OPERATING SYSTEM

 

1️⃣ What is an Operating System?

🔹 Definition (Exam Ready)

An Operating System is system software that acts as an interface between the user and computer hardware.

🔹 Simple Line to Remember

👉 User → OS → Hardware

🔹 Examples

  • Windows

  • Linux

  • macOS

  • Android

  • iOS


🔹 Main Functions (Mnemonic: PM-FD-SH-UI)

FunctionMeaning
Process ManagementCPU allocation, multitasking
Memory ManagementRAM usage, allocation
File ManagementCreate, delete, store files
Device ManagementPrinter, keyboard, mouse
SecurityPassword, protection
User InterfaceGUI / CLI

📌 Exam Fact:
Kernel performs process + memory management


3️⃣ Types of Operating System

🔹 1. Batch Operating System

  • Jobs executed in batches

  • No user interaction

📌 Example: Old IBM systems

🔹 2. Time Sharing Operating System

  • Multiple users share CPU

  • Fast response time

📌 Example: UNIX, Linux

🔹 3. Multiprogramming OS

  • Multiple programs in memory

  • CPU always busy

📌 MCQ: Improves CPU utilization ✔️

🔹 4. Multiprocessing OS

  • Uses multiple CPUs

📌 Example: Modern servers

🔹 5. Real-Time Operating System (RTOS)

  • Time critical

  • Used in missiles, robots

📌 Example: VxWorks

🔹 6. Distributed OS

  • Multiple computers connected

  • Work as one system

📌 Example: Network OS



4️⃣ Process & Thread

    

🔹 Process

  • Program in execution

  • Has its own memory

🔹 Thread

  • Lightweight process

  • Shares memory

📌 MCQ Favorite:
Thread is faster than process ✔️



5️⃣ Process States

🔹 Five States (Mnemonic: N-R-R-W-T)

New
Ready
Running
Waiting
Terminated

📌 UP Exam Repeated Question

6️⃣ CPU Scheduling Algorithms 

🔹 Algorithms

AlgorithmKey Point
FCFSFirst Come First Serve
SJFShortest Job First
PriorityHigh priority first
Round RobinTime quantum based

📌 MCQ:
Round Robin is used in Time Sharing OS


7️⃣ Deadlock

🔹 Definition

Deadlock is a situation where processes wait forever for resources.

🔹 Four Conditions (Mnemonic: M-H-N-C)

  1. Mutual Exclusion

  2. Hold and Wait

  3. No Preemption

  4. Circular Wait

📌 Important:
All four must occur for deadlock.


8️⃣ Memory Management

🔹 Techniques

  • Paging

  • Segmentation

  • Virtual Memory

📌 MCQ:
Paging removes external fragmentation


9️⃣ File System

🔹 File Operations

  • Create

  • Delete

  • Read

  • Write

🔹 File Access Methods

  • Sequential

  • Direct

  • Indexed

📌 Exam Fact:
NTFS → Windows file system


🔟 Kernel

🔹 Kernel Types

  • Monolithic Kernel

  • Micro Kernel

📌 MCQ:
Kernel is heart of OS

🔁 1-Minute Revision Chart

TopicKeyword
OS  Interface
KernelCore
ProcessExecution
ThreadLightweight
DeadlockInfinite wait
PagingNo external fragmentation
Round RobinTime quantum

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