155 - Introduction to Operating System - Chapter1 (1)

8/13/2019 155 - Introduction to Operating System - Chapter1 (1)

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Chapter 1

Introduction to Operating Systems

Prepared by: Romel L. Emperado


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Grading System

Major Exams - 60% Participation - 10% Attendance - 10% Quizzes - 20%


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1.1 General Definition

An OS is a program which acts as aninterface between computer system usersand the computer hardware.

It provides a user-friendly environment inwhich a user may easily develop andexecute programs.

So, it can be said that an OS hides thecomplexity of hardware from uninterestedusers.


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In general, a computer system has someresources :

Memory Processor(s) I/O

o A hardware interruptis an electronic alerting signalsent to the processor from an external device.

o A software interruptis caused either by anexceptional condition in the processor itself.

File System and etc.


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The OS manages these resources and allocates them tospecific programs and users. With the management ofthe OS, a programmer is rid of difficult hardwareconsiderations.

An OS provides services for:

Processor Management

Memory Management File Management

Device Management

Concurrency Control


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Another aspect for the usageof OS is that; it is used as apredefined library for

hardware-software interaction.

This is why, system programsapply to the installed OS

since

they cannot reach hardware

directly.


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Since we have an already written library,namely the OS, to add two numbers wesimply write the following line to our program:

c = a + b ;


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in a system where there is no OS installed, we shouldconsider some hardware work as:

(Assuming an MC 6800 computer hardware)

LDAA $80 - Loading the number at memory location 80

LDAB $81 - Loading the number at memory location 81

ADDB - Adding these two numbers

STAA $55 - Storing the sum to memory location 55

As seen, we considered memory locations and used ourhardware knowledge of the system.


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With the advantage of easier programmingprovided by the OS, the hardware, itsmachine language and the OS constitutes a

new combination called as a virtual(extended) machine.


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In a more simplistic approach, in fact, OS itself is aprogram.

But it has a priority which application programs dont

have.

OS uses the kernel modeof the microprocessor,whereas other programs use the user mode. Thedifference between two is that; all hardware instructionsare valid in kernel mode, where some of them cannotbe used in the user mode.


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The processor switches between the two modesdepending on what type of code is running on theprocessor.

When you start a user-mode application, it creates aprocess for the application. The process provides theapplication with a private virtual address space and aprivate handle table.

A processor running in user mode cannot access virtualaddresses that are reserved for the operating system.

All code that runs in kernel mode shares a single virtualaddress space.


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This diagram illustrates communication between user-mode and kernel-mode

components


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1.2 History of Operating Systems


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It all started with computer hardware in about1940s.

ENIAC


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ENIAC (Electronic Numerical Integrator andComputer), at the U.S. Army's AberdeenProving Ground in Maryland.

o built in the 1940s,o weighed 30 tons,

o was eight feet high, three feet deep, and 100 feetlong

o contained over 18,000 vacuum tubes that werecooled by 80 air blowers.


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Programs were loaded into memory manuallyusing switches, punched cards, or papertapes.


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Finally, the idea of multiprogrammingcame.

Multiprogramming means sharing of

resources between more than oneprocesses.

By multiprogramming the CPU time is notwasted, because, while one process moveson some I/O work, the OS picks anotherprocess to execute till the current oneasses to I/O o eration.


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With the development of interactivecomputation in 1970s, time-sharingsystemsemerged.

In these systems, multiple users haveterminals (not computers) connected to amain computer and execute the task in the

main computer.


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Another computer system is themultiprocessor system having multipleprocessors sharing memory and peripheral

devices. With this configuration, they have greater

computing power and higher reliability.


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Multiprocessor systems are classified into two as tightly-coupledand loosely-coupled(distributed).

In the tightly-coupled one, each processor is assigned aspecific duty but processors work in close association,possibly sharing the same memory.

In the loosely coupled one, each processor has its ownmemory and copy of the OS.


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In network systems, each process runs in its ownmachine but the OS have access to other machines.By this way, file sharing, messaging, etc. becamepossible.

Distributed systemsare similar to networks. Howeverin such systems, there is no need to exchangeinformation explicitly, it is handled by the OS itselfwhenever necessary.

Most distributed processing systems containsophisticated software that detects idle CPUs on thenetwork and parcels out programs to utilize them.

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155 - Introduction to Operating System - Chapter1 (1)