os_3_osservices.pdf

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Chapter 2: OperatingChapter 2: Operating--System StructuresSystem Structures


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2.2 Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition , Jan 14, 2005

Chapter 2: OperatingChapter 2: Operating--System StructuresSystem Structures

Operating System Services

User Operating System Interface

System Calls Types of System Calls

System Programs

Operating System Design and Implementation

Operating System Structure

Virtual Machines

Operating System Generation

System Boot


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ObjectivesObjectives

To describe the services an operating system provides to users,

processes, and other systems

To discuss the various ways of structuring an operating system

To explain how operating systems are installed and customized

and how they boot


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Operating System ServicesOperating System Services

O/S provides certain services to programs and to the users of those programs.One set of operating-system services provides functions that are helpful to the

user:

z User interface - Almost all operating systems have a user interface (UI)

z Command Line Interface (CLI) uses text commands and method for entering them

z Batch Interface: in which commands are entered into files, and those files are

executed

z Graphical User Interface (GUI) most commonly used. Interface is window system

with a pointing device to direct I/O, choose from menus, and make selections and akeyboard to enter text.

z Some systems provide two or three or all of these variations.

z Program execution - The system must be able to load a program into memory and

to run that program, end execution, either normally or abnormally (indicating error)z I/O operations - A running program may require I/O, which may involve a file or an

I/O device. For efficiency and control users cannot control I/O devices directly,

therefore O/S must provide a means to do I/O


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Operating System ServicesOperating System Servicesz

File-system manipulation - The file system is of particular interest. Obviously,programs need to read and write files and directories, create and delete them, search

them, list file Information, permission management.


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Operating System Services (Cont.)Operating System Services (Cont.) Another set of OS functions exists for ensuring the efficient operation of the system

itself via resource sharing

z Resource allocation - When multiple users or multiple jobs running

concurrently, resources must be allocated to each of them

Many types of resources - Some (such as CPU cycles,main memory, and file

storage) may have special allocation code, others (such as I/O devices) mayhave general request and release code.

E-g in CPU scheduling routines that take into account the speed of the CPU.

There are routines to allocate printers, modems, USP storage devices, and

other peripheral devices.

z Accounting - To keep track of which users use how much and what kinds of

computer resources

z Protection and security - The owners of information stored in a multiuser or

networked computer system may want to control use of that information,

concurrent processes should not interfere with each other Protection involves ensuring that all access to system resources is controlled

Security of the system from outsiders requires user authentication, extends

to defending external I/O devices from invalid access attempts

If a system is to be protected and secure, precautions must be institutedthroughout it. A chain is only as strong as its weakest link.


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System CallsSystem Calls Programming interface to the services provided by the OS

System calls are generally available in a high-level language (C or C++)

Mostly accessed by programs via a high-levelApplication Program Interface (API)

rather than direct system call use

Three most common APIs are Win32 API for Windows, POSIX API for POSIX-based

systems (including virtually all versions of UNIX, Linux, and Mac OS X), and JavaAPI for the Java virtual machine (JVM)

May need to be written in a low level language such as assembly language


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Example of System CallsExample of System Calls Writing a simple program to read data from one file and copy them to another file.

System call sequence to copy the contents of one file to another file


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Standard C Library ExampleStandard C Library Example

C program invoking printf() library call, which calls write() system call


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Types of System CallsTypes of System Calls

Process control

File management

Device management Information maintenance

Communications


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Layered ApproachLayered Approach

The operating system is divided into a number of layers (levels),

each built on top of lower layers. The bottom layer (layer 0), is the

hardware; the highest (layer N) is the user interface.

With modularity, layers are selected such that each uses functions(operations) and services of only lower-level layers


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Layered Operating SystemLayered Operating System


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UNIXUNIX

UNIX limited by hardware functionality, the original UNIX operating

system had limited structuring. The UNIX OS consists of two

separable parts

z Systems programs

z The kernel

Consists of everything below the system-call interface and

above the physical hardware

Provides the file system, CPU scheduling, memory

management, and other operating-system functions; a large

number of functions for one level


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UNIX System StructureUNIX System Structure


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Microkernel System StructureMicrokernel System Structure As UNIX expanded, the kernel became large and difficult to manage.

In the mid 1980s, researchers at Carnegie Mellon University developed an Operatingsystem, called Mach, that modularized the kernel using microkernel approach.

This method structures the operating system by removing all nonessential

components from kernel and implementing them as kernel level or user level

programs. Microkernels provide minimal process and memory management, in addition to

communication facility.

Communication takes place between user modules using message passing

Benefits:z Easier to extend a microkernel

z Easier to port the operating system to new architectures

z More reliable (less code is running in kernel mode)

z More secure


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Virtual MachinesVirtual Machines The idea is to abstract the hardware of a single computer (the CPU, memory,

disk drive, network interface cards, and so forth) into several different execution

environments, thereby creating an illusion that each separate execution

environment is running its own private computer

A virtual machine takes the layered approach to its logical conclusion. It treats

hardware and the operating system kernel as though they were all hardware

CPU is multiprogrammed among different virtual machines

VMWare

VMWare runs as an application on host operating system, such as Windows or

Linux and allows this host system to concurrently run several different guest

operating systems as independent virtual machines.

Example: A developer has designed an application and would like to test it on

Linux, FreeBSD, Windows NT and Windows XP

One option to is to take separate computer and run separate operating systemon that

Second option is to use a single computer and first install one operating

system, run software, then uninstall and install the other operating system and

so on. Same physical machine, but process is time consuming


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Such testing could be accomplished on the same physical computer

using VMWare

In this case programmer could test the application on host operatingsystem and three guest operating systems with each system running a

separate virtual machine.

Fundamental idea behind a virtual machine is to abstract the hardware of a

single computer (the CPU, memory, disk drives, network interface cards and soforth) into several execution environments thereby creating the illusion that

each separate execution environment is running its own private computer.

A virtual machine provides an interface identical to the underlying bare

hardware

The operating system creates the illusion of multiple processes, each executingon its own processor with its own (virtual) memory


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Virtual Machines (Cont.)Virtual Machines (Cont.)

The resources of the physical computer are shared to create the

virtual machines

z CPU schedul ing can create the appearance that users have

their own processor

z Spooling and a file system can provide virtual card readers and

virtual line printers

z A normal user time-sharing terminal serves as the virtual

machine operators console


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Virtual Machines (Cont.)Virtual Machines (Cont.)

(b) virtual machine(a) Non virtual machine


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Virtual MachinesVirtual Machines (Cont.)(Cont.)

The virtual-machine concept provides complete protection of system resources sinceeach virtual machine is isolated from all other virtual machines. This isolation,

however, permits no direct sharing of resources.

A virtual-machine system is a perfect vehicle for operating-systems research and

development. System development is done on the virtual machine, instead of on a

physical machine and so does not disrupt normal system operation.

The virtual machine concept is difficult to implement due to the effort required to

provide an exact duplicate to the underlying machine


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VMware ArchitectureVMware Architecture


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The Java Virtual MachineThe Java Virtual Machine


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Operating System GenerationOperating System Generation

OS kernel is downloadable in from www.kernel.org


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End of Chapter 2End of Chapter 2

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