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CareerAces

Operating system



Operating System

Definitionso A program that manages hardware and

software resources of a computer

o It is the first thing that is loaded into memorywhen one turn ones computer on.

o A program that acts as an intermediary

between a user of a computer and computerhardware

o A software that manages the sharing of theresources of a computer and provides

programmers with an interface used to accessthose resources.



• ser pera ng ys emInterfaceo Command Line Interface – CLI

o Graphical User Interface – GUI

Types

• Process control• File management• Device management• Information maintenance• Communications

•System Calls



• Virtual Machine



•System Boot

o Operating system must be made available tohardware so

o hardware can start it.

o Bootstrap loader is a small piece of codeo which locates the kernel, loads it into memory, and

o starts it.



Kernel The kernel's primary purpose is to managethe computer's resources and

allow other programs to run and use theresources like the CPU, memory and the

I/O devices in the computer.

The facilities provides by the kernel are:

o Memory management - The kernel has fullaccess to the system's memory and must allowprocesses to access safely this memory as theyrequire it.

o Device management - To perform useful



Types of Kernel:

oMonolithic kernels - Every part which is tobe accessed by most programs which

cannot be put in a library is in the kernelspace:

oDevice drivers

o Scheduler

oMemory handling

o File systems

oNetwork stacks



File Systemso File systems are an integral part of any

operating systems with the capacity for longterm storage.

o Two distinct parts of a file system

o The mechanism for storing files and

o The directory structure into which they areorganized



Implementation strategy

Contiguous allocationo First implementation strategy was that of a

contiguous allocation

o

Lay out of various files are in contiguous diskblocks.

o Used in VM/CMS - an old IBM interactivesystem

o Quick and easy calculation of block holdingdata - just offset from start of file

o For sequential access, almost no seeks

required.



Linked allocation

o Next implementation strategy was that of a

linked allocation.

o All files stored in fixed size blocks. Linktogether adjacent blocks like a linked list.

o No more variable-sized file allocationproblems. Everything takes place in fixed-sizechunks, which makes memory allocation a loteasier.

o No more external fragmentation.

o No need to compact or relocate files.

o Potentially terrible performance for direct

access files - have to follow pointers from one



FAT allocation

o Next implementation strategy is FAT (FileAllocation Table) allocation




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3rd ClassProcess, Threads



Process

The term "process" was first used by thedesigners of the MULTICS in 1960's.

The process has been given many definitionsfor instance

o A program in Execution.

o An asynchronous activity.

o The 'animated sprit' of a procedure in

execution.

o The entity to which processors are assigned.

o The 'dispatch able' unit.

o There is no universally agreed upon definition,



Process stateso New The process is being created

o Running Instructions are being executedo Waiting/Blocking The process is waiting for some event to

occur

o Ready The process is waiting to be assigned to a

processo Terminated The process has finished execution



Process Control Blocks (PCB)

A process in an operating system is represented

by a data structure

known as a process control block (PCB) orprocess descriptor.

The PCB contains important information aboutthe specific process including

o The current state of the process i.e., if it is inready, running, waiting.

o Unique identification of the process in order totrack the concerned process.

o A pointer to parent process.

o A pointer to child process (if it exists).



ProcessesCooperating Processes

A process which can affect or can be affectedby other processes is called co-operatingprocess

Concurrent ProcessA sequential computer program consists of a

series of instructions to be executed one afteranother. A concurrent program consists of

several sequential programs to be executed inparallel. Each of the concurrently executingsequential programs is called a process

Independent Process

Inde endent rocess cannot affect or be

Th d



Threado Process has is a thread of execution

o Threads add to the process model is to allowmultiple executions to take place in the sameprocess environment, to a large degreeindependent of one another.

o Although a thread must execute in someprocess, the thread and its process aredifferent concepts and can be treated

separately. Processes are used to groupresources together; threads are the entitiesscheduled for execution on the CPU.

o The threads share an address space, open

files, and other resources, processes share



Three processeswith one thread

One process withthree threads



rocesses sThreadsSimilarities

o Like processes threads share CPU and onlyone thread active (running) at a time.

o Like processes, threads within processes,

threads within a process execute sequentially.

o Like processes, thread can create children.

o And like process, if one thread is blocked,

another thread can run.Differences

o Unlike processes, threads are not independent

of one another.



