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Introduction to Operating Systems

Introduction to Operating Systems

What is an Operating System?

An Operating System (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. It acts as an intermediary between users and computer hardware.

Functions of an Operating System

1. Process Management

  • Creation and deletion of processes
  • Scheduling of processes
  • Synchronization and communication between processes
  • Deadlock handling

2. Memory Management

  • Allocation and deallocation of memory
  • Keeping track of memory usage
  • Virtual memory management
  • Memory protection

3. File System Management

  • Creation and deletion of files and directories
  • Mapping files onto secondary storage
  • File access control and permissions
  • Backup and recovery

4. Device Management (I/O Management)

  • Managing device drivers
  • Buffering, caching, and spooling
  • Device allocation and deallocation
  • I/O scheduling

5. Security and Protection

  • User authentication
  • Access control mechanisms
  • Protection against unauthorized access
  • Encryption and data integrity

6. User Interface

  • Command Line Interface (CLI)
  • Graphical User Interface (GUI)
  • Batch interface

Types of Operating Systems

1. Batch Operating System

  • Jobs are collected and processed in batches
  • No user interaction during execution
  • Jobs with similar requirements are batched together
  • Examples: Payroll systems, bank statements

Advantages:

  • Efficient for large repetitive jobs
  • Reduced idle time

Disadvantages:

  • No interaction with user
  • Difficult to debug
  • One job failure can stall entire batch

2. Multiprogramming Operating System

  • Multiple programs loaded in memory simultaneously
  • CPU switches to another program when current one waits (e.g., for I/O)
  • Improves CPU utilization
  • Uses job scheduling

Key Concept:

  • When Process A waits for I/O → CPU executes Process B
  • No time slicing (non-preemptive)

3. Multitasking / Time-Sharing Operating System

  • Extension of multiprogramming with time slicing
  • Each process gets a fixed time quantum (time slice)
  • Fast context switching creates illusion of parallel execution
  • Interactive system with quick response time

Examples: UNIX, Linux, Windows

Advantages:

  • Quick response time
  • Reduced CPU idle time
  • Supports multiple users

Disadvantages:

  • Context switching overhead
  • Security concerns with multiple users

4. Multiprocessing Operating System

  • Uses multiple CPUs/processors
  • Parallel processing of multiple tasks
  • Increased throughput and reliability

Types:

Type Description
Symmetric Multiprocessing (SMP) All processors are equal, share memory
Asymmetric Multiprocessing (AMP) Master-slave relationship between processors

Advantages:

  • High throughput
  • Fault tolerance (if one CPU fails, others continue)
  • Better performance for parallel tasks

5. Distributed Operating System

  • Multiple independent computers connected via network
  • Appear as a single coherent system to users
  • Resources are shared across the network

Examples: LOCUS, Amoeba

Advantages:

  • Resource sharing
  • Scalability
  • Fault tolerance
  • Load balancing

Disadvantages:

  • Complex to implement
  • Network dependency
  • Security challenges

6. Real-Time Operating System (RTOS)

  • Must respond within strict time constraints
  • Used in time-critical applications
  • Deterministic behavior is essential

Types:

Type Description Example
Hard RTOS Missing deadline = system failure Pacemakers, ABS brakes, flight control
Soft RTOS Missing deadline = degraded performance Video streaming, gaming

Characteristics:

  • Minimal interrupt latency
  • Predictable timing
  • Priority-based scheduling

7. Network Operating System

  • Provides network-related functionality
  • Runs on a server, manages data and users
  • Allows shared access to files, printers, etc.

Examples: Windows Server, Novell NetWare

8. Mobile Operating System

  • Designed for mobile devices (smartphones, tablets)
  • Touch-based interface
  • Battery and resource optimization

Examples: Android, iOS, HarmonyOS

9. Embedded Operating System

  • Designed for embedded systems
  • Limited resources and specific functionality
  • Often real-time

Examples: VxWorks, FreeRTOS, Embedded Linux

Operating System Structure

1. Monolithic Structure

  • All OS services run in kernel space
  • Single large kernel
  • Example: Traditional UNIX

Pros: Fast, efficient
Cons: Difficult to maintain, one bug can crash system

2. Layered Structure

  • OS divided into layers, each built on top of lower layers
  • Each layer only uses services of layer below

Pros: Easy to debug, modular
Cons: Difficult to define layers, performance overhead

3. Microkernel Structure

  • Minimal kernel with basic functions
  • Other services run in user space
  • Example: Minix, QNX

Pros: Extensible, reliable, portable
Cons: Performance overhead due to message passing

4. Hybrid Structure

  • Combines monolithic and microkernel approaches
  • Examples: Windows NT, macOS

5. Exokernel Structure

  • Minimal kernel that provides hardware abstraction
  • Applications manage their own resources
  • Maximum flexibility

System Calls

System calls provide the interface between a process and the operating system.

Categories of System Calls

Category Examples
Process Control fork(), exec(), exit(), wait()
File Management open(), read(), write(), close()
Device Management ioctl(), read(), write()
Information Maintenance getpid(), alarm(), sleep()
Communication pipe(), shmget(), mmap()
Protection chmod(), chown(), umask()

How System Calls Work

  1. User program invokes system call
  2. Trap/interrupt generated (mode switch: user → kernel)
  3. OS identifies system call via number
  4. OS executes the system call
  5. Returns result to user program (mode switch: kernel → user)

User Mode vs Kernel Mode

Aspect User Mode Kernel Mode
Privilege Level Low (Ring 3) High (Ring 0)
Access Limited resources Full hardware access
Crash Impact Only process crashes Entire system can crash
Instructions Limited instruction set All instructions available

Mode Bit

  • 0 = Kernel mode
  • 1 = User mode
  • Hardware provides this mode bit for protection

Key Interview Questions

Q1: What is the difference between multiprogramming and multitasking?

Multiprogramming keeps multiple programs in memory to improve CPU utilization (no time sharing). Multitasking adds time slicing, giving each process a time quantum for interactive user experience.

Q2: Why do we need an operating system?

OS manages hardware resources, provides abstraction, enables multitasking, ensures security, and provides a user interface for interaction with the computer.

Q3: What is the difference between a program and a process?

A program is a passive entity (executable file on disk). A process is an active entity (program in execution with its own memory, registers, and state).

Q4: What is a system call?

A system call is the programmatic way for a process to request a service from the operating system's kernel.

Q5: Explain the difference between monolithic and microkernel architecture.

In monolithic architecture, all OS services run in kernel space as a single large executable. In microkernel architecture, only essential services run in kernel space while others run in user space, communicating via message passing.

Summary

OS Type Key Feature Example
Batch Jobs processed in batches IBM OS/360
Multiprogramming Multiple programs in memory Early mainframes
Time-sharing Time slicing for interactivity UNIX, Linux
Real-time Strict timing constraints VxWorks, FreeRTOS
Distributed Multiple computers as one Amoeba
Network Server-based resource sharing Windows Server
Mobile Touch-based, battery optimized Android, iOS