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Introduction to Basic OS Concepts. Introduction. What is an Operating System? Mainframe Systems Desktop Systems Multiprocessor Systems Distributed Systems Clustered System Real -Time Systems Handheld Systems Computing Environments. What is an Operating System?.

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Introduction to Basic OS Concepts

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Introduction to basic os concepts l.jpg

Introduction to Basic OS Concepts

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  • What is an Operating System?

  • Mainframe Systems

  • Desktop Systems

  • Multiprocessor Systems

  • Distributed Systems

  • Clustered System

  • Real -Time Systems

  • Handheld Systems

  • Computing Environments

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What is an Operating System?

  • A program that acts as an intermediary between a user of a computer and the computer hardware.

  • Operating system goals:

    • Execute user programs and make solving user problems easier.

    • Make the computer system convenient to use.

  • Use the computer hardware in an efficient manner.

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What is OS?

  • Computer systems typically contain:Hardware and SoftwareHardware - electronic, mechanical, optical devicesSoftware – programs

  • Without support software, the computer is of little use..

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What is OS?

  • An interface between Hardware and User Programs

  • An abstraction of the hardware for all the (user) processes

    • Hide the complexity of the underlying hardware and give the user a better view of the computer

  • => A MUST!

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Computer System Components

1.Hardware – provides basic computing resources (CPU, memory, I/O devices).

2.Operating system – controls and coordinates the use of the hardware among the various application programs for the various users.

3.Applications programs – define the ways in which the system resources are used to solve the computing problems of the users (compilers, database systems, video games, business programs).

4.Users (people, machines, other computers).

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Abstract View of System Components

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The OS

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Operating System Definitions

  • Resource allocator – manages and allocates resources.

  • Control program – controls the execution of user programs and operations of I/O devices .

  • Kernel – the one program running at all times (all else being application programs).

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The Goals of an OS

  • Let users run programs:

    • Correctness

      • Memory boundaries, priorities, steady state

    • Convenience

      • User should not handle the tiny details (encapsulate/abstract), provide synchronization primitives, system calls, file system, tools

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The Goals of an OS

  • Let users run programs:

    • Efficiency

      • Resource Utilization, resource Sharing, Multitasking

    • Fairness (in resource allocation)

      • Among: users, tasks, resources

      • The tradeoff between efficiency and fairness

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An OS is a Resource Allocator

“Mama says: It’s good to share!”

  • Multiple users (?) get all computing resources “simultaneously”:

    • Cpu time

    • Memory (ram, swap, working set, virtual,..)

    • File system (storage space)

    • I/O devices (display, printers, mouse,..)

    • Clock

  • The OS should give every user the illusion that she is getting all resources to herself (not sharing!)

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What an OS does for a living..

loop forever {

run the process for a while.

stop process and save its state.

load state of another process.


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Virtual Continuity

  • A process can get “switched in” or “switched out”.

  • OS should give the illusion for the process as if it exists in the CPU continuously=> Context Switching

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Context switching

  • When an eventoccurs, the operating system saves the state of the active process and restores the state of the new process.

  • This mechanism is called a Context Switch.

  • What must get saved? Everything that the next process could or will damage. For example:

    • Program counter (PC)

    • Program status word (PSW)

    • CPU registers (general purpose, floating-point)

    • File access pointer(s)

    • Memory (perhaps?)

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Scheduling and Context switch

  • A process can give up the cpu:

    • A. by performing I/O (e.g. getchar())

    • B. by entering a waiting state (e.g. semaphore)

    • C. by entering a suspended state (e.g. sleep())

  • Give up the CPU == switch out the current process+switch in another process

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Preemptive Scheduling

  • There are OS’s where a process is forced to give up the cpu (e.g. when stayed for too long).

  • Such systems are implementing a “preemptive scheduling” policy.

    • Examples include Windows NT, Unix, - BUT NOT - Windows prior to Win95 ! or Macintosh!

    • Xinu?

      Should a real-time system implement preemptive scheduling?

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Using Priorities

  • Most OS’s provide the priority mechanism

  • Priorities are associated with processes

  • Priority are used to help the OS to reach fairness

    Can you think of processes (e.g. in Windows) for which you willgive especially high/low priority ??

