OS

• Operating system


  Integrated set of programs that controls the resources (the CPU, memory, I/O devices, etc.) of a computer system
• Operating system

  • Provides its users with an interface or virtual machine that is more convenient to use than the bare machine
  • Two primary objectives are: making a computer system convenient to use and managing the resources of a computer system
• The operating system layer hides the details of the hardware from the programmer and provides the programmer with convenient interface for using the system
• Main functions of an operating system

  • Process management
  • Memory management
  • File management
  • Security
  • Command interpretation
• Throughput

  Amount of work that the system is able to do per unit time
• Turnaround time

  Interval from the time of submission of a job to the system for processing to the time of completion of the job
• Response time

  Interval from the time of submission of a job to the system for processing to the time the first response for the job is produced by the system
• Process

  A program in execution
• Process management


  • Manages the processes submitted to a system in a manner to minimize idle time of processors (CPUs, I/O processors, etc.) of the system
• Manual loading mechanism

  Jobs were manually loaded one after another in a computer by the computer operator
• Batch processing mechanism

  Batch of jobs was submitted together to the computer and job-to-job transition was done automatically by the operating system
• Job Control Language (JCL)

  Control statements were used to facilitate job loading and unloading
• Uniprogramming

  Only one job is processed at a time and all system resources are available exclusively for the job until its completion
• Multiprogramming


  Interleaved execution of two or more different and independent programs by a computer
• Types of Multiprogramming

  • Multiprogramming with fixed tasks (MFT): Fixed number of jobs can be processed concurrently
  • Multiprogramming with variable tasks (MVT): Number of jobs can vary
• Memory partition


  Area occupied by each job residing simultaneously in the main memory
• Types of jobs

  • CPU bound: Mostly perform computations with little I/O operations
  • I/O bound: Mostly perform I/O operations with little computation
• Uniprogramming system

  • Only one job is processed by the system at a time and all the system resources are exclusively available for the job until it completes
• Multiprogramming system

  • Interleaved execution of multiple jobs (often referred to as tasks of same user) in a single-user system
  • Computer systems used for multitasking are uniprocessor systems (having only one CPU)
  • Treated differently from multiprogramming that refers to interleaved execution of multiple jobs in a multi-user system
• Thread

  Basic unit of CPU utilization. Threads share a CPU in the same way as processes do
• Multithreading

  • All threads of a process also share the same set of operating system resources
  • All threads of a process inherit parent's address space and security parameters
  • Each thread of a process has its own program counter, its own register states, and its own stack
• Tightly-coupled multiprocessing


  Single system-wide primary memory shared by all processors
• Loosely-coupled multiprocessing


  Each processor has its own local memory
• Multiprocessing is a system with two or more CPUs having ability to execute multiple processes concurrently
• Simultaneous interactive use of a computer system by many users is called multiuser
• Time-sharing

  Simultaneous interactive use of a computer system by many users in such a way that each one feels that he/she is the sole user of the system
• Advantages of Time-sharing Systems

  • Reduces CPU idle time
  • Provides advantages of quick response time
  • Offers good computing facility to small users
• Memory Management

  • Keeps track of parts of memory in use and parts not in use
  • Allocates memory to processes as needed and deallocates when no longer needed
• Uniprogramming Memory Model

  Used in systems that process one job only at a time, and all system resources are available exclusively for the job until it completes
• Multiprogramming with Fixed Number of Memory Partition


  User area of the memory is divided into a number of fixed-sized partitions
• Multiprogramming with Variable Number of Memory Partitions

  Number, size and location of the partitions vary dynamically as processes come and go
• Virtual Memory

  Memory management scheme that allows execution of processes that might not be completely loaded in the main memory
• Virtual Memory Realization

  1. On-line secondary storage

  2. Swapping

  3. Demand paging
• Advantages of Virtual Memory

  • Provides a large virtual memory to programmers on a system having smaller physical memory
  • Enables execution of a process on a system whose main memory size is less than the total memory required by the process
  • Enables a process's execution to be started even when sufficient free memory for loading the entire process is not available
  • Makes programming easier there no longer need to worry about the memory size limitations
  • Often leads to less I/O activity resulting in better throughput, turnaround time, and response time
• Disadvantages of Virtual Memory

  • Difficult to implement because it requires algorithms to support demand paging
  • If used carelessly, it may substantially decrease performance due to high page fault rate
• File Management

  • File's name uniquely identifies it in the system and is used by its users to access it
  • File's data is its contents
  • File's attributes contain information such as date & time of its creation, date & time of last access, date & time of last update, its current size, its protection features, etc.
• File Access Methods

  • Sequential access: Information stored in a file can be accessed sequentially (in the order in which they are stored, starting at the beginning)
  • Random access: Information stored in a file can be accessed randomly irrespective of the order in which the bytes or records are stored
• External security

  Deals with securing computer against external factors such as fires, floods, earthquakes, stolen disks/tapes, etc. by maintaining adequate backup, using security guards, allowing access to sensitive information to only trusted employees/users
• Internal security

  Deals with user authentication, access control, and cryptography mechanisms
• User authentication

  Deals with the problem of verifying the identity of a user (person or program) before permitting access to the requested resource