SLIDES_OS_1

1ER/CORP/CRS/OS94ER/CORP/CRS/OS94 Ver. No.: 1.0Ver. No.: 1.0 Copyright 2008, Infosys Technologies Ltd.Copyright 2008, Infosys Technologies Ltd.

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ConfidentialConfidential

Infosys

Operating System Concepts Operating System Concepts Part -1Part -1

Welcome to this course on Operating System Concepts

Copyright 2008, Infosys Technologies Ltd. Confidential

Infosys

General GuidelineGeneral Guideline

(2008) Infosys Technologies Ltd.

This document contains valuable confidential and proprietary information of Infosys. Such confidential and proprietary information includes, amongst others, proprietary intellectual property which can be legally protected and commercialized. Such information is furnished herein for training purposes only. Except with the express prior written permission of Infosys, this document and the information contained herein may not be published, disclosed, or used for any other purpose.

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Confidential InformationConfidential Information

This Document is confidential to Infosys Technologies Limited. This document contains information and data that Infosys considers confidential and proprietary (Confidential Information).

Confidential Information includes, but is not limited to, the following: Corporate and Infrastructure information about Infosys Infosys project management and quality processes Project experiences provided included as illustrative case studies

Any disclosure of Confidential Information to, or use of it by a third party, will be damaging to Infosys.

Ownership of all Infosys Confidential Information, no matter in what media it resides, remains with Infosys.

Confidential information in this document shall not be disclosed, duplicated or used in whole or in part for any purpose other than reading without specific written permission of an authorized representative of Infosys.

This document also contains third party confidential and proprietary information. Such third party information has been included by Infosys after receiving due written permissions and authorizations from the party/ies. Such third party confidential and proprietary information shall not be disclosed, duplicated or used in whole or in part for any purpose other than reading without specific written permission of an authorized representative of Infosys.

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Learning approachLearning approach The following are strongly suggested for a better learning and

understanding of this course:

Noting down the key concepts in the class, explained by the educator Analyze all the examples / code snippets provided Study and understand the self study topics Completion and submission of all the assignments, on time Completion of the self review questions in the lab guide Study and understand all the artifacts including the reference materials / e-

learning / supplementary materials specified Completion of the project (if applicable for this course) on time inclusive of

individual and group activities Taking part in the self assessment activities Participation in the doubt clearing sessions

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Course PrerequisiteCourse Prerequisite

Knowledge of Basic computer Hardware Introduction to computer system

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6Copyright 2008, Infosys Technologies Ltd. Confidential

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Course ObjectivesCourse Objectives


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Learning OutcomesLearning Outcomes

Basic understanding of OS Understanding of Memory management functions of operating

system. Concept of Paging and Virtual memory Understanding of Process states and Process scheduling

policies. Understanding of Interprocess communication , Producer

Consumer problem and Critical section. Understanding of Basic File concept & File management Understanding of Device management and data transfer

using DMA.

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Course AgendaCourse AgendaPart 1

Introduction to OS course and Scope of the subject Basic OS concept :

Goals and Major Functions of Operating System. Multiprogramming ,Multitasking & Multiprocessing

Memory Management schemes Simple Paging Demand Paging Page Replacements Algorithms Segmentation Paged Segmentation

Process Management Process Definition Process State Transition Process Control Block

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Course AgendaCourse Agenda

Part 2 Process Management

Process Scheduling Preemptive scheduling Non Pre-emptive scheduling

Threads Interprocess Communication

Concurrency Critical Section problem Mutual Exclusion Semaphore

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Course AgendaCourse Agenda

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ReferencesReferences

Silberschatz and Galvin, Operating System Concepts, John Wiley & Sons ,Sixth edition

Andrew Tanenbaum, Modern Operating Systems, Pearson Education

Milan Milenkovic, Operating Systems: concepts and design, McGraw-Hill

Charles Crowley, Operating Systems: A Design-Oriented Approach

D.A. Patterson and J.L. Hennessy, Computer Architecture: A Quantitative Approach, 2nd edition, Morgan-Kaufmann, 1996.