Multi tasking, Multi

programming & Multithreading



Context Switch To give each process on a multi programmed

machine a fair share of the CPU, a hardwareclock generates interrupts periodically. Thisallows the operating system to schedule allprocesses in main memory (using scheduling

algorithm) to run on the CPU at equal intervals.Each time a clock interrupt occurs, the interrupthandler checks how much time the currentrunning process has used. If it has used up its

entire time slice, then the CPU schedulingalgorithm (in kernel) picks a different process torun. Each switch of the CPU from one process toanother is called a context switch.




CareerAces

4th Class – CPU Scheduling and IPC

CPU S h d li



CPU Scheduling

o The objective of multiprogramming is to havesome process running all the time, tomaximize CPU utilization Maximum CPUutilization obtained with multiprogramming.

o

CPU scheduling is the basis of multiprogrammed operating systems. By switchingthe CPU among processes, the operatingsystem can make the computer moreproductive.

o Scheduling schemes can be divided into twoparts:

o Non-preemptive scheduling scheme and

o Preemptive scheduling scheme.



scheduling schemeNon-preemptive scheduling is a scheme

where once a process has control of the CPU noother processes can preemptively take the CPUaway. The process retains the CPU until either itterminates or enters the waiting state.

There are two algorithms that can be used fornon-preemptive scheduling.

o First-Come, First-Served algorithm(FCFS)

o Shortest-Job-First (SJF)



o First-Come, First-Served algorithm(FCFS)

The first process to request the CPU is theone that is allocated the CPU first and is verysimple to implement. It can be managed usinga First-In, First-Out (FIFO) queue. When the CPUis free, it is allocated to the first process waitingin the FIFO queue. Once that process is finishedthe CPU goes back to the queue and selects thefirst job in the queue.

o Shortest-Job-First (SJF)

In this scheduling scheme the process withthe shortest next CPU burst will get the CPU

first. By moving all the short jobs ahead of the

i h d li



Preemptive scheduling schemePreemptive scheduling is the second

scheduling scheme. In preemptive schedulingthere is no guarantee that the process using theCPU will keep it until it is finished. This isbecause the running task may be interrupted

and rescheduled by the arrival of a higherpriority process.

There are two preemptive schedulingalgorithms that are preemptive.

o The Round Robin (RR) and

o The Shortest Remaining Time First (SRTF)



Round-Robin (RR)

o The Round-Robin scheduling scheme is similar to that of FCFS exceptpreemption is added to it. In the RR scheduling scheme the CPU picks aprocess from the ready queue and sets a timer to interrupt after one time

quantum. During this scheme two things may happen.

o The process may need less than one time quantum to execute.

o The process needs more than one time quantum.

In the first case when a process is allocated the CPU it executes. Becausethe time required by the process is less than one time quantum, the processgives up the CPU freely. This causes the scheduler to go and select anotherprocess from the ready queue.

In the second case if a process needs more than one quantum time toexecute it must wait. In the RR scheme each process is given only one time

quantum. The only way for the process to gain access to the CPU for more thanone time quantum is if it is the only process left. If that is not the case, thenafter one time quantum the process will be interrupted by the timer. This willcause the process to go to the end of the ready queue. The next process in linewill get allocated the CPU and will be allotted one time quantum.

