os chapter i

7/29/2019 OS Chapter I

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Chapter 1 Introduction to OS

What is an Operating System?

Operating System is a Resource Manager. Handles multiple computer resources: CPU,

Internal/External memory, Processes, Tasks,Applications, Users, etc

Manages and allocates resources to multipleusers or multiple jobs running at the same time(e.g., processor time, memory space, I/Odevices)

Arranges to use the computer hardware in anefficient manner (maximize throughput,minimize response time) and in a fair manner.

It is a Control Program. Manages all the components of a complex

computer system in an integrated manner. Controls the execution of user programs and

I/O devices to prevent errors and improper useof the computer resources.

Looks over and protects the computer.

It is an extended/virtual machine

An interface between the user and hardwarethat hides the details of the hardware (e.g., I/O).

Constructs higher-level (virtual) resources outof lower-level (physical) resources (e.g., files).

Definition: Is a collection of softwareenhancements, executed on the barehardware, culminating in a high-level virtual

machine that serves as an advancedprogramming environment

Prepared by Dr. Amjad Mahmood 1.1


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Why Operating System?

Computer hardware is developed to execute userprograms and make solving user problems easier.

An operating system makes a computer moreconvenient to use.

It acts as an interface between user andcomputer hardware. Therefore, the end-usersare not particularly concerned with thecomputers architecture, and they view thecomputer system in terms of an application.

To programmers, it provides some basic utilitiesto assist him in creating programs, themanagement of files, and the control of I/Odevices.



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Operating System Objectives

Convenience

Makes the computer more convenient to use Efficiency

Allows computer system resources to be usedin an efficient manner

Ability to evolve

Permit effective development, testing, andintroduction of new system functions without

interfering with service

Services Provided by Operating Systems

Facilities for program creation Editors, compilers, linkers, debuggers, etc.

Program execution

Loading in memory, I/O and file initialization.

Access to I/O and files Deals with the specifics of I/O and file formats.

System access

Resolves conflicts for resource contention.

Protection in access to resources and data. Error detection and response

internal and external hardware errors

memory error

device failure

software errors

arithmetic overflow access forbidden memory locations



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operating system cannot grant request ofapplication

Accounting collect statistics

monitor performance

used to anticipate future enhancements

used for billing users

Computer System Components

A computer system can be divided in to fourcomponents.

The Hardware : Provides basic computingresources (CPU, memory, I/O devices).

The Operating System : Controls andcoordinates the use of the hardware among thevarious application programs for the varioususers.

The Application Programs : Define the ways inwhich the system resources are used to solvethe computing problems of the users(compilers, database systems, video games,

business programs).

The Users : Users (people, machines, othercomputers).



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These components can be viewed as layers, whereeach layer uses the services provided by the layerbeneath it.

A Static View of System Components

Dynamic View of System Components



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Another view of computer system components

History of Operating Systems

Lets see how operating systems evolve over time. This will help us to identify some common features

of operating systems and how and why thesesystems have been developed as they are.

Evolution of Operating Systems

Early Systems (1950) Simple Batch Systems (1960)

Multiprogrammed Batch Systems (1970)

Time-Sharing and Real-Time Systems (1970)

Personal/Desktop Systems (1980)

Multiprocessor Systems (1980)

Networked/Distributed Systems (1980)

Handheld Systems (1990)

Prepared by Dr. Amjad Mahmood

End

User

Programmer

Operating-

System

Designer

Computer Hardware

Operating-System

Utilities

Application

Programs

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Early Systems

Structure Single user system. Large machines run from

console. Programmer/User as operator.

Paper Tape or Punched cards.

No tapes/disks in computer.

Early software: Assemblers, Libraries of commonsubroutines, Device Drivers, Compilers, Linkers.

Significant amount of setup time.

Low CPU utilization.

But very secure.

Simple Batch Systems

Mainframe machines. Input devices were cardreaders. Output devices were line printer, tapedrives, and card punch.

A job (a single program+ associated data + controlinformation) usually on the punch cards submitted to

the operator. The output consisted of the results of the program

or memory dump in case of error. The operator used to batch together similar

programs and run as a group to reduce setup time. No user interaction while the job is executing.

Current examples include .bat files under Dos

Windows and shell files under Unix/Linux.



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Example of card deck of a job

The operating systems (called resident monitor)manages the execution of each program in thebatch.

Monitor utilities are loaded when needed.

Resident monitor is always in main memoryand available for execution.



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The resident monitor usually has the followingpart.

Control card interpreter responsible for

reading and carrying out instructions on thecards.

Loader loads systems programs andapplications programs into memory.

Device drivers know specialcharacteristics and properties for each ofthe systems I/O devices.



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In batch systems: Initial control is in monitor. Load next program and transfer control to it.

When a job completes, the control transfersback to monitor.

Automatically transfer control from one job toanother (Automatic job sequencing).

Problems

Slow Performance I/O and CPU could not overlap;card reader very slow.

CPU was often idle.

Solutions

1. Off-line Operation

Speed up computation by loading jobs intomemory from tapes while card reading and lineprinting is done off-line using smaller machines.

2. Use spooling (Simultaneous Peripheral OperationOn Line).

Cards are read directly from the card reader

onto a disk and location of card images arekept in a table by the operating system.



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The output is sent to the disk and when the jobis completed then the output was actuallyprinted.

I/O and computations were overlapped. Whileexecuting one job, the OS:

Reads next job from card reader into astorage area on the disk (job queue).

Outputs printout of previous job fromdisk to printer.