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Session Plan Part 1Session Plan Part 1

Introduction to OS course and Scope of the subject Basic OS concept :

Goals and Major Functions of Operating System. Multiprogramming ,Multitasking & Multiprocessing

Memory Management schemes Paging Segmentation Paged Segmentation Page Replacements Algorithms


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Operating SystemOperating System

System Software which acts as an interface between the user and the computer

It provides an environment to execute different types of applications

It is viewed as a Resource Allocator or Resource Manager Memory Processor(s) Peripherals Information

Operating System is a set of system programs which provides an environment to help the user to execute the programs. The OS is a resource manager which allocates and manages various resources like processor(s), main memory, input/output devices and information (files) on secondary memory devices.

The primary goal of Operating System is to provide the execution environment for the system programs and application programs.Secondary Goal of Operating System is to manage and allocate the system resources efficiently to various applications running in the system.

Resource Examples ManagersMemory Primary, Secondary Memory ManagementProcessors CPU, I/O Process ManagementPeripherals Terminal, Printer, Tape Device ManagementInformation Files, Data File Management

Examples:WINDOWS XPWINDOWS VISTALINUXUNIXOS/ 2


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Operating system as an interfaceOperating system as an interface

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Files

Processor

Operating System

System Software which acts as an interface between the user and the computer.It provides an environment to execute different types of applications

It is viewed as a Resource Allocator or Resource ManagerMemoryProcessorsPeripheralsInformation

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Major Functions of Operating SystemMajor Functions of Operating System

Memory Management

Process Management & CPU scheduling

Interprocess Communication

File management

Device and I/O Management

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Layers of Computer System

Computer Hardware

Operating System

System & Application Programs

User 1 User nUser 2 .

Application program e.g. Calculator,

Notepad etc.

System program e.g

C Compiler etc.

Database application e.g

Oracle etc.

Operating System is a set of system programs which provides an environment to help the user to execute the programs.

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Operating System : TerminologyOperating System : Terminology

Introduction to Operating SystemsIntroduction to the Concepts of Memory Management


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Uni-programmingUni-programming

User ProgramArea

OS

Job1

Only one job resides in the memory at any time

Observations?1.CPU utilization2.Memory utilization

unused

In sequential program execution/ Uniprogramming , only one program/job at a time can reside in main memory.CPU execute this program. When program is busy in I/O CPU sits idle for that time.

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MultiprogrammingMultiprogramming More than one job resides in main memory Multiple users can access the system simultaneously

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User ProgramArea

OS

Job1

Job2

Other jobs can be stored

Observations?

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MultiprocessingMultiprocessing

More than one processor executes one or more jobs in parallel

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CPU1 CPU nCPU2

Memory

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Memory ManagementMemory Management

Introduction to Operating SystemsIntroduction to the Concepts of Memory Management

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Memory ManagementMemory Management

Manages memory usage efficiently

Allocates memory to as many user jobs as possible Respond to changes in memory demand by user jobs Prevent unauthorized changes to a user jobs memory

The computer must keep several processes in memory at the same time to improve the CPU utilization and the speed of the response of the computer to its users. Memory management discusses various ways to manage memory.

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Memory Management SchemesMemory Management Schemes

Simple Paged allocation

Demand Paging

Segmentation

Paged Segmentation

There are various memory management schemes as mentioned in the slide above. Each scheme has its own advantage and disadvantage. Selection of a particular technique depends on various factors such as hardware support, extent of memory available etc.

Logical address is the address generated by the CPU and the range of these addresses is called Address Space/ User Dddress Space.Where as physical address is the actual address of the process in the main memory and the range of these addresses is called Memory Space /Physical Address Space .

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Simple Paged AllocationSimple Paged Allocation

All pages for a job should reside in memory

Divides the jobs address space into pages of the same size

Divides the main memory address space into blocks/frames

Pages are units of memory that are swapped in and out of primary memory

Pages are grouped together for a given user job and are placed in page tables

Simple paged allocation is a solution to fragmentation.

