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
• Process management mechanisms in early systems

  • Manual loading mechanism
  • Batch processing mechanism
  • Job Control Language (JCL)
• 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)
  • Multiprogramming with variable tasks (MVT)
• Memory partition

  Area occupied by each job residing simultaneously in the main memory
• In a multiprogramming system, multiple jobs are executed concurrently with the CPU switching between them
• Requirements of multiprogramming systems

  • Large memory
  • Memory protection
  • Job status preservation
  • Proper job mix (CPU and I/O bound jobs)
  • CPU scheduling
• Process Control Block (PCB)

  Used to preserve the job status of each loaded process in a multiprogramming system
• Multitasking

  Interleaved execution of multiple jobs (often referred to as tasks of same user) in a single-user system
• Thread

  Basic unit of CPU utilization, threads share a CPU in the same way as processes do
• All threads of a process share the same set of operating system resources and 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
• Multiprocessing

  System with two or more CPUs having ability to execute multiple processes concurrently
• Types of multiprocessing

  • Tightly-coupled
  • Loosely-coupled
• Multiprocessing allows multiple CPUs to process either instructions from different and independent programs or different instructions from the same program simultaneously
• Simultaneous interactive use of a computer system by many users is called time-sharing
• Tightly-coupled multiprocessing

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

  Each processor has its own local memory
• 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
• 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


  A collection of related information with a name, data and attributes
• File Management


  Takes care of file-related activities such as structuring, accessing, naming, sharing, and protection of files
• File Access Methods


  • Sequential access
  • Random access
• File Naming


  Rules for naming files in an operating system, including maximum number of characters, allowed special characters, case sensitivity, and file extensions