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Abhishek Gupta
Abhishek Gupta

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OPERATING SYSTEM — DETAILED EXAM NOTES (UNIT-WISE, CIITM Dhanbad)

🟦 UNIT – I : OS Basics, Types, Services, Structure, System Calls

⭐ 1. Operating System – Definition

An Operating System (OS) is system software that manages computer hardware, executes programs, and provides a user-friendly environment for running applications.

It acts as an intermediary between user and hardware.

⭐ 2. Functions of Operating System

  1. Process Management
  • Creates, schedules, and terminates processes.
  1. Memory Management
  • Allocates memory to programs.
  • Performs deallocation when program ends.
  1. File Management
  • Creates, deletes, and organizes files.
  1. Device Management
  • Controls hardware devices (printers, disks, keyboard).
  1. Security & Protection
  • Prevents unauthorized access.
  1. Error Detection
  • Detects and handles system errors.
  1. Resource Allocation
  • Manages CPU, memory, disk, I/O.
  1. User Interface
  • Command-line (CLI)
  • Graphical (GUI)

⭐ 3. Types of Operating Systems

✔ 1. Batch Operating System

  • Jobs grouped into batches.
  • No user interaction during execution.
  • Example: Banking batch jobs.

✔ 2. Multiprogramming Operating System

  • Multiple programs kept in memory at same time.
  • CPU switches between them.
  • Increases CPU utilization.

✔ 3. Time Sharing Operating System

  • CPU time divided into time slices.
  • Many users use computer simultaneously.
  • Example: UNIX, Linux.

✔ 4. Single-User Operating System

  • Only one user at a time.
  • Simple, low-security.
  • Example: Windows 98.

✔ 5. Multiuser Operating System

  • Multiple users logged in at same time.
  • Strong protection.
  • Example: Linux, Unix server.

⭐ 4. Components of an Operating System

  • Process Manager – handles processes
  • Memory Manager – allocates memory
  • File Manager – manages files/folders
  • I/O Manager – controls devices
  • Security Manager – permissions & protection
  • Kernel – core of the OS
  • Shell – command interpreter

⭐ 5. Operating System Services

  • Program execution
  • File system access
  • Device access
  • Communication
  • Error handling
  • Resource allocation
  • I/O operations
  • Authentication & security

⭐ 6. System Calls

System calls allow user-level programs to interact with OS.

Types of System Calls:

  1. Process Control – create, terminate process
  2. File Management – open, read, write files
  3. Device Management – request or release device
  4. Information Maintenance – get system info
  5. Communication – send/receive messages

⭐ 7. System Programs

Useful programs included with OS:

  • Text editors
  • Compilers & Assemblers
  • File management tools
  • Command interpreters (shell)
  • Utilities (backup, compression)

⭐ 8. System Structure

1. Monolithic Structure

  • Entire OS is one large program.

2. Layered Structure

  • OS divided into multiple layers.

3. Microkernel

  • Minimal kernel; rest runs in user mode.

4. Modules

  • OS built in modular form.

5. Virtual Machines

  • Hardware is virtualized.

🟦 UNIT – II : Processes, Scheduling, Threads

⭐ 1. Process Concept

A process is a program in execution.
It includes:

  • Program counter
  • Stack
  • Data section
  • CPU registers

⭐ 2. Process States

  1. New – process created
  2. Ready – waiting for CPU
  3. Running – executing
  4. Waiting – waiting for I/O
  5. Terminated – finished execution

⭐ 3. Process Control Block (PCB)

A data structure used by OS to store information about a process.

Contains:

  • Process ID
  • CPU registers
  • Process state
  • Program counter
  • Scheduling info
  • Memory allocation info

⭐ 4. Process Scheduling

Decides which process runs next.

⭐ 5. Scheduling Criteria

  • CPU utilization
  • Throughput
  • Turnaround time
  • Waiting time
  • Response time

⭐ 6. Scheduling Algorithms

✔ 1. FCFS (First Come First Serve)

  • Simple
  • Non-preemptive
  • Process served in arrival order

✔ 2. SJF (Shortest Job First)

  • Selects shortest process first
  • Preemptive version → SRTF

✔ 3. Priority Scheduling

  • Highest priority runs first
  • Can cause starvation

✔ 4. Round Robin

  • Time-sharing
  • Each process gets fixed time quantum

✔ 5. Multilevel Queue Scheduling

  • Different queues for different process types

⭐ 7. Multiple Processor Scheduling

  • Used in multicore CPUs
  • Load balancing between processors

  • Two types:

    • Asymmetric multiprocessing
    • Symmetric multiprocessing (SMP)

⭐ 8. Real-Time Scheduling

  • Hard real-time → deadlines must be met
  • Soft real-time → occasional misses allowed

⭐ 9. Threads

A thread is a lightweight process.