Advantage:As each page is separately allocated, the users job need not be contiguous.Disadvantages:

Extra memory required for storing page tablesConsiderable amount of hardware support is required for address transformations etc.All pages of entire job must be in memory

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Simple Paged AllocationSimple Paged Allocation

Page 0Page 1Page 2Job A

Memory Space

Frame 1

Frame 2

Frame 3

Free Frame Frame 4

Frame 5

Free Frame Frame 6

Frame 7

Free Frame Frame 8

Page 0Page 1Job BA

- 0

A- 1

A -2

B- 0

B-

1

Frame 0Free Frame

The example in the slide above shows a page map table for 2 jobs viz., page number and frame number which essentially shows the mapping between page number and block number. JOBA has 3 pages viz., 0,1,2. Page 0 maps to frame 5 in the OS, page 1 to frame 3 and page 2 to frame 1 in the OS. JOB 2 has page 0 in frame 2 and page1 in frame7.

Thus, we can see that pages of a job need not be located contiguously in the memory.

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Page Table and Address Translation Page Table and Address Translation

f

An address in jobs address space is considered to be of the form(p,d) where p is the page number containing the memory location and d is thedisplacement (offset) of the memory location from the start of the page. This page number acts as index to a page table which is created for every job. This page table gives the corresponding frame number in the main memory for the particular page. This frame number with the displacement would give the physical address.

The Page table has two purposes:

It has got entries which map each page number in the program (in the secondary memory) and the corresponding page frame number (or block number present in the primary memory).

Since there is one to one mapping between the page number and the frame number , even if the pages are not contiguous in the primary memory, there exists a link between all the pages of a job through this mapping in the page table.

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Internal Fragmentation due to Simple Paged AllocationInternal Fragmentation due to Simple Paged Allocation

Page size=4K

Fragmentation is wastage of memory, it occurs when memory is divided in to many small chunks/free blocks and these free blocks are not able to satisfy any request. Fragmentation is of two type: 1. Internal fragmentation 2. External fragmentation

Internal Fragmentation: As shown in the diagram in this slide, internal fragmentation is wastage of the space inside the allocated memory blocks. Because page size / memory block is of fixed size. Job may need less space but minimum 1 memory block is allocated(space allocated to a job is always multiple of frame) , so the wastage of memory happens is internal fragmentation. Limitation because of fixed size memory block.

External Fragmentation: Memory that is not allocated to any job but still unusable because it is divided in to small chunks. These chunks are too small to satisfy the request of any job.

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Illusion of infinite memory available to the user (Virtual Memory) Job operating under demand paging A page is brought to the main memory only when there is a

demand

Demand PagingDemand Paging

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Demand PagingDemand Paging

Page number Frame number

JOB 12 3 1 0

Status

1000 2000 3000 4000

7000

OS

Page Table

Modify

The diagram in the slide above shows the page map table for demand paging. The page map table (PMT) has two additional columns viz., status and modify or judgment. Initially, all the pages have status field as NA (Not Available) implying that all the pages are in the secondary device (disk). As and when a page is loaded from the secondary to primary, the status is updated to A (Available) from NA. Now, if the same page is required again in the main memory, the status bit will indicate the presence of it in the primary memory. The judgment field decides if a page has to be moved back to the secondary memory or not.

1000, 2000, 3000 are the frame starting addresses

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Page ReplacementPage Replacement

Suppose a scenario when all frames in memory are full and a Page Fault occurs.

We need to bring in the required page but there in no free frame.

where we will load it?

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Page Replacement AlgorithmsPage Replacement Algorithms

Algorithms used for page replacement are First in First out (FIFO ) Least Recently Used ( LRU ) Least Frequently Used (LFU) Not Recently Used (NRU ) (Discussed in Appendix) Most Recently Used (MRU) (Discussed in Appendix) Most Frequently Used (MFU ) (Discussed in Appendix)

