🖲️Operating Systems Unit 1 – Introduction to Operating Systems

What's an Operating System Anyway?

• Serves as an interface between computer hardware and software applications
• Manages and coordinates computer resources (CPU, memory, storage, I/O devices)
• Provides a platform for running applications and executing user commands
• Ensures efficient utilization of system resources by allocating them to processes
• Handles low-level tasks (memory management, process scheduling, file system operations)
• Provides a layer of abstraction, hiding hardware complexities from users and developers
• Enables multiple applications to run concurrently on a single computer system
• Maintains system stability, security, and performance through various mechanisms

Key Components of an OS

• Kernel: Core component that manages system resources and facilitates communication between hardware and software
  • Runs in privileged mode with full access to hardware
  • Handles process scheduling, memory management, and I/O operations
• Device Drivers: Software modules that enable the OS to communicate with and control hardware devices
  • Provide a standardized interface for accessing devices (keyboards, mice, printers, network adapters)
  • Translate OS commands into device-specific instructions
• File System: Component responsible for organizing, storing, and retrieving data on storage devices
  • Defines a logical structure for storing files and directories
  • Handles file creation, deletion, reading, and writing operations
• User Interface: Provides a means for users to interact with the OS and applications
  • Can be a command-line interface (CLI) or graphical user interface (GUI)
  • Accepts user input and displays output from the system and applications
• System Libraries: Collection of pre-compiled code modules that provide common functions to applications
  • Offer a consistent API for developers to utilize OS features and services
• Shell: Interprets user commands and executes corresponding system functions or applications
  • Can be a command-line shell (Bash, PowerShell) or graphical shell (Windows Explorer, macOS Finder)

How Operating Systems Manage Resources

• Process Management: OS allocates CPU time to processes using scheduling algorithms
  • Creates and terminates processes as needed
  • Assigns priorities to processes based on their importance and resource requirements
• Memory Management: OS controls the allocation and deallocation of main memory (RAM) to processes
  • Employs techniques like paging and segmentation to optimize memory usage
  • Implements virtual memory to extend the available memory beyond physical RAM
• I/O Management: OS manages input/output operations between processes and devices
  • Schedules and prioritizes I/O requests to ensure fair access to shared resources
  • Buffers data to minimize the impact of slow I/O devices on system performance
• File System Management: OS organizes and manages files and directories on storage devices
  • Allocates disk space to files and keeps track of their locations
  • Enforces file access permissions and handles file sharing among processes
• Network Management: OS facilitates communication between processes on different computers over a network
  • Implements network protocols (TCP/IP) and manages network connections
  • Provides APIs for developers to create network-aware applications

Types of Operating Systems

• Batch Operating Systems: Designed to process a series of jobs (batches) without user interaction
  • Jobs are submitted to a queue and executed sequentially by the OS
  • Suitable for tasks that do not require real-time user input (data processing, scientific simulations)
• Time-Sharing Operating Systems: Allow multiple users to share computer resources simultaneously
  • Allocate CPU time to each user in small time slices, creating the illusion of concurrent execution
  • Enable interactive computing and remote access to shared resources
• Distributed Operating Systems: Manage resources across multiple interconnected computers
  • Provide a unified view of distributed resources, making them appear as a single system
  • Facilitate resource sharing, load balancing, and fault tolerance across nodes
• Real-Time Operating Systems (RTOS): Designed to meet strict deadlines for time-critical applications
  • Prioritize tasks based on their timing constraints and deterministic behavior
  • Used in embedded systems (automobiles, industrial control, medical devices)
• Mobile Operating Systems: Optimized for mobile devices (smartphones, tablets)
  • Designed to be energy-efficient and adapt to limited screen sizes and input methods
  • Provide a touch-friendly user interface and support mobile-specific features (GPS, accelerometer)

