What is an Operating System? GATE Computer Science

What is an Operating System?

Definition

An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. It is an essential component of the system software in a computer system. Without an operating system, a user cannot run application programs on their computer.

Functions of an Operating System

1. Process Management

Definition: Process management involves handling the creation, scheduling, and termination of processes. It ensures efficient execution of processes, resource allocation, and process isolation.

Key Functions:

• Process Creation and Termination:
  • Processes are created through system calls such as fork() in UNIX-based systems.
  • The OS allocates resources like memory, CPU time, and I/O devices to the newly created process.
  • Processes are terminated when they complete their execution or are explicitly killed.
• Process Scheduling:
  • The OS uses scheduling algorithms to determine the order of process execution.
  • Scheduling Algorithms:
    • First-Come, First-Served (FCFS): Processes are executed in the order they arrive.
    • Shortest Job Next (SJN): The process with the shortest execution time is selected next.
    • Round Robin (RR): Each process is given a fixed time slice in rotation.
    • Priority Scheduling: Processes are assigned priorities, and the highest priority process is executed first.
• Synchronization:
  • Ensures that multiple processes can operate safely without interfering with each other.
  • Mechanisms:
    • Semaphores: Used to signal and wait operations for process synchronization.
    • Monitors: High-level synchronization construct that allows safe access to shared resources.
• Inter-process Communication (IPC):
  • Processes need to communicate to share data and synchronize their actions.
  • Methods:
    • Shared Memory: Multiple processes can access a common memory space.
    • Message Passing: Processes exchange messages through the OS.
    • Pipes: Unidirectional data channels used for communication between related processes.
• Deadlock Handling:
  • Deadlock: A situation where two or more processes are unable to proceed because each is waiting for the other to release resources.
  • Techniques:
    • Deadlock Prevention: Ensuring that at least one of the necessary conditions for deadlock does not hold.
    • Deadlock Avoidance: Dynamically examining the resource-allocation state to ensure that there can never be a circular-wait condition.
    • Deadlock Detection: The OS periodically checks for deadlocks and takes action to resolve them.
    • Deadlock Recovery: Once a deadlock is detected, the OS can recover by terminating one or more processes or preempting resources.

Complex Concepts:

• Process Control Block (PCB): Contains information about a process, such as its state, program counter, CPU registers, memory management information, and I/O status.
• Context Switching: The process of saving the state of a currently running process and loading the state of the next process to be executed.

2. Memory Management

Definition: Memory management involves the allocation and deallocation of memory space to various programs to ensure efficient memory utilization and isolation.

Key Functions:

• Memory Allocation and Deallocation:
  • The OS allocates memory to processes when they are created and deallocates it when they terminate.
  • Dynamic Allocation Techniques:
    • First Fit: Allocates the first hole that is big enough.
    • Best Fit: Allocates the smallest hole that is big enough.
    • Worst Fit: Allocates the largest hole.
• Memory Tracking:
  • The OS keeps track of each byte in memory to ensure efficient use.
  • Memory Tables:
    • Free List: Keeps track of free memory blocks.
    • Allocation Table: Keeps track of allocated memory blocks.
• Virtual Memory:
  • Extends physical memory by using disk space, allowing larger processes to run.
  • Paging: Divides the process’s virtual memory into fixed-size pages and the physical memory into frames.
  • Segmentation: Divides the process’s memory into variable-sized segments.
• Page Replacement Algorithms:
  • FIFO (First-In-First-Out): Replaces the oldest page.
  • LRU (Least Recently Used): Replaces the page that has not been used for the longest period.
  • Optimal: Replaces the page that will not be used for the longest period in the future.
• Memory Protection:
  • Ensures that a process cannot access memory allocated to another process.
  • Protection Mechanisms:
    • Base and Limit Registers: Define the range of legal addresses a process may access.
    • Paging and Segmentation: Use protection bits associated with each frame or segment.

Complex Concepts:

• Translation Lookaside Buffer (TLB): A cache used to reduce the time taken to access the page table in a paging system.
• Thrashing: A condition where excessive paging operations are causing a significant slowdown in system performance.

3. File System Management

Definition: File system management involves the organization, storage, retrieval, naming, sharing, and protection of files on a computer.

Key Functions:

• File Operations:
  • The OS handles the creation, deletion, reading, and writing of files.
  • File Attributes: Metadata about files, such as name, type, size, creation date, and permissions.
• Directory Management:
  • Directories provide a hierarchical structure to organize files.
  • Directory Operations:
    • Creation and Deletion: Handles adding and removing directories.
    • Navigation: Allows users to traverse the directory structure.
    • Search: Enables finding files within directories.
• File Access Control:
  • Ensures that only authorized users can access certain files.
  • Access Control Mechanisms:
    • Access Control Lists (ACLs): Specify which users or system processes can access objects.
    • File Permissions: Define read, write, and execute permissions for files.
• File System Implementation:
  • File Allocation Methods:
    • Contiguous Allocation: Files are stored in contiguous blocks.
    • Linked Allocation: Files are stored in linked blocks scattered throughout the disk.
    • Indexed Allocation: Uses an index block to keep track of the blocks allocated to a file.
• Free Space Management:
  • Tracks available disk space to allocate to new files.
  • Techniques:
    • Bit Map: Uses a bit array to represent free and allocated blocks.
    • Linked List: Uses a linked list of free blocks.

Complex Concepts:

• File System Mounting: The process of making a file system accessible at a certain point in the directory structure.
• Journaling File Systems: Keep a log of changes that will be made to the file system to ensure data integrity in case of a crash.

4. Device Management

Definition: Device management involves controlling and coordinating the use of hardware devices.

