Operating Systems -ch1-F08

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Operating Systems

408351 Section 1

4 October 2009


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Chapter 1 Operating Systems 2

Chapter 1: Introduction

What is an Operating System?

What Operating Systems Do

Computer-System Organization

Computer-System Architecture

Operating-System Structure

Operating-System Operations

Process Management, Memory Management, Storage Management Protection and Security

Mainframe Systems

Desktop Systems

Multiprocessor Systems

Distributed Systems

Clustered System

Real -Time Systems

Handheld Systems

Special-Purpose Systems

Computing Environments


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Objectives

To provide a grand tour of the major operatingsystems components

To provide coverage of basic computer systemorganization


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Computer Software

Software

Application

Software

System

Software

Compilers Operating System Assembler

Virtual File I/O Device

Memory System Driver

Process

Management

LaTex Word Games


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What is an Operating System?

A program that acts as an intermediarybetween a user of a computer and thecomputer hardware.

Operating system goals:

Execute user programs and makesolving user problems easier.

Make the computer systemconvenient to use.

Use the computer hardware in anefficient manner.


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Computer System Components

1. Hardware provides basic computingresources (CPU, memory, I/O devices).

2. Operating system controls and coordinatesthe 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 solvethe computing problems of the users(compilers, database systems, video games,business programs).

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


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Four Components of a Computer System


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Operating System Definitions

No universally accepted definition Resource allocator manages and allocates resources.

Decides between conflicting requests for efficient andfair resource use

Control program Controls execution of programs to

prevent errors and improper use of the computer.

Kernel the one program running at all times (all elsebeing application programs).

As a virtual machine: Operating system provides a"new" machine. Allows many users to believe theyhave an entire piece of hardware to themselves


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Computer Startup

bootstrap program is loaded at power-up orreboot

Typically stored in ROM or EPROM, generallyknown as firmware

Initializates all aspects of system Loads operating system kernel and starts

execution


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Computer System Organization

Computer-system operation

One or more CPUs, device controllersconnect through common bus providingaccess to shared memory

Concurrent execution of CPUs and devicescompeting for memory cycles


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Computer-System Operation

I/O devices and the CPU can executeconcurrently.

Each device controller is in charge of aparticular device type.

Each device controller has a local buffer. CPU moves data from/to main memory to/from

local buffers

I/O is from the device to local buffer of

controller.Device controller informs CPU that it has

finished its operation by causing an interrupt.


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Why are operating systems important?

Use up a small percentage of the CPU time, but supports many

machines all the time. They perform more functions for more users than any other

program. They are necessary for any use of the computer.

When "the (operating) system" is down, the computer is down. They are used by many users. Lifetime - the systems remain

around longer than the programmers who wrote them. Complexity - the system must do difficult things.

Deal with ugly I/O devices, multiplexing-juggling act, handleerrors (hard!).

Asynchronous - must do several things at once. Handles interrupts, and must change what it is doing

thousands of times a second - and still get work done.

General purpose - must do many different things. Run Doom, Java, Fortran, Lisp, Trek, Databases, Web Servers,

etc. Everybody wants their stuff to run well. Need to adapt to changes very well: New devices, processors,

applications.


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Device-Status Table


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Storage Structure

Main memory only large storage media thatthe CPU can access directly.

Secondary storage extension of main memorythat provides large nonvolatile storage capacity.

Magnetic disks rigid metal or glass platterscovered with magnetic recording material

Disk surface is logically divided into tracks,which are subdivided into sectors.

The disk controllerdetermines the logicalinteraction between the device and thecomputer.


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Storage Hierarchy

Storage systemsorganized inhierarchy.

Speed

Cost

Volatility Caching copying

information intofaster storagesystem; main

memory can beviewed as a lastcache for secondarystorage.


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Caching

Important principle, performed at many levels in acomputer (in hardware, operating system,software)

Information in use copied from slower to fasterstorage temporarily

Faster storage (cache) checked first to determine ifinformation 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


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Performance of Various Levels of Storage

Movement between levels of storage hierarchycan be explicit or implicit


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Migration of Integer A from Disk to Register

Multitasking environments must be careful to use mostrecent value, no matter where it is stored in the storagehierarchy

Multiprocessor environment must provide cachecoherency in hardware such that all CPUs have the most

recent value in their cache

Distributed environment situation even more complex

Several copies of a datum can exist

Various solutions covered in Chapter 17


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Operating System Structure

Multiprogramming needed for efficiency

Single user cannot keep CPU and I/O devices busy at all times

Multiprogramming organizes jobs (code and data) so CPU alwayshas one to execute

A subset of total jobs in system is kept in memory

One job selected and run viajob scheduling

When it has to wait (for I/O for example), OS switches to another

job Timesharing (multitasking) is logical extension in which CPU

switches jobs so frequently that users can interact with each job whileit is running, creating interactive computing