OS Architecture and Design

• Monolithic Architecture: All OS components are tightly integrated into a single large program
  • Kernel, device drivers, and system services run in the same memory space
  • Offers performance benefits due to direct communication between components
  • Difficult to maintain and modify due to high complexity and interdependencies
• Microkernel Architecture: Minimal kernel that provides only essential services (IPC, memory management)
  • Other OS components (device drivers, file systems) run as separate user-space processes
  • Enhances modularity, flexibility, and fault isolation at the cost of increased IPC overhead
• Layered Architecture: OS is divided into hierarchical layers, each with a well-defined interface
  • Lower layers provide services to upper layers, abstracting hardware details
  • Promotes modularity and simplifies development, but can impact performance due to layer boundaries
• Exokernel Architecture: Minimal kernel that provides low-level resource multiplexing
  • Exposes hardware resources directly to user-space applications through library operating systems (libOSes)
  • Allows applications to customize OS services based on their specific needs
  • Increases flexibility and performance but requires careful resource management by applications

Processes and Threads

• Process: An instance of a program in execution, with its own memory space and resources
  • Consists of code, data, and execution context (registers, stack, program counter)
  • Processes are isolated from each other to ensure system stability and security
• Thread: A lightweight unit of execution within a process, sharing the same memory space
  • Multiple threads can exist within a single process, executing concurrently
  • Threads share process resources (code, data, files) but have their own execution context
• Process States: A process can be in one of several states during its lifetime
  • New: Process is being created and initialized
  • Ready: Process is waiting to be assigned to a CPU for execution
  • Running: Process is currently executing on a CPU
  • Waiting (Blocked): Process is waiting for an event (I/O, synchronization) to occur
  • Terminated: Process has finished execution or has been forcefully terminated
• Context Switching: The act of saving the execution context of a process and loading another
  • Occurs when the OS decides to preempt a running process to allow another process to execute
  • Involves saving registers, stack pointer, and program counter of the current process
  • Restores the execution context of the next process to be run
• Inter-Process Communication (IPC): Mechanisms for processes to exchange data and synchronize their execution
  • Shared Memory: Processes access a common memory region to read and write data
  • Message Passing: Processes send and receive messages through OS-provided communication channels
  • Pipes: Unidirectional data channels that allow processes to stream data between them
  • Sockets: Bidirectional communication endpoints used for inter-process and network communication

Memory Management Basics

• Memory Hierarchy: Computer systems employ a hierarchy of memory devices with varying speeds and capacities
  • Registers: Fastest and most expensive memory, located within the CPU
  • Cache: Fast memory between the CPU and main memory, used to store frequently accessed data
  • Main Memory (RAM): Volatile memory that stores currently executing programs and data
  • Secondary Storage: Non-volatile storage devices (hard disks, SSDs) that store persistent data
• Memory Allocation: The process of assigning memory to processes as they require it
  • Static Allocation: Memory is allocated to processes at compile-time, before execution begins
  • Dynamic Allocation: Memory is allocated to processes during runtime, as needed
• Memory Partitioning: Techniques for dividing main memory into smaller, manageable units
  • Fixed Partitioning: Memory is divided into fixed-size partitions, each assigned to a single process
  • Variable Partitioning: Memory is divided into variable-size partitions based on process requirements
• Paging: A memory management scheme that divides physical memory into fixed-size pages
  • Logical memory (used by processes) is divided into pages, mapped to physical memory frames
  • Allows non-contiguous memory allocation and efficient memory utilization
• Segmentation: A memory management scheme that divides memory into variable-size segments
  • Each segment corresponds to a logical unit of a program (code, data, stack)
  • Provides a more natural and flexible memory model compared to paging
• Virtual Memory: A technique that allows processes to use more memory than physically available
  • Portions of a process's memory are swapped between main memory and secondary storage
  • Enables the execution of large programs and efficient memory utilization
• Memory Protection: Mechanisms to prevent processes from accessing unauthorized memory regions
  • Base and Limit Registers: Define the valid memory range for a process
  • Memory Management Unit (MMU): Hardware component that translates logical addresses to physical addresses
  • Access Control: Enforces read, write, and execute permissions for memory regions

