Lecture 1 Computer Operating Systems

7/31/2019 Lecture 1 Computer Operating Systems

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Computer Operating Systems

Technical Elective 1


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What is an Operating System?

Operating System (OS) Is a program that acts as an intermediary between a

user of a computer and the computer hardware.

Purpose Provide an environment in which a user can execute programs in

a convenientand efficientmanner.

Thus, the primary goal of an OS is to make the computer

system convenientto use A secondary goal is to use the computer hardware in

an efficientmanner.


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Computer System

Can be divided roughly into four components:

1. The hardware

2. The operating system (OS)

3. The application programs (APs)

4. And the users


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assembler Text editor Databasesystems

User 1 User 2 User 3 User n. . .

compiler . . .

System & application programs

Operating system

Computer hardware

Figure 1. Abstract View of the components of a computer system


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The Hardware provides the basic computing resources

1. Central processing unit (CPU)

2. The memory

3. The I/O devices

TheApplication programs define the ways in w/cthese resources are used to solve the computingproblems of the users.

1. word processors

2. spreadsheets,

3. compilers,

4. webbrowsers


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The operating system (OS) controls and

coordinates the use of the hardware among

the various application programs for the

various users.

1. Windows

2. Linux

3. Mac OS

4. etc


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The users

1. Encoder

2. Programmer

3. Analyst

4. Etc


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Different views for an OS

Resource allocator

A computer has many resources (hardware &

software) that may be required to solve a problem:

CPU time, memory space, file-storage space, I/O devices,and so on.

Control program

Controls the execution of user programs to prevent

errors and improper use of the computer. It is

especially concerned with the operation and control

of the I/O devices


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Note:

In general, there is no completely adequate

definition of an OS (accdg to: Siberschatz, Galvin,

Gagne)

A more common definition is that OS is the one

program running at all times on the computer

(usually called the kernel), with all else beingapplication programs.


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Batch Systems

Operating

systems

User

programarea

Fig. 2 Memory layout for a simple batch system

- To speed up processing, operators batchedtogether jobs with similar needs

and ran them through the computer as a group.- Early computers were (physically) enormously large machines run from a

console.- Common input devices were card readers and tape drives and line

printers, tape drives, and card punches for output devices.


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cont

The introduction of disk technology allowed the OS to keepall jobs on a disk, rather than a in a serial card reader.

With direct access to several jobs, it could dojobscheduling to use resources and perform tasks efficiently.

The most important aspect of job scheduling is the abilityto multiprogram.

A single user cannot, in general, keep either the CPU or theI/O devices busy at all times.

Multiprogramming increases CPU utilization by organizing

jobs such that the CPU always has one to execute.


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cont

Operating

systems

Job

1

Job2

Job

3

Job 4

0

512K

Fig 3 Memory Layout for a Multiprogramming System


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..cont,

The OS keeps several jobs in memory simultaneously (fig 3)

This set of jobs is a subset of the jobs kept in the job pool(since the number of jobs that can be kept simultaneouslyin memory is usually much smaller than the number of jobsthat can be in the job pool).

The OS picks up and begins to execute one of the jobs inthe memory.

Eventually, the job may have to wait for some task, such asan I/O operation to complete.

In a non-multiprogrammed system, the CPU would sit idle. In a multiprogramming system, the OS simply switches to

and executes another job.


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...cont

When that job needs to wait, the CPU is switched toanother job and so on.

Eventually, the first job finishes waiting and gets the CPUback.

As long as there is always at least one job to execute, theCPU is never idle.

Multiprogramming is the first instance where the OS mustmake decisions for the users.

Multiprogrammed OS are therefore fairly sophisticated.

All the jobs that enter the system are kept in the job pool. This pool consists of all processes residing on the disk

awaiting allocation of main memory.


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cont

If several jobs are ready to be brought into memory, and there isnot enough room for all of them, then the system must chooseamong them.

Making this decision isjob scheduling.

When the OS selects a job from the job pool, it loads that job into

memory for execution. Having several programs in memory at the same time requires

some form ofmemory management.

In addition, if several jobs are ready to run at the same time, thesystem must choose among them.

Making this decision is CPU scheduling. Multiple jobs running concurrently require that their ability to affect

one another be limited in all phases of the OS, includingprocessscheduling, disk storage, and memory management.


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Time Sharing Systems

Time sharing or multitasking, is a logicalextension of multiprogramming.

CPU executes multiple jobs by switching amongthem, but the switches occurs so frequently thatthe users can interact with each program while itis running.

An interactive, or hands-on computer systemprovides direct communication between the user

and the system. Response time should be short-typically within a

second or so.


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cont

Time-shared OS allows the many users to share thecomputer simultaneously.

As the system switches rapidly from one user to thenext, each user is given the impression that the entire

computer system is dedicated to her use, whereasactually one computer is being shared among manyusers.

A time-shared system uses CPU scheduling andmultiprogramming to provide each user with a small

portion of a time-shared computer. Each user has at least one separate program in

memory.