Inter Process



Inter ProcessCommunication (IPC)

Since processes frequently need tocommunicate with other processes therefore,there is a need for a well-structuredcommunication, without using interrupts,among processes. It is a set of techniques forthe exchange of data among multipleprocesses. Processes may be running on one ormore computers connected by a network. IPCtechniques are divided into methods formessage passing, synchronization, sharedmemory, and remote procedure calls (RPC). Themethod of IPC used may vary based on thebandwidth and latency of communication

between the process, and the type of data



synchronizationProcess synchronization is the task of

organizing the access of several concurrentprocesses to shared (i.e. jointly used) resourceswithout causing any conflicts. The sharedresources are most often memory locations

(shared data) or some hardware. Processsynchronization can be divided into twosubcategories:

o Synchronizing competing processes: Severalprocesses compete for one exclusiveresource. This is solved by one of the mutualexclusion mechanisms.

o

Synchronizing cooperating processes: Severalrocesses have to notif each other of their



Race conditionsIn operating systems, processes that are

working together share some common storage(main memory, file etc.) that each process canread and write. When two or more processesare reading or writing some shared data and

the final result depends on who runs preciselywhen, are called race conditions.

Avoiding Race conditions

The key to preventing trouble involvingshared storage is find some way to prohibitmore than one process from reading and writingthe shared data simultaneously. That part of theprogram where the shared memory is accessed

is called the Critical Section.

dl k



Deadlock A set of process is in a deadlock state if each

process in the set is

waiting for an event that can be caused by onlyanother process

in the set.

Conditions for Deadlock

o Mutual exclusion

o Hold and wait

o No preemption

o Circular wait



Deadlock Preventiono Elimination of “Mutual Exclusion” Condition

o Elimination of “Hold and Wait” Condition

o Elimination of “No-preemption” Condition

o Elimination of “Circular Wait” Condition

Virtual memory



Virtual memory

Virtual memory is hardware technique where

the system appears to have more memory thatit actually does. This is done by time-sharing,the physical memory and storage parts of thememory one disk when they are not actively

being used.Cache memory Cache memory is random access memory

(RAM) that a computer microprocessor canaccess more quickly than it can accessregular RAM. As the microprocessorprocesses data, it looks first in the cache

memory and if it finds the data there (from

Paging



Pagingo Paging is an aspect of virtual memory

addressing whereby relatively inactive pagescan be temporarily removed from physicalmemory if necessary.

o If these pages have been modified, they must

be saved to a temporary storage area on disk,called a paging file or swap space.

o The operation of writing one inactive page ora cluster of inactive memory pages to disk is

called a page out, and the correspondingoperation of reading them in again later whenone of the pages is referenced is called a pagein.

Paging operation



Paging operationo Basic idea is to allocate physical memory to

processes in fixed size chunks called pageframes.

o Inside machine, break address space of application up into fixed size chunks called

pages.o When process generates an address,

dynamically translate to the physical pageframe which holds data for that page.

o So, a virtual address now consists of twopieces: a page number and an offset withinthat page.

o To access a piece of data at a given



o System performs translation using a pagetable. Page table is a linear array indexed

by virtual page number that gives thephysical page frame that contains thatpage. It contains:

o

Extract page number.o Extract offset.

o Check that page number is within address spaceof process.

o Look up page number in page table.

o Add offset to resulting physical page number

o Access memory location.

r ua o ys ca



r ua o ys camemory

F t ti



Fragmentation

Fragmentation occurs in a dynamic memoryallocation system when many of the free blocksare too small to satisfy any request.

External Fragmentation: ExternalFragmentation happens when a dynamicmemory allocation algorithm allocates somememory and a small piece is left over that

cannot be effectively used. If too much externalfragmentation occurs, the amount of usablememory is drastically reduced. Total memoryspace exists to satisfy a request, but it is not

contiguous.

Th hi



Thrashingo If the number of frames allocated to a low

priority process is lower than the minimumnumber required by the computer architecturethen we must suspend the execution of thislow priority process.

o We should page out all of its remaining pagesand free all of its allocated frames. Thisprovision introduces a swap in, swap-out levelof intermediate CPU scheduling.

o If the process does not have the number of frames it needs to support pages in activeuse, it will quickly page fault. The only optionremains here for process is to replace some

active pages with the page that requires a



System(RTOS)o

Real-time computing (RTC) is the study of hardware and software systems which aresubject to a "real-time constraint"—i.e.,operational deadlines from event to systemresponse.

o A real time system is one in which thecorrectness of the computations not onlydepends upon the logical correctness of the

computation but also upon the time at whichthe result is produced. If the timingconstraints of the system are not met, systemfailure is said to have occurred



oNetworking



o Q. What is OSI model ? List layers.