Page Fault: If a user job accesses a page and the page is not available in the main memory, a page fault is said to occurPage Replacement: If the memory is full then the inactive pages which are not needed currently for execution are removed and are replaced by those pages from the secondary device which are to be executed. This is called Page Replacement.FIFO: If the algorithm decides that the page has been first moved to the memory should be moved out to secondary memory first, then it is using FIFO.LRU: If the algorithm decides to move a page from main memory and store it in secondary memory based on the fact that it is not used often in the recent times, then it is called LRU. For every page a timestamp is maintained in the judgment field.LFU: If the algorithm decides to move a page from main memory and store it in secondary memory based on the fact that it is not used often, then it is called LFU. For every page a reference counter is maintained in the judgment field.NRU: If the algorithm decides to move a page from main memory and store it in secondary memory based on the fact that it is not used at all in the recent times, then it is called NRU. A reference bit is associated with each page.MRU: If the page which has been used recently will be replaced by the incoming page.MFU: This technique replaces the page which is used most frequentlyDemand Paging technique could lead to a scenario where in a job might try to access a particular page which is not available in main memory. This would lead to a fault called page fault. Consider the figure discussed in slide 34 (All frames are already occupied), suppose that the page 2 of job A, which is currently in memory, tries to access page 1 of job A then a page fault would occur because page 1 is not loaded in memory. Once this page fault is caused the OS would replace one of the existing pages in physical memory with page 1 of job A . the process of removing some pages from physical memory to accommodate new pages is called Page Replacement.

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Page request sequence 3 1 3 4 2 4 1 2 3 1 2 4 2 3 1 3

FIFO (Assume frame size 3 and No frames preloaded)

Number of page faults=10Number of page replacement = 7

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Page request sequence3 1 3 4 2 4 1 2 3 1 2 4 2 3 1 3LRU

Number of Page faults=9

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Page request sequence3 1 3 4 2 4 1 2 3 1 2 4 2 3 1 3LFU (use FIFO in case of tie)

Number of Page faults=9Ask the participants to solve the problem using LFU with LRU technique instead of FIFO.Another variation could be not to reset the counter even as a page is swapped out.

Refer to the examples in the course material for more examples on different types of page replacement algorithms.

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SegmentationSegmentation

Segment is a grouping of information that is treated as a logical entity

A process is divided into different segments each of its own length. Example: one segment can correspond to a single subroutine or a

group of closely related subroutines Segmentation uses unequal chunks Each individual segment can be protected Segmentation support the user view of memory

In paging , job is divided into fixed length pages , it does not provide support to user view of memory.

User view of memory means , user like to see job is divided into variable length segments.For example, a program consist of main program , one or more then functions/ subroutines etc. Each of these modules are referred to by a name and each of these segments may be of different length. The length of each segment is calculated from the code written in the segment or from the logical structure of the program.

Thus segmentation supports the user view of memory. A programmer has a say on the number of segments in a process and this division is dependent on the logical structure of the process.

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SegmentationSegmentation

2(Segment 2)

24(24 Bytes)

Segment No. (s) Displacement (d)

s

2048 Bytes(2K) 20480

8192 Bytes(8K) 11264

4096 Bytes(4K) 1024

Length Byte Address

0

1

2

Segment Table

Is d > length

1048

1024

Memory

Error

Yes

No

Byte address + thedisplacement gives the

physical address

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Paged SegmentationPaged Segmentation Requires two-level translation : Segment tables to page tables

and then to main memory

Paging leads to internal fragmentation

Segmentation leads to external fragmentation

Trying to reduce external fragmentation increases internal fragmentation and vice versa

In practice most operating systems use the combination of pagingand segmentation. This is called as Paged Segmentation


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Logical address space of a process is divided into three parts Segment Number(S)

Page Number(P)

Offset(d)

Intel 386 and later architecture use segmentation with paging

Paged SegmentationPaged Segmentation

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+

+

Segment Table

Page Table

Main Memory

FrameNumber Offset

Segment Number

PageNumber

PageFrame

d

Virtual/Logical Address

Segment Map Table

SegmentNumber

PageNumber Offset

Paged SegmentationPaged Segmentation

Suppose a process references a virtual/logical address V=(S, P, d) ,then a search will be performed to locate (S,P). There are three possibilities for it

(i)If (S,P) found in associative storage then , we fine page frame and then move as per offset to get real memory address.(ii)If (S,P) is not found in associative storage then we should follow these steps

(i) Find base address b of segment map table ( in primary memory)(ii) Add b to segment number S ( Say we set S1)(iii) Then we add S1 to P to get frame number(iv) Finally we add offset d to get real memory address.