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  • A process is a program in execution.

  • The components of a process are:

    • the program to be executed,

    • the data on which the program will execute,

    • the resources required by the program—such as memory and file(s)—and

    • the status of the execution.

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תהליכים מקבילים







Process Interleaving

תהליכים עוקבים

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Mainframe Systems

  • Reduce setup time by batching similar jobs

  • Automatic job sequencing – automatically transfers control from one job to another. First rudimentary operating system.

  • Resident monitor

    • initial control in monitor

    • control transfers to job

    • when job completes control transfers pack to monitor

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Memory Layout for a Simple Batch System

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Multiprogrammed Batch Systems

Several jobs are kept in main memory at the same time, and the

CPU is multiplexed among them.

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OS Features Needed for Multiprogramming

  • I/O routine supplied by the system.

  • Memory management – the system must allocate the memory to several jobs.

  • CPU scheduling – the system must choose among several jobs ready to run.

  • Allocation of devices.

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Time-Sharing Systems–Interactive Computing

  • The CPU is multiplexed among several jobs that are kept in memory and on disk (the CPU is allocated to a job only if the job is in memory).

  • A job swapped in and out of memory to the disk.

  • On-line communication between the user and the system is provided; when the operating system finishes the execution of one command, it seeks the next “control statement” from the user’s keyboard.

  • On-line system must be available for users to access data and code.

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Desktop Systems

  • Personal computers – computer system dedicated to a single user.

  • I/O devices – keyboards, mice, display screens, small printers.

  • User convenience and responsiveness.

  • Can adopt technology developed for larger operating system’ often individuals have sole use of computer and do not need advanced CPU utilization of protection features.

  • May run several different types of operating systems (Windows, MacOS, UNIX, Linux)

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Parallel Systems

  • Multiprocessor systems with more than on CPU in close communication.

  • Tightly coupled system – processors share memory and a clock; communication usually takes place through the shared memory.

  • Advantages of parallel system:

    • Increased throughput

    • Economical

    • Increased reliability

      • graceful degradation

      • fail-soft systems

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Parallel Systems (Cont.)

  • Symmetric multiprocessing (SMP)

    • Each processor runs and identical copy of the operating system.

    • Many processes can run at once without performance deterioration.

    • Most modern operating systems support SMP

  • Asymmetric multiprocessing

    • Each processor is assigned a specific task; master processor schedules and allocated work to slave processors.

    • More common in extremely large systems

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Symmetric Multiprocessing Architecture

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Distributed Systems

  • Distribute the computation among several physical processors.

  • Loosely coupled system – each processor has its own local memory; processors communicate with one another through various communications lines, such as high-speed buses or telephone lines.

  • Advantages of distributed systems.

    • Resources Sharing

    • Computation speed up – load sharing

    • Reliability

    • Communications

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Distributed Systems (cont)

  • Requires networking infrastructure.

  • Local area networks (LAN) or Wide area networks (WAN)

  • May be either client-server or peer-to-peer systems.

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General Structure of Client-Server

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Clustered Systems

  • Clustering allows two or more systems to share storage.

  • Provides high reliability.

  • Asymmetric clustering: one server runs the application while other servers standby.

  • Symmetric clustering: all N hosts are running the application.

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Real-Time Systems

  • Often used as a control device in a dedicated application such as controlling scientific experiments, medical imaging systems, industrial control systems, and some display systems.

  • Well-defined fixed-time constraints.

  • Real-Time systems may be either hard or soft real-time.

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Real-Time Systems (Cont.)

  • Hard real-time:

    • Secondary storage limited or absent, data stored in short term memory, or read-only memory (ROM)

    • Conflicts with time-sharing systems, not supported by general-purpose operating systems.

  • Soft real-time

    • Limited utility in industrial control of robotics

    • Useful in applications (multimedia, virtual reality) requiring advanced operating-system features.

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Handheld Systems

  • Personal Digital Assistants (PDAs)

  • Cellular telephones

  • Issues:

    • Limited memory

    • Slow processors

    • Small display screens.

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Migration of Operating-System Concepts and Features

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Computing Environments

  • Traditional computing

  • Web-Based Computing

  • Embedded Computing

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Let’s fillin’ the bits..

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