Uniprogramming Until Now

I/O operations are exceedingly slow (compared toinstruction execution).

A program containing even a very small number ofI/O operations will spend most of its time waiting for

them. Hence: poor CPU usage when only one program is

present in memory.



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Memory Layout of Uniprogramming

Memory layout of a simple batch processing system

Multiprogrammed Batch Systems

Several jobs are kept in main memory at the sametime, and the CPU is multiplexed among them.

If memory can hold several programs, then CPUcan switch to another one whenever a program iswaiting for an I/O to complete This ismultiprogramming.



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OS Features Needed for Multiprogramming

I/O routine supplied by the system.

Memory management the system must allocatethe memory to several jobs.

CPU scheduling the system must choose amongseveral jobs ready to run.

Allocation of devices.

Time Sharing Systems (Interactive Systems)

TSS extends Batch multiprogramming to handlemultiple interactive jobs Its InteractiveMultiprogramming.

Multiple users simultaneously access the systemthrough terminals. Processors time is shared among multiple users,

that is, the CPU is multiplexed among several jobsthat are kept in memory and on disk (the CPU isallocated to a job only if the job is in memory).

On-line communication between the user and the

system is provided; when the operating system



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finishes the execution of one command, it seeks thenext control statement from the users keyboard.

TS system provides each user with her/her own

virtual machine.

Multitasking

TS eventually supports multitasking.

A time share system that supports multipleprocesses (program in execution) per user is calleda multitasking system.

Why Does Time Sharing Work?

Because of slow human reaction time, a typical userneeds 2 seconds of processing time per minute.

Then many users should be able to share the samesystem without noticeable delay in the computerreaction time.



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Batch MultiprogrammingVs. Time Sharing

Batch Multi-Prog. Time SharingPrinciple obj. Max. Processor use Min. response time

Source of inst.To OS

JCL provided withthe job

Commands enteredat the terminal

OS Features Needed for Time Sharing Systems

On-line file system must be available for users to

access data and code. Should do memory management

Should do CPU scheduling Should do job synchronization and have

communication facilities.

Should ensure that dead lock and indefinite waitingdoes not occur.

Should allow sharing of computer resources.

Personal Computer Systems

Personal computers computer system dedicatedto a single user.

Have a wide variety of I/O devices keyboards,

mice, display screens, small printers. User convenience and responsiveness are of prime

importance. 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.



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

Two Categories of Computer Systems

Single Instruction Single Data (SISD) Single processor executes a single instruction

sequence to operate on data stored in a singlememory.

This is a Uniprocessor.

Multiple Instruction Multiple Data (MIMD) A set of processors simultaneously execute

different instruction sequences on different datasets.

This is a Multiprocessor.

Multiprocessor Systems

Multiprocessor systems have more than one CPU inclose communication.

Tightly coupled system processors sharememory and a clock; communication usuallytakes place through the shared memory.

Advantages of parallel system: Increased throughput

Economical Increased reliability

Graceful degradation



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Multiprocessor architecture

Symmetric Multiprocessing (SMP)

Each processor runs an identical copy of theoperating system.

Each processor can perform the same functions andshare same main memory and I/O facilities(symmetric).

The OS schedules processes/threads across all theprocessors (real parallelism).

Existence of multiple processors is transparent tothe user.

Incremental growth: just add another CPU!

Robustness: a single CPU failure does not halt thesystem, only the performance is reduced.

Many processes can run at once withoutperformance deterioration.

Most modern operating systems support SMP

Asymmetric multiprocessing

Each processor is assigned a specific task; masterprocessor schedules and allocated work to slaveprocessors.

More common in extremely large systems



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Distributed Systems

Distribute the computation among several physically

separated processors. Loosely coupled system each processor has

its own local memory; processors communicatewith one another through variouscommunications lines, such as high-speedbuses or telephone lines.

Advantages of distributed systems.

Resources Sharing

Computation speed up load sharing

Reliability and fault tolerance

Communications

Requires networking infrastructure - Local areanetworks (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

Peer-to-peer systems

Network Operating System

Provides file sharing Provides communication scheme

Runs independently from other computers on thenetwork

Distributed Operating System

Less autonomy between computers



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Gives the impression there is a single operatingsystem controlling the network.

Clustered Systems

Clustering allows two or more systems to shareexternal storage and balance CPU load.

Asymmetric clustering: one server runs theapplication while other servers standby.

Symmetric clustering: all N hosts are running the

application.

Real-Time Systems

Note that not all Operating Systems are general-purpose systems.

Real-Time (RT) systems are dedicated systems thatneed to adhere to deadlines, i.e., time constraints.

Correctness of the computation depends not only onthe logical result but also on the time at which theresults are produced.



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Often used as a control device in a dedicatedapplication such as controlling scientificexperiments, medical imaging systems, industrial

control systems, and some display systems. Real-Time systems may be eitherhardorsoftreal-

time.



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Hard Real-Time System

Must meet its deadline.

Conflicts with time-sharing systems, not supportedby general-purpose operating systems. Often used as a control device in a dedicated

application such as industrial control and robotics

Secondary storage limited or absent, data stored inshort term memory, or read-only memory (ROM).

Soft Real-Time System A critical real-time task gets priority over the other

tasks (Deadline desirable but not mandatory).

Limited utility in industrial control of robotics

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

Hand Held Systems

Personal Digital Assistants (PDAs)

Cellular telephones

Issues:

Limited memory

Slow processors

Small display screens.



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Migration of Operating-System Concepts andFeatures

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