Com sci

• Operating System


  A fundamental software component that serves as an intermediary between computer hardware and user applications
• Operating System


  A program that controls the execution of application programs and tells the CPU when to execute other programs
• Operating System


  An interface between the user and hardware that maximizes the efficiency of a system
• Functions of the Operating System


  • Provide User Interface
  • Control the Processor Hardware
• Operating System


  A program that acts as an interface between the user and the computer hardware and controls the execution of all kinds of programs
• Some popular Operating Systems


  • Linux Operating System
  • Windows Operating System
  • iOS
  • Android
• Types of Operating Systems


  • Single User OS
  • Batch OS
  • Multiprogramming OS
  • Multiprocessing OS
  • Multitasking OS
  • Real-time OS
  • Networking OS
  • Time-Sharing OS
• Single User OS


  • Allows only one user or process to access the system at a time
• Single User OS


  • MS DOS
• Batch OS


  • Jobs with similar requirements are grouped and executed as a group to speed up processing. The system put all the jobs in a queue based on first come first serve and then executes the jobs one by one.
• Multiprogramming OS


  • The CPU is always kept busy. Each process needs two types of system time: CPU time and IO time. When a process does its I/O, the CPU can start the execution of other processes. Improves the efficiency of the system.
• Multiprocessing OS


  • More than one processor is present and can execute more than one process at the same time. Parallel computing is achieved.
• Multitasking OS


  • Enables multiple programs to be executed simultaneously. Allows a user to perform more than one computer task at the same time.
• Real-time OS


  • Whenever response must closely match clock time. System must respond to an event within a well-defined time. Used in modern multimedia systems.
• Networking OS


  • Includes software and associated protocols to communicate with other computers via a network conveniently and cost-effectively.
• Time-Sharing OS


  • The CPU is switched among multiple programs given by different users on a scheduled basis. Allows many users to be served simultaneously, so sophisticated CPU scheduling schemes and Input/output management are required.
• Important functions of the Operating System


  • Coordination between other software and users
  • Process management
  • Processor Management
  • Device Management
  • Memory Management
  • File Management
  • Security
• BIOS (Basic Input Output System)


  It runs checks on all the hardware components and prepares them for use by the application software. It is firmware permanently stored in a computer chip (ROM) and not part of the Operating System.
• Bootloader


  After the BIOS checks the system, it calls the bootloader program which looks on the hard-drive (or any attached drive) for bootable kernels. It is part of the Operating System.
• Kernel


  The part of the Operating system responsible for the interface between the application software and the hardware resources that it uses. It manages the use of the CPU, writing and reading data from RAM and hard drive, and the GPU.
• Bootstrapping


  The process of loading a set of instructions when a computer is first turned on or booted. Diagnostic tests are performed, configurations are set, and the bootloader or bootstrap program is then loaded to initialize the OS.
• Typical programs that load the OS


  • GNU Grand Unified Bootloader (GRUB)
  • NT Loader (NTLDR)
  • Linux Loader (LILO)
  • Network Interface Controller (NIC)
• Process


  A program in execution. It includes the program counter, stack, registers, and any other resources it's using.
• Process Control Block (PCB)


  A data structure created by the operating system to store information about a process, stored in kernel space.
• Attributes of a Process Control Block


  • Process-ID
  • Program Counter
  • Process State
  • Priority
  • General Purpose registers
  • List of open files
  • List of open devices
• Process Table


  PCBs are stored in the form of a LinkedList in the memory. The process image consists of program, data, stack, and attributes.
• Process States


  • New
  • Ready
  • Running
  • Blocked/Wait
  • Suspended ready
  • Completion/Termination
• Five-State Process Model


  Simplified model of the process states: New, Ready, Running, Exit, Blocked
• Process Creation


  1. Assign a unique process identifier

  2. Allocate space for the process

  3. Initialize process control block

  4. Set up appropriate linkage to the scheduling queue
• Process Scheduling


  The Operating system manages various types of queues for each of the process states. The PCB related to the process is also stored in the queue of the same state.
• Job Queue


  All the processes get stored in the job queue, maintained in the secondary memory. They then transition to the Ready queue.
• Ready Queue


  Ready queue is maintained in primary memory. The short-term scheduler picks the job from the ready queue and dispatches it to the CPU for execution.
• Wait Queue


  When the process needs some IO operation to complete its execution, the OS changes the state of the process from running to waiting.
• Process Scheduling Algorithms


  1. First Come First Serve

  2. Round Robin

  3. Shortest Job First

  4. Shortest remaining time first

  5. Priority based scheduling
• Interrupts


  Signals emitted by software or hardware when a process or event requires immediate attention. Tell the CPU to stop its current activities and execute the appropriate part of the operating system.
• Types of Interrupts


  • Hardware Interrupts
  • Software Interrupts
  • Traps
• Deadlock


  A situation where each of the computer processes waits for a resource which is being assigned to some another process, so none of the processes get executed.
• Strategies for handling Deadlock


  • Deadlock Ignorance
  • Deadlock prevention
  • Deadlock avoidance
• In this situation, none of the process gets executed since the resource it needs, is held by some other process which is also waiting for some other resource to be released
• Deadlock


  • Process 2 needs Resource 1 but Resource 1 is being used by Process 1
  • Process 1 needs Resource 2 but Resource 2 is being used by Process 2
  • No process releases the required resource for the other process, so a deadlock occurs