-DOS (Microsoft Disk Operating System) was a CLI-only OS. These commands are still asked in exams:

Exam Tip: MD = Make Directory, RD = Remove Directory, REN = Rename, DEL = Delete.

Linux Distributions

Linux is an open-source kernel; a distribution (distro) bundles the kernel with software, package manager, and desktop environment.

Android Version History

Android versions were named after desserts alphabetically until Android 10. From Android 10 onwards, Google switched to just numbers.

Exam Tip: Android is based on the Linux kernel and is developed by Google. Dessert names were dropped from public branding starting with Android 10.

Windows Version Timeline

File System Management (Quick Reference)

The OS uses a file system to organize data on storage devices. Different OS use different file systems:

For detailed file system comparison (FAT32 vs NTFS vs ext4), see the Storage, Memory & File Systems lesson.

Key Takeaways

• OS is the bridge between user and hardware — manages processes, memory, files, devices, and security
• Types of OS: Batch, Multi-programming, Multi-tasking, Time-sharing, Real-time, Distributed, Embedded, Mobile
• Multiprogramming = CPU switches on I/O wait; Multitasking = rapid time slicing; Multiprocessing = multiple CPUs; Multithreading = threads within a process
• Hard RTOS = deadline miss causes failure (missiles, pacemaker); Soft RTOS = degraded performance (video, VoIP)
• Kernel is the core of OS — Monolithic (Linux), Micro (QNX), Hybrid (Windows/macOS)
• CLI = text commands (faster, less resources); GUI = graphical icons (easier for beginners)
• DOS commands: DIR (list), COPY, DEL, REN, MD (mkdir), CD, RD (rmdir), TYPE, CLS, FORMAT
• Process states: New → Ready → Running → Waiting → Terminated; Context Switching saves/loads state
• Spooling = queue management for slow devices (print queue); Thrashing = excessive paging kills performance
• Virtual Memory uses disk as extra RAM; Paging = fixed-size pages/frames; Segmentation = variable-size
• Booting: Power On → BIOS/UEFI → POST → Boot Loader → OS loads
• UEFI is modern replacement of BIOS — faster boot, Secure Boot, GPT > 2 TB, 128 partitions
• Deadlock needs 4 conditions: Mutual Exclusion, Hold & Wait, No Preemption, Circular Wait
• Linux distros: Ubuntu (beginners), Fedora (cutting-edge), CentOS (enterprise), Debian (stable), Kali (cybersecurity)
• Windows: XP (2001, most popular), 7 (2009), 10 (2015, Cortana/Edge), 11 (2021, TPM 2.0)
• Android: based on Linux kernel, by Google; dessert names until Android 10; latest = Android 15 (2024)
• File systems: FAT32 (4 GB max, universal), NTFS (Windows), ext4 (Linux), APFS (macOS)

Summary Cheat Sheet