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					OPERATING SYSTEMS

    OVERVIEW



      CS301
 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.
                OPERATING SYSTEM
                   OVERVIEW
           WHAT IS AN OPERATING SYSTEM?

•   An interface between users and hardware - an environment "architecture”
•   Allows convenient usage; hides the tedious stuff
•   Allows efficient usage; parallel activity, avoids wasted cycles
•   Provides information protection
•   Gives each user a slice of the resources
•   Acts as a control program.




                                                                              3
          OPERATING       The Layers Of
                            A System
       SYSTEM OVERVIEW
Humans




Program Interface



User Programs


O.S. Interface

O.S.

Hardware Interface/
Privileged Instructions

Disk/Tape/Memory
                                      4
OPERATING SYSTEM                                       Components
   OVERVIEW
A mechanism for scheduling jobs or processes. Scheduling can be as simple
   as running the next process, or it can use relatively complex rules to pick
   a running process.

A method for simultaneous CPU execution and IO handling. Processing is
  going on even as IO is occurring in preparation for future CPU work.

Off Line Processing; not only are IO and CPU happening concurrently, but
   some off-board processing is occurring with the IO.




                                                                         5
OPERATING SYSTEM                                           Components
   OVERVIEW
The CPU is wasted if a job waits for I/O. This leads to:

    • Multiprogramming ( dynamic switching ). While one job waits for a
      resource, the CPU can find another job to run. It means that several jobs
      are ready to run and only need the CPU in order to continue.

CPU scheduling is the subject of Chapter 6.

All of this leads to:
     •     memory management
     •     resource scheduling
     •     deadlock protection

which are the subject of the rest of this course.

                                                                          6
    OPERATING SYSTEM
                                                                  Characteristics
       OVERVIEW
Other Characteristics include:
•   Time Sharing - multiprogramming environment that's also interactive.

•   Multiprocessing - Tightly coupled systems that communicate via shared memory. Used
    for scientific applications. Used for speed improvement by putting together a number of off-
    the-shelf processors.




•   Distributed Systems - Loosely coupled systems that communicate via message passing.
    Advantages include resource sharing, speed up, reliability, communication.




•   Real Time Systems - Rapid response time is main characteristic.         Used in control of
    applications where rapid response to a stimulus is essential.
                                                                                          7
OPERATING SYSTEM
                                                             Characteristics
   OVERVIEW
Interrupts:
•   Interrupt transfers control to the interrupt service routine generally, through the
    interrupt vector, which contains the addresses of all the service routines.
•   Interrupt architecture must save the address of the interrupted instruction.
•   Incoming interrupts are disabled while another interrupt is being processed to prevent
    a lost interrupt.
•   A trap is a software-generated interrupt caused either by an error or a user request.
•   An operating system is interrupt driven.




                                                                                     8
OPERATING SYSTEM       Hardware
   OVERVIEW             Support

   These are the
 devices that make
up a typical system.




    Any of these
 devices can cause
    an electrical
interrupt that grabs
the attention of the
        CPU.



                                  9
OPERATING SYSTEM   Hardware
   OVERVIEW         Support

 Sequence
 of events
     for
processing
    an IO
  request.



 Comparing
Synchronous
     and
Asynchronous
IO Operations




                              10
OPERATING SYSTEM   Hardware
   OVERVIEW         Support
                         This is O.S.
                    Bookkeeping. These
                        structures are
                      necessary to keep
                   track of IO in progress.