File Systems and Storage

• File: A named collection of related data stored on a storage device
  • Identified by a unique name and file extension (document.txt, image.jpg)
  • Organized in a hierarchical directory structure for easy navigation and management
• Directory: A special type of file that contains information about other files and directories
  • Provides a logical structure for organizing and accessing files
  • Can contain subdirectories, forming a tree-like hierarchy
• File Metadata: Additional information about a file, stored alongside its data
  • Includes file name, size, creation/modification timestamps, access permissions, and file type
  • Used by the OS to manage and organize files efficiently
• File Operations: Basic operations supported by file systems for manipulating files and directories
  • Create: Create a new file or directory
  • Open: Open an existing file for reading, writing, or both
  • Read: Read data from an opened file
  • Write: Write data to an opened file
  • Close: Close an opened file, releasing associated resources
  • Delete: Remove a file or directory from the file system
• File Access Methods: Different ways in which data can be accessed and read from a file
  • Sequential Access: Data is accessed in a sequential manner, from beginning to end
  • Random Access: Data can be accessed in any order, by specifying an offset from the beginning of the file
• File Allocation Methods: Techniques used by file systems to allocate storage space for files
  • Contiguous Allocation: Each file occupies a contiguous block of storage, simplifying sequential access
  • Linked Allocation: Each file is stored as a linked list of disk blocks, allowing flexible space management
  • Indexed Allocation: An index table is used to store the addresses of disk blocks allocated to a file
• Directory Structures: Different ways of organizing directories in a file system
  • Single-Level Directory: All files are stored in a single directory, without any subdirectories
  • Two-Level Directory: Each user has their own directory, containing their files and subdirectories
  • Tree-Structured Directory: Directories are organized in a hierarchical tree structure, allowing nested subdirectories
• Disk Scheduling: Algorithms used by the OS to optimize disk I/O performance
  • First-Come, First-Served (FCFS): Requests are serviced in the order they arrive
  • Shortest Seek Time First (SSTF): Requests with the shortest seek time from the current head position are serviced first
  • SCAN (Elevator): The disk head moves in one direction, servicing requests along the way, then reverses direction
  • C-SCAN (Circular SCAN): Similar to SCAN, but the disk head moves in only one direction, then returns to the beginning

User Interface and Experience

• Command-Line Interface (CLI): A text-based interface where users interact with the OS using commands and parameters
  • Users type commands and receive text output from the OS
  • Provides direct control over the system and is often used by advanced users and system administrators
• Graphical User Interface (GUI): A visual interface that allows users to interact with the OS using graphical elements
  • Uses windows, icons, menus, and pointers (WIMP) to represent and manipulate system objects
  • Provides a more intuitive and user-friendly experience, suitable for novice and casual users
• Shell: A program that provides the user interface for interacting with the OS
  • Command-Line Shell: Interprets user commands and executes corresponding system functions or applications
  • Graphical Shell: Provides a visual interface for managing files, launching applications, and configuring system settings
• Window Manager: A component of the GUI that controls the placement and appearance of windows
  • Handles window creation, movement, resizing, and decoration (title bars, borders)
  • Coordinates the interaction between windows and the underlying OS
• Desktop Environment: A collection of software components that provide a complete GUI experience
  • Includes a window manager, file manager, application launcher, and system utilities
  • Examples include GNOME, KDE, and Microsoft Windows Desktop
• User Account Management: Mechanisms for creating, modifying, and deleting user accounts
  • Each user is assigned a unique identifier (username) and authenticates using a password
  • User accounts have associated permissions and resource limits, enforced by the OS
• Access Control: Methods for restricting access to system resources based on user identity and permissions
  • Discretionary Access Control (DAC): Resource owners determine access permissions for other users
  • Mandatory Access Control (MAC): The OS enforces access policies based on predefined security labels
  • Role-Based Access Control (RBAC): Users are assigned roles, which determine their access rights to resources
• User Experience Design: The process of designing OS interfaces to be intuitive, efficient, and visually appealing
  • Involves user research, interaction design, and usability testing
  • Aims to create interfaces that are easy to learn, navigate, and use, reducing cognitive load and increasing productivity