Slide Title | CONFIDENTIAL 2006




OSI (Open Systems

Interconnect)Model• Standards that ensure varying devices and products can communicate with each other over any

network. This set of standards is called a model.

• The International Standards Organization (ISO) created an industry wide model, or framework,for defining the rules networks should employ to ensure reliable communications. This model was

termed as OSI

• This network model is broken into layers, with each layer having a distinctive job in thecommunication process.

• Groping into Layers reduces the complexity in implementing the Network Architecture

• Provides Compatibility and allows multi-vendor Integration

•

Facilitates Modularization and allows developer to swap out new changes at a particular Layer without affecing the other Layers



OSI Model Overview

Data Flow

Layers

Transport Layer

Data Link

Network Layer

Physical

Application

(Upper)

Layers

Session

Presentation

Applicatio

n

OS d l



OSI Model


SD

P111

Pro fe ssi o n a l W o rksta t io n1111

Presentation

Session

Transport

Network

Data link

Physical

Application

LAN 1 LAN 2



o Q. Explain working of each layer.




How to transmit signal; codingHardware means of sending andreceiving data on a carrier

Two party communication: Ethernet 1

Routing and Forwarding Address: IP 1

End-to-end control & error checking(ensure complete data transfer): TCP 1

Establish/manage connection 1

ASCII Text, Sound (syntax layer) 1

File Transfer, Email, Remote Login 1

to transmit signal; codingare means of sending and

ving data on a carrier

o party communication: Ethernet 1

uting and Forwarding Address: IP 1

-to-end control & error checkingsure complete data transfer): TCP 1

Establish/manage connection1

ASCII Text, Sound (syntax layer) 1

le Transfer, Email, Remote Login 1

The Application Layer



The Application Layer

4/18/12

Application

Presentatio

n

Session

Transport

Network

Data Link

Physical

Application

SDP111

ProfessionalWorkstat ion1111

• Provides the user interface.

• Connects the user to the network.

• Provides file transfer service, mail service,

Th S i L



4/18/12Slide Title | CONFIDENTIAL 2006

The Sessions Layer Application

Presentatio

n

Session

Transport

Network

Data Link

Physical

Session

• Establishes and maintains connection.

•

Manages upper layer errors.

• Handles remote procedure calls.

• Synchronizes communicating nodes.

Th T t L




The Transport Layer Application

Presentatio

n

Session

Transport

Network

Data Link

Physical

Transport

R e t u r n r e c

e i p t f o r

s e n d e r

• Takes action to correct faulty transmission.

•

Controls the flow of data.

• Acknowledges successful receipt of data.

• Fragments and reassembles data.

Th N t k L




The Network Layer Application

Presentatio

n

Session

Transport

Network

Data Link

Physical

NetworkLAN 1 LAN 2

the network.

• Moves information to the correct address.

• Assembles and disassembles packets.

• Addresses and routes data packets.

• Determines best path for moving data through

Th D t Li k L




The Data Link Layer Application

Presentatio

n

Session

Transport

Network

Data Link

Physical

Data Link

0 1 0 1 0 1 0 1 0 0 1 1 0 0

1

Data from

upper layers

• Controls access to the communication channel.

• Controls the flow of data.

• Organizes data into logical frames.• Identifies specific computer on the network.

• Detects errors.

10010011 0000011 1010011 0010111

The Ph sical La er




The Physical Layer Application

Presentatio

n

Session

Transport

Network

Data Link

PhysicalPhysical

• Provides electrical and mechanicalinterfaces for a network.

network devices.