Response time should be < 1 second

Each user has at least one program executing in memoryprocess

If several jobs ready to run at the same time CPU scheduling

If processes dont fit in memory, swapping moves them in andout to run

Virtual memory allows execution of processes not completely inmemory


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Memory Layout for Multiprogrammed System


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Operating-System Operations

Interrupt driven by hardware

Software error or request creates exception or trap

Division by zero, request for operating system service

Other process problems include infinite loop, processesmodifying each other or the operating system

Dual-mode operation allows OS to protect itself andother system components

User mode and kernel mode

Mode bit provided by hardware

Provides ability to distinguish when system isrunning user code or kernel code

Some instructions designated as privileged, onlyexecutable in kernel mode

System call changes mode to kernel, return fromcall resets it to user


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Common Functions of Interrupts

Interrupt transfers control to the interruptservice routine generally, through the interruptvector, which contains the addresses of all theservice routines.

Interrupt architecture must save the address ofthe interrupted instruction.

Incoming interrupts are disabledwhile anotherinterrupt is being processed to prevent a lostinterrupt.

A trap is a software-generated interrupt causedeither by an error or a user request.

An operating system is interruptdriven.


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Interrupt Handling

The operating system preserves the state of theCPU by storing registers and the program counter.

Determines which type of interrupt has occurred:

Polling (no information is passed with the interrupt,thus the CPU polls (asks) every device if it is the one whoinitiate the interrupt)

vectoredinterrupt system (the device who issuedthe interrupt inform the CPU by adding its ID to theinterrupt request)

Separate segments of code determine what actionshould be taken for each type of interrupt


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Interrupt Timeline


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I/O Structure

After I/O starts, control returns to user program

only upon I/O completion. Wait instruction idles the CPU until the next

interrupt

Wait loop (contention for memory access).

At most one I/O request is outstanding at a

time, no simultaneous I/O processing. After I/O starts, control returns to user program

without waiting for I/O completion.

System call request to the operating systemto allow user to wait for I/O completion.

Device-status table contains entry for each I/Odevice indicating its type, address, and state.

Operating system indexes into I/O device tableto determine device status and to modify tableentry to include interrupt.


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Two I/O Methods

Synchronous Asynchronous


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Transition from User to Kernel Mode

Timer to prevent infinite loop / process hoggingresources

Set interrupt after specific period

Operating system decrements counter

When counter zero generate an interrupt

Set up before scheduling process to regaincontrol or terminate program that exceedsallotted time


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Process Management

A process is a program in execution. It is a unit of work withinthe system. Program is apassive entity, process is an activeentity.

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 specifyinglocation 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 theprocesses / threads


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Process Management Activities

The operating system is responsible for thefollowing activities in connection with processmanagement:

Creating and deleting both user and systemprocesses

Suspending and resuming processes

Providing mechanisms for processsynchronization

Providing mechanisms for processcommunication

Providing mechanisms for deadlock handling


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Memory Management

All data in memory before and after processing

All instructions in memory in order to execute

Memory management determines what is inmemory when (placement and replacement)

Optimizing CPU utilization and computerresponse to users

Memory management activities

Keeping track of which parts of memory are currentlybeing used and by whom

Deciding which processes (or parts thereof) and datato move into and out of memory

Allocating and deallocating memory space as needed


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Storage Management

OS provides uniform, logical view of information storage Abstracts physical properties to logical storage unit - file

Each medium is controlled by device (i.e., disk drive, tapedrive) Varying properties include access speed, capacity, data-

transfer rate, access method (sequential or random)

File-System management

Files usually organized into directories

Access control on most systems to determine who canaccess what

OS activities include Creating and deleting files and directories

Primitives to manipulate files and dirs

Mapping files onto secondary storage

Backup files onto stable (non-volatile) storage media


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Mass-Storage Management

Usually disks used to store data that does not fit in mainmemory or data that must be kept for a long period oftime.

Proper management is of central importance

Entire speed of computer operation hinges on disk

subsystem and its algorithms OS activities

Free-space management

Storage allocation

Disk scheduling

Some storage need not be fast Tertiary storage includes optical storage, magnetic tape

Still must be managed

Varies between WORM (write-once, read-many-times) and RW(read-write)


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I/O Subsystem

One purpose of OS is to hide peculiarities ofhardware devices from the user

I/O subsystem responsible for

Memory management of I/O including

buffering (storing data temporarily while itis being transferred), caching (storing partsof data in faster storage for performance),spooling (the overlapping of output of onejob with input of other jobs)

General device-driver interface

Drivers for specific hardware devices


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Protection and Security

Protection any mechanism for controlling access ofprocesses or users to resources defined by the OS

Security defense of the system against internal and externalattacks

Huge range, including denial-of-service, worms, viruses,identity theft, theft of service

Systems generally first distinguish among users, to determinewho can do what User identities (user IDs, security IDs) include name and

associated number, one per user

User ID then associated with all files, processes of that user todetermine access control

Group identifier (group ID) allows set of users to be defined andcontrols managed, then also associated with each process, file

Privilege escalation allows user to change to effective ID withmore rights


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Computing Environments

Traditional computer Blurring over time

Office environment

PCs connected to a network, terminals

attached to mainframe or minicomputersproviding batch and timesharing

Now portals allowing networked and remotesystems access to same resources

Home networks Used to be single system, then modems

Now firewalled, networked


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Computing Environments (Cont.)