                                   11
         Computer-System
           Architecture
• Most systems use a single general-
  purpose processor (PDAs through
  mainframes)
  • Most systems have special-purpose
    processors as well
• Multiprocessors systems growing in use
  and importance
  • Also known as parallel systems, tightly-
    coupled systems
Symmetric Multiprocessing Architecture
A Dual-Core Design
        Clustered Systems
• Like multiprocessor systems,                 but
  multiple systems working together
  • Usually sharing storage via a storage-area
    network (SAN)
  • Provides a high-availability service which
    survives failures
    • Asymmetric clustering has one machine in hot-
      standby mode
    • Symmetric clustering has multiple nodes
      running applications, monitoring each other
OPERATING SYSTEM                                                  Storage
   OVERVIEW                                                      Hierarchy
Very fast storage is very expensive. So the Operating System manages a hierarchy of
storage devices in order to make the best use of resources. In fact, considerable effort
goes into this support.

Fast and Expensive




   Slow an Cheap

                                                                                  16
OPERATING SYSTEM    Storage
   OVERVIEW        Hierarchy
Performance:




                           17
OPERATING SYSTEM                                       Storage
   OVERVIEW                                           Hierarchy
Caching:
•Important principle, performed at many levels in a computer (in hardware,
operating system, software)
•Information in use copied from slower to faster storage temporarily
•Faster storage (cache) checked first to determine if information is there
     • If it is, information used directly from the cache (fast)
     • If not, data copied to cache and used there
•Cache smaller than storage being cached
     • Cache management important design problem
     • Cache size and replacement policy




                                                                     18
OPERATING SYSTEM                                      Protection
   OVERVIEW
The goal is protecting the Operating System and
others from malicious or ignorant users.

The User/Supervisor      Mode    and    privileged
instructions.

Concurrent threads might interfere with others.
This leads to protection of resources by
user/supervisor mode. These resources include:

    I/O Define I/O instructions as privileged; they
         can be executed only in Supervisor
         mode. System calls get us from user to
         supervisor mode.




                                                               19
OPERATING SYSTEM                                   Protection
   OVERVIEW
 Memory       A user program can only access its own logical memory. For
    instance, it can't modify supervisor code.   Depends on an address
    translation scheme such as that shown here.




                                                                  20
OPERATING SYSTEM                                        Protection
   OVERVIEW
 CPU         A clock prevents programs from using all the CPU time. This
    clock causes an interrupt that causes the operating system to gain control
    from a user program.


 For machines connected together, this protection must extend across:
     Shared resources,
     Multiprocessor Architectures,
     Clustered Systems
 The practice of this is called “distributed operating systems”.




                                                                        21
      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
         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
•
              Systems (Cont.)
    Parallel multiprocessing (SMP)
    Symmetric
    • 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.
Symmetric Multiprocessing Architecture
    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.
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.
•
                    Systems
       Real-Timecontrol device in a
    Often used as 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.
    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
Operating System
Concepts
                   Handheld Systems
• Personal Digital Assistants (PDAs)
• Cellular telephones
• Issues:
     • Limited memory
     • Slow processors
     • Small display screens.


Operating System
Concepts
•
          Process Management
    A process is a program in execution. It is a unit of work within the system.
    Program is a passive entity, process is an active entity.
•   Process needs resources to accomplish its task
     • CPU, memory, I/O, files
     • Initialization data
•   Process termination requires reclaim of any reusable resources

•   Single-threaded process has one       program counter
    specifying location of next instruction to execute
     • Process executes instructions sequentially, one at a time, until
        completion
•   Multi-threaded process has one program counter per thread
•   Typically system has many processes, some user, some operating system
    running concurrently on one or more CPUs
     • Concurrency by multiplexing the CPUs among the processes / threads
     Process Management
           Activities
The operating system is responsible for
  the following activities in connection
  with process management:
• Creating and deleting both user and
  system processes
• Suspending and resuming processes
• Providing mechanisms for process
  synchronization
• Providing mechanisms for process
    Open-Source Operating
          Systems
• Operating systems made available in
  source-code format rather than just
  binary closed-source
• Counter to the copy protection and
  Digital Rights Management (DRM)
  movement
• Started by Free Software Foundation
  (FSF), which has “copyleft” GNU Public
  License (GPL)

				
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Description: operating system lecture