• Specifies type of media used to connect




Q. What is TCP/IP



o The Transmission Control Protocol (TCP) is one of the core

protocols of the Internet Protocol Suite. o TCP is one of the two original components of the suite,

complementing the Internet Protocol (IP), and therefore the entiresuite is commonly referred to as TCP/IP.

o TCP is the protocol that major Internet applications rely on,

applications such as the World Wide Web,e-mail, and file transfer. o TCP provides a point-to-point channel for applications that require

reliable communications.

o The Hypertext Transfer Protocol (HTTP), File Transfer Protocol (FTP)are all examples of applications that require a reliable

communication channel.


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

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

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

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

http://en.wikipedia.org/wiki/E-mail

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

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

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

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

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

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

http://en.wikipedia.org/wiki/E-mail

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

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

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

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



o Q . What is connection less and connectionoriented protocol?




Connection-Oriented

o Connection-Oriented means that whendevices communicate, they performhandshaking to set up an end-to-endconnection.

o Connection-Oriented systems can only work in bi-directional communications

environments.o To negotiate a connection, both sides must

be able to communicate with each other. This will not work in a unidirectional

environment.Slide Title | CONFIDENTIAL 2006



Connectionless

o Connectionlessmeans that no effort ismade to set up a dedicated end-to-end

connection.




oWhat Is difference between switch

and hub?




oHub is a multiport repeater it broadcast

information (it receives on any port) to allports hence is called non-intelligent ordumb.

oHUB works on Physical layer where as

SWITCH works on data link layer,HUBbased networks are on one collisiondomain where as in Switch based networkswitch divides networks into multiple

collision domains.Switch also maintainsSlide Title | CONFIDENTIAL 2006



A Simple Example

• Hub - Think of a postman with a letter to deliver

in a row of houses, none of the houses

have numbers so he has to visit each house andask the owner if the letter is for them.

• Switch - All the houses are numbered, so thepostman knows where to go, and doesn't haveto bother any other home owners.



oQ. What is DHCP?





• The Dynamic Host Configuration

Protocol (DHCP) is an automaticconfiguration protocol used on IPnetworks.

• Computers that are connected to IPnetworks must be configured before they

can communicate with other computers onthe network.

• DHCP allows a computer to be configuredautomatically, eliminating the need forintervention by a network administrator.

• It also provides a central database forkeeping track of computers that have beenconnected to the network.

• This prevents two computers from

accidentally being configured with the




Difference between Router and Gateway

In simpler terms a router is like a elevator in thebuilding. It can take you to any floor

[destination] and back again [source]. Thiswould work with any routable protocol [tcp/ip,ipx, decnet..]

Your first door to the elevator is your gateway.

This is all your pc needs to know since the routerwill take it from there and make sure it gets towhere you want and back again. You can accessthe World by going thru that first door [gateway]



Q: What is the PDU of "Network layer" and"Data link layer”.





o PDU(Protocol Data Unit) for Network Layer is:"Packet" and PDU for Data Link Layeris :"Frame"




oWhy we use cross cable to connectsame devices?



o Same devices like PC-2-PC, it uses(NIC for PC)1,2 for transmission & 3,6 for reception. If wedon't use cross cable then we can't transferdata.

o While in the case of switch/hub they receive(NIC of SWITCH/HUB)data on 1,2 & transmit on

3,6.

o Thats why we use straight cable for de-similarhost & cross cable for similar hosts.




oQ. What are associated TCP/IP Protocols &

its Services?




Associated TCP/IP Protocols& ServicesHTTP This protocol, the core of the World Wide Web, facilitates retrieval

and transfer of hypertext (mixed media) documents. Stands for theHyperText Transfer protocol

Telnet A remote terminal emulation protocol that enables clients to log onto remote hosts on the network.

SNMP Used to remotely manage network devices. Stands for the SimpleNetwork Management Protocol.

DNS Provides meaningful names like achilles.mycorp.com for computers to replace numerical addresses like 123.45.67.89.Stands for the Domain Name System.

SLIP/PPP

SLIP (Serial Line Internet Protocol) and PPP (Point to PointProtocol) encapsulate the IP packets so that they can be sent over a dial up phone connection to an access provider’s modem.

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