Unit 1 introduction to operating system

Introduction

To

Operating System

Prepared By

Prof. Bhushan Pawar

www.bhushanpawar.com

Basics

Prof.Bhushan Pawar www.bhushanpawar.com

Operating System

• It is a collection of software that managescomputer hardware resources and providescommon services for computer programs.

• The operating system is a vital component ofthe system software in a computer system

• Definition- “An operating system is a programthat acts as an interface between the user andthe computer hardware and it controls theexecution of all kinds of programs”


Operating System


History of UNIX

• In mid-1960s, Massachusetts Institute ofTechnology, AT&T Bell Labs and GeneralElectric were (Ken Thompson, Dennis Ritchie,Doug McIlroy, and Joe Ossanna) developing anexperimental time-sharing & hierarchical filesystem operating system called Multics.

• In 1970, Peter Neumann coined the project nameUNICS (UNiplexed Information and ComputingService) as a pun on Multics (MultiplexedInformation and Computer Services)


History of UNIX (Continued…)• In 1972, Unix was rewritten by Dennis Ritchie in

the C programming language .

• In 1980, Bell developed multiple versions of Unixfor internal use, such as CB UNIX, PWB/UNIX(Programmer's Workbench)

• Microcomputer Unix became commerciallyavailable in 1980, when Onyx Systems released itsZilog Z8000-based C8002 and Microsoft announcedits first Unix for 16-bit microcomputers calledXenix, which the Santa Cruz Operation (SCO)ported to the 8086 processor in 1983


http://en.wikipedia.org/wiki/16-bit



http://en.wikipedia.org/wiki/Xenix

http://en.wikipedia.org/wiki/Santa_Cruz_Operation





History of UNIX (Continued…)

• Can find out more at:

http://en.wikipedia.org/wiki/History_of_Unix


http://en.wikipedia.org/wiki/History_of_Unix

Unique Features of Unix

• Multitasking

• Multiuser

• Portability

• Application Libraries and Tools (e.g

shell, gedit etc.)


Benefits of Unix• High level language “C” which makes it easier to

read understand and update.

• Complex programs can be built on simple programs.

• Hierarchal file system which allows easy

maintenance and efficient implementation.

• Provides simple and consistent interface to the

peripheral devices.

• It completely hides the machine architecture from the

user which makes it easier for the programmers to

write programs independent of hardware.


System Structure


System Structure

Prof.Bhushan Pawarwww.bhushanpawar.com

System Structure


System Structure

• It can be viewed as a set of layers

• Lowermost layer is Hardware Layer which is notthe part of the UNIX operating system

• Middle layer is called the System Kernel or thekernel which is a actual operating system code andfunctionality. It is totally isolated from user’sprogram. The programs interact with the kernel byusing the system calls

• User programs can be built on top of the lowerlevel programs using these lower level programsand system calls


User Perspective

• User perspective is basically based

on 3 scenarios

–The File System

–Processing Environment

–Building Block Primitives


File System

• Characteristics of UNIX file system

– A hierarchal structure.

– Consistent treatment of data

– Ability to create and delete files

– Dynamic growth of files

– Peripheral devices are also treated as files


File System- Diagram 1


File System- Diagram 2


File System

• A full path name starts with the root directory i.e.a slash character ( “/” ) and specifies the file thatcan be found by travestying the tree.

• The path that starts from the root directory iscalled the Absolute Path.

• Alternatively we can give path of any file relativeto any other directory. This path will be calledRelative Path.

• The files are just stream of bytes it is up-to the program to interpret these bytes.


File System

• Permission to any file is governed by the file

access permissions.

• Access permissions are set independently for

read, write and execute. These permissions are

set independently for the file user, file group

and others.

– E.g. rwx rwx rwx–

u g o


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Processing Environment• Program is an executable file and Process is an

instance of program in execution.

• Many processes can execute simultaneously on

UNIX systems (sometimes this feature is

known as Multiprogramming or Multitasking)

• Various system calls allow processes to create

new processes, terminate processes,

synchronize stages of process execution, and

control reaction to various events


Processing Environment


Building Block Primitives

• Unix allows user to write small, modular

programs, which are later on use as a building

block for complex programs.

• It has 3 standard files

– Standard Input File

– Standard Output File

– Standard Error File


Building Block Primitives

• e.g

– One primitive building block available to the shell

user is the redirect I/O

• ls – this command gives the list of all files in current

directory

– The second building block primitive is the PIPE

• ls | more


Operating System Services

• Memory Management

• Processor Management

• Device Management

• File Management

• Security

• Control over system performance

• Job accounting

• Error detecting aids

• Coordination between other software and users



• Process Controlling– Controlling the Creating, Termination and Suspension

of processes.

• Scheduling Processes– Since many programs can execute simultaneously in

UNIX the process scheduling is also done by thekernel.

• Main Memory Management– allocating main memory to the user programs and

protecting the memory region where kernel is running.Also, protecting the memory region of one processfrom another process.



• Secondary Memory Management

– Managing the secondary storage for the efficient andtimely retrieval and storage of data.

• Virtual Memory

– Managing the swap device and handling the swappingsystem. Controlling the pages in the pagingsystem(memory allocation)

• Controlling Peripheral Devices

– Kernel controls the peripheral devices such asterminals, disk drives and network devices.


Assumptions about the Hardware

• When a process executes on UNIX it executes

on two levels or we can say it executes in two

modes

– User level

– Kernel level


User Level Vs. Kernel Level

• Processes in user mode can access their own

instructions and data but not kernel

instructions and data (or those of other

processes).

• Processes in kernel mode, can access kernel

data and instructions as well as user data and

instructions.


Assumption 1

Interrupts and exceptions

• UNIX supports asynchronous interrupt coming

facility.

• After receiving interrupt kernel saves the context and

services the interrupt, After servicing the interrupt it

reloads its context and resumes whatever it was

doing.

• There might be a possibility that kernel is servicing

one interrupt and another interrupt may occur. So

whether or not to service that interrupt and stop

whatever kernel was doing is decided by the interrupt

priority(or interrupt levels).Prof.Bhushan Pawar

www.bhushanpawar.com

Interrupts and exceptions• If a high priority interrupt occurs the kernel stops the

previous one and jumps onto second. If a lower priority

interrupt occurs the kernel will not stop what it was

doing and that interrupt will have to wait. In other words

the lower priority interrupt is blocked if kernel is

servicing some high priority interrupt.

• An exception occurs when a process does something

unexpected. Exceptions are different from interrupts

they occur as events. If an interrupt occurs in the middle

of instruction, that instruction will be restarted after

handling the exception. If the exception is not caused by

the instruction but because of some other reasons and

between two instructions then the next instruction is

processed after handling the exception.

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Interrupts and exceptions


Assumption 2

Processor Execution Levels

• Computers typically have a set of privileged

instructions that set the processor execution level in

the processor status word. Setting the processor

execution level to certain values masks off interrupts

from that level and lower levels, allowing only

higher-level interrupts


Assumption 3

Memory management

• It is functionality of an operating system which

handles or manages primary memory.

• It keeps track of each and every memory location

either it is allocated to some process or it is free.

• It checks how much memory is to be allocated to

processes, decides which process will get memory at

what time.

• It tracks whenever some memory gets freed or

unallocated and correspondingly it updates the status.


Assumption 3

Memory management

• It consist of 7 operations– Dynamic Loading

– Dynamic Linking

– Logical versus Physical Address Space

– Swapping

– Memory Allocation

– Fragmentation

– Paging

– Segmentation


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Unit 1 introduction to operating system

Engineering

Transcript of Unit 1 introduction to operating system