Key Functions:

• Device Drivers:
  • Specific software modules that allow the OS to communicate with hardware devices.
  • Functions of Device Drivers:
    • Initialization: Setting up the device and configuring it.
    • Data Transfer: Reading from and writing to the device.
    • Error Handling: Detecting and managing errors.
• Device Communication:
  • Manages interaction between the system and peripheral devices.
  • I/O Control Methods:
    • Programmed I/O: The CPU actively waits for I/O operations to complete.
    • Interrupt-Driven I/O: The CPU is interrupted when the I/O operation completes.
    • Direct Memory Access (DMA): Allows devices to transfer data directly to/from memory without CPU involvement.
• Device Scheduling:
  • Allocates device time efficiently among various processes.
  • Disk Scheduling Algorithms:
    • FCFS (First-Come, First-Served): Requests are processed in the order they arrive.
    • SSTF (Shortest Seek Time First): Selects the request with the shortest seek time.
    • SCAN: The disk arm moves in one direction, fulfilling requests until it reaches the end, then reverses direction.

Complex Concepts:

• Buffering and Caching:
  • Buffering: Temporarily stores data while it is being transferred.
  • Caching: Stores frequently accessed data in a faster storage medium.

5. Security and Access Control

Definition: Security and access control involve protecting data and resources from unauthorized access and ensuring data integrity.

Key Functions:

• User Authentication:
  • Verifies the identity of users before granting access.
  • Methods:
    • Passwords: Secret words or phrases used to authenticate.
    • Biometric Authentication: Uses unique biological characteristics.
    • Two-Factor Authentication: Combines two methods of authentication.
• Access Control:
  • Implements policies to control who can access specific resources.
  • Access Control Models:
    • Discretionary Access Control (DAC): The owner of the resource sets permissions.
    • Mandatory Access Control (MAC): The system enforces access policies based on security labels.
    • Role-Based Access Control (RBAC): Access permissions are based on user roles.
• Data Encryption:
  • Protects data by converting it into a secure format that is only accessible to authorized users.
  • Encryption Techniques:
    • Symmetric Encryption: Uses the same key for encryption and decryption.
    • Asymmetric Encryption: Uses a pair of keys – public and private – for encryption and decryption.

Complex Concepts:

• Intrusion Detection Systems (IDS): Monitors network or system activities for malicious activities or policy violations.
• Security Policies: Rules that govern the protection of information and information systems.

6. User Interface

Definition: The user interface is the means by which users interact with the operating system.

Key Functions:

• Command-Line Interface (CLI):
  • Allows users to interact with the OS by typing commands.
  • Examples: Bash, PowerShell.
• Graphical User Interface (GUI):
  • Provides a visual interface with graphical elements like windows, icons, and menus.
  • Examples: Windows OS, macOS, GNOME.

Complex Concepts:

• Shells: Command interpreters that execute user commands. Common shells include Bourne shell, C shell, and Korn shell.
• Window Managers: Software that manages the placement and appearance of windows within a GUI.

Types of Operating Systems

1. Batch Operating Systems

Definition: Batch operating systems execute jobs in batches without user interaction.

Key Characteristics:

• Jobs with similar needs are batched together and executed simultaneously.
• The OS collects jobs, executes them, and returns the results without human intervention.

Example: Early IBM mainframe systems.

Complex Concepts:

• Job Scheduling: Determines the order in which jobs are executed.
• Spooling: Simultaneous peripheral operations with online job entry.

2. Time-Sharing Operating Systems

Definition: Time-sharing systems allow multiple users to access the system simultaneously by sharing CPU time.

Key Characteristics:

• The CPU switches between user programs so rapidly that it appears each user has their own dedicated system.
• Time slices are allocated to each user, and context switching is used to manage multitasking.

Example: UNIX.

Complex Concepts:

• Multitasking: Running multiple tasks (processes) simultaneously.
• Context Switching: The CPU switches from one process to another, saving the state of the current process and loading the state of the next process.

3. Distributed Operating Systems

Definition: Distributed operating systems manage a group of distinct computers and make them appear as a single computer.

Key Characteristics:

• Distributed systems share resources and data processing tasks across multiple computers.
• Provides fault tolerance, scalability, and resource sharing.

Example: LOCUS.

Complex Concepts:

• Distributed File Systems: Allows files to be accessed and managed across multiple computers.
• Remote Procedure Calls (RPCs): Enables a program to cause a procedure to execute in another address space.

4. Network Operating Systems

Definition: Network operating systems provide services to computers connected to a network.

Key Characteristics:

• Manages network resources such as file servers, printers, and network security.
• Supports multiple users connected to a local area network (LAN) or wide area network (WAN).

Example: Microsoft Windows Server.

Complex Concepts:

• Network Protocols: Rules that govern data communication over a network (e.g., TCP/IP, HTTP).
• Client-Server Architecture: A network architecture where clients request services, and servers provide them.

5. Real-Time Operating Systems (RTOS)

Definition: RTOS provide strict timing constraints for processing, ensuring timely task completion.

Key Characteristics:

• Designed for applications requiring precise timing and high reliability.
• Tasks are prioritized, and high-priority tasks preempt lower-priority ones.

Example: VxWorks, RTLinux.

Complex Concepts:

• Hard Real-Time Systems: Systems where failing to meet timing constraints leads to catastrophic consequences.
• Soft Real-Time Systems: Systems where timing constraints are important but not critical.

6. Mobile Operating Systems

Definition: Mobile operating systems are designed specifically for mobile devices.

Key Characteristics:

• Optimized for touch inputs, power management, and mobile connectivity.
• Supports mobile applications and provides a platform for app developers.

Example: Android, iOS.

Complex Concepts:

• Mobile Security: Measures to protect mobile devices from threats.
• Mobile App Ecosystem: The marketplace and infrastructure for mobile applications (e.g., Google Play Store, Apple App Store).