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Chapter 3 Operating Systems 1

Chapter 3: Operating-System Structures

This Chapter covers:1. System Components2. Operating System Services3. System Calls4. System Programs5. System Structure6. Virtual Machines

7. System Design and Implementation8. System Generation

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1. Common System Components

Process ManagementMain Memory ManagementFile ManagementI/O System ManagementSecondary ManagementNetworkingProtection System

Command-Interpreter System

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2-Command-Interpreter System (The Shell)

Interface between users and OS.Many commands are given to the operatingsystem by control statements which deal with:

process creation and management

I/O handlingsecondary-storage managementmain-memory managementfile-system accessprotectionnetworking

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Chapter 3 Operating Systems 4

Command-Interpreter System (Cont.)

The program that reads andinterprets control statementsis called variously:

command-line interpretershell (in UNIX)MS-DOS prompt

Its function is to get andexecute the next command

statement.

OS

System Calls

Shell

Request or

ControlStatement

Sequenceof

User

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Chapter 3 Operating Systems 5

3. System Calls

System calls provide the interface between a runningprogram and the operating system.Generally available as assembly-languageinstructions.Languages defined to replace assembly language for

systems programming allow system calls to be madedirectly (e.g., C, C++)Three general methods are used to pass parametersbetween a running program and the operating system.

Pass parameters in registers .

Store the parameters in a table in memory , and thetable address is passed as a parameter in a register.Push (store) the parameters onto the stack by theprogram, and pop off the stack by operating system.

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Passing of Parameters As A Table

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Types of System Calls

Process controlFile managementDevice managementInformation maintenance

Communications

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Examples :MS-DOS and UNIX

At System Start-up Running a Program

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Process Communication Models

Message Passing Shared Memory

Communication may take place using eithermessage passing or shared memory .

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4. System Programs

System programs provide a convenientenvironment for program development andexecution. They can be divided into:

File manipulationStatus informationFile modificationProgramming language supportProgram loading and executionCommunicationsApplication programs

Most users view of the operation system isdefined by system programs, not the actualsystem calls.

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Chapter 3 Operating Systems 11

5. MS-DOS System Structure

MS-DOS written toprovide the mostfunctionality in the leastspace

not divided intomodulesAlthough MS-DOS hassome structure, itsinterfaces and levels of functionality are notwell separated

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Chapter 3 Operating Systems 12

UNIX System Structure

UNIX limited by hardware functionality,the original UNIX operating system hadlimited structuring. The UNIX OS consists of two separable parts.

Systems programsThe kernel

Consists of everything below thesystem-call interface and above thephysical hardware

Provides the file system, CPUscheduling, memory management, andother operating-system functions; alarge number of functions for onelevel.

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UNIX System Structure

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Layered Approach

The operating system is divided into a number of layers(levels), each built on top of lower layers. The bottomlayer (layer 0), is the hardware; the highest (layer N) isthe user interface.With modularity, layers are selected such that each usesfunctions (operations) and services of only lower-levellayers.

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OS/2 Layer Structure

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Microkernel System Structure

Moves as much from the kernel into user space.Communication takes place between usermodules using message passing.

Benefits:- easier to extend a microkernel- easier to port the operating system to newarchitectures

- more reliable (less code is running in kernelmode)- more secure

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Windows NT Client-Server Structure

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6. Virtual Machines

A virtual machine takes the layered approachto its logical conclusion. It treats hardwareand the operating system kernel as thoughthey were all hardware.

A virtual machine provides an interfaceidentical to the underlying bare hardware.The operating system creates the illusion of multiple processes, each executing on its ownprocessor with its own (virtual) memory.

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Virtual Machines (Cont.)

The resources of the physical computer areshared to create the virtual machines.CPU scheduling can create the appearancethat users have their own processor.

Spooling and a file system can providevirtual card readers and virtual line printers.A normal user time-sharing terminal servesas the virtual machine operators console.

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System Models

Non-virtual Machine Virtual Machine

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Advantages/Disadvantages of Virtual Machines

The virtual-machine concept provides completeprotection of system resources since each virtualmachine is isolated from all other virtualmachines. This isolation, however, permits nodirect sharing of resources.A virtual-machine system is a perfect vehicle foroperating-systems research and development.System development is done on the virtualmachine, instead of on a physical machine and

so does not disrupt normal system operation.The virtual machine concept is difficult toimplement due to the effort required to providean exact duplicate to the underlying machine.

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Chapter 3 Operating Systems 22

Java Virtual Machine

Compiled Java programs areplatform-neutral byte codesexecuted by a Java VirtualMachine (JVM).JVM consists of

- class loader- class verifier- runtime interpreterJust-In-Time (JIT) compilersincrease performance

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Chapter 3 Operating Systems 23

7. System Design Goals

User goals operating system should beconvenient to use, easy to learn, reliable, safe,and fast.System goals operating system should beeasy to design, implement, and maintain, aswell as flexible, reliable, error-free, andefficient.

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Mechanisms and Policies

Mechanisms determine how to do something,policies decide what will be done.The separation of policy from mechanism is avery important principle, it allows maximumflexibility if policy decisions are to be changedlater.

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System Implementation

Traditionally written in assembly language,operating systems can now be written inhigher-level languages.Code written in a high-level language:

can be written faster.is more compact.is easier to understand and debug.

An operating system is far easier to port (move

to some other hardware) if it is written in ahigh-level language.

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8. System Generation (SYSGEN)

Operating systems are designed to run on anyof a class of machines; the system must beconfigured for each specific computer site.SYSGEN program obtains informationconcerning the specific configuration of thehardware system.Booting starting a computer by loading thekernel.Bootstrap program code stored in ROM thatis able to locate the kernel, load it intomemory, and start its execution.