Introduction to Basic OS Concepts

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Introduction to Basic OS Concepts
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Introduction

• 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 containHardware 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
• gt 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 Its 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 continuouslygt Context
  Switching

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Context switching

• When an event occurs, 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 OSs 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 OSs 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 will give especially high/low
  priority ??

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Process

• 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 programsuch as
  memory and file(s)and
• the status of the execution.

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Process Interleaving
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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 SystemsInteractive 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 users
  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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The PC-XINU OS

• Lets fillin the bits..