Client-Server Computing Dumb terminals supplanted by smart PCs

Many systems now servers, responding to requests generated byclients

Compute-server provides an interface to client to requestservices (i.e. database)

File-server provides interface for clients to store and retrievefiles


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Peer-to-Peer Computing

Another model of distributed system

P2P does not distinguish clients and servers

Instead all nodes are considered peers

May each act as client, server or both

Node must join P2P network Registers its service with central lookup

service on network, or

Broadcast request for service and respond

to requests for service via discoveryprotocol

Examples include Napsterand Gnutella


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Web-Based Computing

Web has become ubiquitous

PCs most prevalent devices

More devices becoming networked to allow webaccess

New category of devices to manage web trafficamong similar servers: load balancers

Use of operating systems like Windows 95,client-side, have evolved into Linux and

Windows XP, which can be clients and servers


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1- 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

back to monitor


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2- Multiprogrammed Batch Systems

Several jobs are kept in main memory at the sametime, 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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3- Time-Sharing SystemsInteractive Computing

The CPU is multiplexed among several jobsthat are kept in memory and on disk (theCPU is allocated to a job only if the job is inmemory).

A job swapped in and out of memory to the

disk. On-line communication between the user

and the system is provided; when theoperating system finishes the execution ofone command, it seeks the next control

statement from the users keyboard.

On-line system must be available for usersto access data and code.


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5- Parallel Systems

Multiprocessor systems with more than on CPUin close communication.

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 an 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 allocatedwork to slave processors.

More common in extremely large systems


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Multiprocessor Operating Systems

ChallengesMutual exclusion

Ensure that only one process can access the

shared resource at one time

Cache coherence problem Occurs when a processor modifies data in its

Cache, the data is also cached by other processors


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6- Distributed Systems

Distribute the computation amongseveral physical processors.

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)

Distributed Operating Systems Provide transparencies of locations of resources, program

execution Challenge: the lack of shared memory, global clock, and

unpredictable delays May be either client-serverorpeer-to-peersystems.


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7-Clustered Systems

Clustering allows two or more systems to sharestorage.

Provides high reliability.

Asymmetric clustering: one server runs the

application while other servers are standby. Symmetric clustering: all N hosts are running

the application.


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Advanced Operating Systems

Database Operating System Supports transactions, concurrency control

and failure recovery

Real-time Operating System Ensures that jobs are completed by deadlines

Maximizes number of jobs satisfyingdeadlines


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8- Real-Time Systems

Often used as a control device in a dedicated

application such as controlling scientificexperiments, medical imaging systems,industrial control systems, and some displaysystems.

Well-defined fixed-time constraints. Real-Time systems may be either hardor

softreal-time.

Hard real-time system: Disaster occurs iftasks are not done by the deadlines

Soft real-time: Users are less satisfied

Ex. Video streaming servers


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9- Handheld Systems

Personal Digital Assistants (PDAs)

Cellular telephones

Issues:

Limited memory

Slow processors Small display screens.


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Advanced Operating Systems

1960-1970 OS designs focus on a stand-alone

computer with a single processor

Later OS designs focus on an advanced OS

Classification of a advanced OS is driven by

advances in high-speed architectures new applications

Advanced OSes

Architecture driven Application driven

Multiprocessorsystems Distributed systems Database systems Real-time systems


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Migration of Operating-System Concepts and Features


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Layered approach

divides the OS into several layers. Each layer has well-defined functions

Example: MULTICS Drawback: Functions must be carefully assigned to a

layer because a layer can only use functionality of thelayer directly beneath it.

Kernel-based approach There is a collection of primitives, the kernel, over

which the rest of the OS is built Example: Linux, Unix, Windows 2000

Virtual machine approach

Creates an illusion that the entire machine is at thesole disposal of each user; this illusion is created bytime-multiplexing of resources among all users.

Example: IBM VM/370; users run CMS (ConversationalMonitor System) OS on top of VM/370

OS Design Approaches


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Whats Next

Detailed discussion of the various topicsintroduced today in succeeding meetings

Please check the course homepage for news

and new lecture notes.

See you next time

Operating Systems -ch1-F08

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