Types:

  • User-level threads
  • Kernel-level threads

Advantages:

  • Faster context switching
  • Efficient resource usage
  • Useful in web servers, games, browsers

🟦 UNIT – III : Critical Section, Synchronization, Deadlocks

⭐ 1. Critical Section Problem

Section of code where shared resources are accessed.

Requirements

  • Mutual Exclusion – only one process
  • Progress – no unnecessary waiting
  • Bounded Waiting – limit on wait time

⭐ 2. Semaphores

Used for synchronization.

Types

  • Binary semaphore
  • Counting semaphore

Operations 

wait(S); // decrements S
signal(S); // increments S
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⭐ 3. Classical Problems of Synchronization

  1. Producer–Consumer Problem
  2. Readers–Writers Problem
  3. Dining Philosophers Problem
  4. Sleeping Barber Problem

⭐ 4. Deadlock

Occurs when a set of processes are waiting for each other forever.

Deadlock Conditions (Necessary & Sufficient):

  1. Mutual exclusion
  2. Hold & wait
  3. No preemption
  4. Circular wait

⭐ 5. Methods for Handling Deadlock

✔ 1. Deadlock Prevention

Break one of the 4 conditions.

✔ 2. Deadlock Avoidance

  • Use extra information
  • Banker’s algorithm

✔ 3. Deadlock Detection

  • Resource allocation graph
  • Periodic checking

✔ 4. Deadlock Recovery

  • Kill process
  • Rollback process
  • Resource preemption

🟦 UNIT – IV : Memory Management, Paging, Segmentation, Virtual Memory

⭐ 1. Logical vs Physical Address

  • Logical address → generated by CPU
  • Physical address → actual location in RAM
  • MMU translates logical → physical

⭐ 2. Contiguous Allocation

Two methods:

✔ Fixed Partitioning

  • Memory divided into fixed-sized blocks
  • Internal fragmentation

✔ Variable Partitioning

  • Dynamic partitions
  • External fragmentation

⭐ 3. Swapping

Entire process swapped between RAM and disk.

⭐ 4. Paging

  • Memory divided into pages and frames
  • Removes external fragmentation

⭐ 5. Segmentation

Memory divided into variable-sized segments:

  • Code
  • Data
  • Stack

More user-friendly than paging.

⭐ 6. Virtual Memory

Allows running programs larger than actual physical memory.

Techniques:

  • Demand paging
  • Page replacement

⭐ 7. Demand Paging

Loads page into memory only when needed.

Page Fault:

Occurs when page is not in memory.

⭐ 8. Page Replacement Algorithms

✔ FIFO

Replace oldest page.

✔ LRU (Least Recently Used)

Replace page not used for longest time.

✔ Optimal

Replace page that will not be used in future.

🟦 UNIT – V : Disk Scheduling, File Systems, Directories, Protection

⭐ 1. Disk Scheduling Algorithms

✔ FCFS

Serve in arrival order.

✔ SSTF

Serve request closest to current head position.

✔ SCAN

Arm moves like elevator.

✔ C-SCAN

Circular version of SCAN.

✔ LOOK / C-LOOK

SCAN/C-SCAN with smart stopping.

⭐ 2. Disk Management

Includes:

  • Partitioning
  • Formatting
  • Boot block
  • Bad block management

⭐ 3. Swap Space Management

Used to store swapped-out pages.
Located on fast disk space.

⭐ 4. Disk Reliability

Techniques:

  • Mirroring
  • RAID
  • Backups

⭐ 5. Stable Storage

Data stored safely using redundant devices.

⭐ 6. File Concepts

A file is a named collection of related information.

Attributes:

  • Name
  • Type
  • Size
  • Location
  • Permissions
  • Owner

⭐ 7. Directory Structure

  • Single-level
  • Two-level
  • Tree-structured
  • Acyclic graph
  • General graph

⭐ 8. File Protection

  • Access control lists
  • Read/Write/Execute permissions
  • Password protection
  • Encryption

📝 IMPORTANT EXAM QUESTIONS (DETAILED)

⭐ UNIT – I

  1. Explain functions of operating system.
  2. Describe different types of operating systems.
  3. What are system calls? Explain types.
  4. Explain OS services.

⭐ UNIT – II

  1. Explain process states and PCB with diagram.
  2. Explain scheduling algorithms.
  3. What are threads? Explain their advantages.

⭐ UNIT – III

  1. What is critical section? Explain its requirements.
  2. Explain semaphores with examples.
  3. Explain deadlock detection, prevention & avoidance.

⭐ UNIT – IV

  1. Explain paging & segmentation.
  2. Explain page replacement algorithms.
  3. What is demand paging? Explain page faults.

⭐ UNIT – V

  1. Explain disk scheduling algorithms.
  2. Explain directory structures.
  3. Explain file protection techniques.

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