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cont

A program that is loaded into memory and isexecuting is commonly referred to as aprocess.

When a process executes, it typically executes for

only a short time before it either finishes orneeds to perform I/O.

I/O may be interactive; that is, output is to adisplay for the user and input is from a user

keyboard, mouse, or other device. Since interactive I/O typically runs at people

speeds, it may take a long time to complete.


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cont

Time-sharing systems are even more complexthan aremultiprogrammed OS.

In both, several jobs must be kept simultaneously inmemory, so the system must have memory

managementandprotection. So that a reasonable response time can be obtained,

jobs may have to be swappedin and out of mainmemory to the disk that now serves as a backing storefor main memory.

A common method of achieving this goal is virtualmemory, w/c is a technique that allows the executionof a job that may not be completely in memory.


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cont

Obvious advantage of this scheme is that

programs can be larger than physical memory.

Further, it abstracts main memoryinto a large,

uniform array of storage, separating logical

memory as viewed by the user from physical

memory.

This arrangement frees programmers from

concern over memory-storage limitations.


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cont

Time-sharing systems must also provide afile system.

The file system resides on a collection of disks; hence,disk managementmust be provided.

Time-sharing systems must also provide a mechanismfor concurrent execution, w/c requires sophisticatedCPU-scheduling schemes

To ensure orderly execution, the system must providemechanisms forjob synchronization andcommunication, and it may ensure that jobs do not getstuck in a deadlock, forever waiting for one another.


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cont

Idea of time-sharing was demonstrated as

early as 1960, but since TS systems are

difficultand expensive to build, they did not

become common until 1970s.

Although batch processing is still done, most

systems today are time-sharing.

Multiprogramming and time-sharing are the

central themes of modern OS.


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Personal Computer Systems

Appeared in the 1970s

1st decade, PCs lacked the features needed toprotect the OS from user programs

PC OS therefore were neither multiuser normultitasking

Goals of these OS have changed with time;

instead of maximizing CPU and peripheralutilization, the systems opt for maximizinguser convenience and responsiveness.


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Parallel Systems

Most systems to date are single-processorsystems; that is, they have only one main CPU.

However, there is trend toward multiprocessor

systems Such systems have more than one processor

in close communication, sharing the computer

bus, the clock, and sometimes memory andperipheral devices referred as tightly coupledsystems.


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contd

Advantages:

Increased throughput

Less expensive

Increased reliability

Graceful degradation (a.k.a.fault tolerant)

Most Common Multiple-processor systems

1. Symmetric multiprocessing

2. Asymetric multiprocessing


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contd

Symmetric multiprocessing Each processor runs an identical copy of the OS, and

these copies communicate with one other as needed.

Asymmetric Multiprocessing Each processor is assigned a specific task.

A master processorcontrols the system; the otherprocessors either look to the master for instruction orhave predefined tasks

This scheme defines a master-slave relationship. Themaster processor schedules an allocates work to theslave processors.


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contd

The difference between symmetric and

asymmetric multiprocessing may be the result of

either hardware/software.

Special hardware can differentiate themultiprocessors, or the software can be written

to allow only one master and multiple slaves.

Example, Suns OS SunOS version 4 providesasymmetric multiprocessing, whereas Version 5

(Solaris 2) is symmetric on the same hardware.


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contd

As microprocessor(P) become less expensive andmore powerful, additional OS functions are off-loadedto slave processor, or back-ends.

Example, it is fairly easy to add a P with its own

memory to manage a disk system The P could receive a sequence of requests from the

main CPU and implement its own disk queue andscheduling algorithm.

This arrangement relieves the main CPU of theoverhead of disk scheduling

PCs contain a P in the KB to convert the key strokesinto codes to be sent to the CPU.


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Real Time Systems

Anotherform of a special-purpose OS

Used when there are rigid time requirementson the operation of the processoror the flow

of data; Often used as a control device in a dedicated

application.

Sensors bring data to the PC The PC must analyze the data and possibly

adjust controls to modify the sensor inputs.


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contd

Systems that control scientific experiments, medicalimaging systems, industrial control systems, andcertain display systems are real-time systems

Also included are some automobile-engine fuel

injection systems, home appliance controllers, andweapon systems.

A real-time system has well defined, fixed timeconstraints.

Contrast this requirement to a time-sharing system,where it is desirable (but not mandatory) to respondquickly, or to a batch system, where they may be notime constraints at all.


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Distributed Systems

Growth of computer networks-especially the internetand the WWW

PCs & workstations are capable of running a webbrowser for accessing a hypertext document in a web

Inclusion of system software such as TCP/IP and PPPthat enables a computer to access the internet

Loosely coupled systems, or distributed systems,processors do not share memory or a clock, insteadeach processor has its own local memory

Processors communicate with one another althoughvarious communication lines, such as high-speed busesor telephone lines

Documents

Lecture 1 Computer Operating Systems