(iii)If segment map table indicates that segment S is not in primary memory, a segment fault is generated

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Process ManagementProcess Management

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Processor Hardware component that is capable of executing

instructions

Process An instance of a program in execution

Is an active entity whereas a program is a passive entity

Needs certain resources for execution in which a processor is one of them

The computer system today allows multiple programs to be loaded into memory and to be executed concurrently but at one point of time only one program is in execution or rather at most one instruction is executed on behalf of the process.

A source program stored on disk is called passive because it will not demand any resource.A process is an active entity i.e. program in execution is called process during execution process demand resource like CPU , memory , I/O etc. for the execution.

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Why Process ManagementWhy Process Management

The four applications are to be served by one processor

OS allocates the processor to each application one at a time

This is done by the process management module of OS

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MS Outlook Calculator

IE MS Word

User wants to work with more than one application.

Operating System

User feels that all of them run simultaneously, however operating system allocates one process at a time.

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States of a Process States of a Process

New : Created

Ready : Assign a process to a processor

Run : Instruction execution

Blocked : For an event to occur (Ex: I/O)

Terminated : Finished execution

Here, the instructor must draw the state diagram in a piecemeal fashion slowly adding the complexity and inviting more questions.

New Here the operating system recognizes the process but does not assign resources to it. Ready The process is ready/waiting to be assigned to a processor.Run When a process is selected by the CPU for execution, it moves to the run state.Blocked When a process is waiting for some event to occur, say for e.g I/O completion or reception of a signal then it is in the blocked state i.e. when a running process does not have immediate access to a resource it is said to be in the blocked state.Terminated This is the state reached when the process finishes execution.

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Process State DiagramProcess State Diagram

New

Ready

Waiting Terminated

Running

Admitted

Exit

IO or Event wait(Blocked)

Interrupt

I/O or Event Completion Processor

Allocated

Here, the instructor must draw the state diagram in a piecemeal fashion slowly adding the complexity and inviting more questions.

New Here the operating system recognizes the process but does not assign resources to it. Ready The process is ready and waiting for the processor for execution.Run When a process is selected by the CPU for execution, it moves to the run state.Blocked When a running process does not have immediate access to a resource it is said to be in the blocked state.Terminated This is the state reached when the process finishes execution.

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Includes : Creating and terminating processes

Controlling and saving the state of a process

Acting on exceptional conditions during process execution (Interrupts)

Communication among processes

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Context SwitchingContext Switching

The task of switching the CPU from one process to another process.

Context switching Keeps the CPU busy all the time Occurs when any of the following situations is encountered

An I/O operation is initiated An interrupt occurs An I/O is completed A process terminates

Context switching is a overhead as the system does not do any useful work while switching from one process to another. switching speed varies from machine to machine depends upon the memory speed, the no. of registers to be copied etc.

Since at any point of time there could be more than one process waiting to be serviced by the CPU, these have to be put in hold at some place. A queue is a place where the IN and OUT of the process is controlled. There are different queues for different purposes.

Example:I/O Queue If the process is waiting for an I/O operation to complete, then it goes to the I/O queue. Job queue A process entering the system for the first time goes into the Job queue.

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Process Control BlockProcess Control Block

Structure in the Operating System which keeps track of the status of the process. Process id Process State Program Counter value for the process Register information Accounting information Memory information for the process List of open files, Inter Process communication Information Pointers to the PCB of the next process to run

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Process Control BlockProcess Control Block


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BrainstormingBrainstorming

Suppose one process is under execution, if a high priority job arrives, what decision OS will take?

What are the various possibilities?

Suppose one process is under execution, if a high priority job arrives, what decision OS will take?

What are the various possibilities?

Suppose four applications(Calculator, Outlook, MS Word & Internet Explorer) are to be served by one Processor ,OS allocates the processor to each application one at a time.How OS will allocate Processor to each application one at a time? What will be the order of execution?


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Can you answer these questions?Can you answer these questions?

What is Operating System ? What are the functions of Operating System?

What do you mean by Frame? Which data structure is used to convert logical address to

physical address? Virtual address is address of hard Disk(TRUE/FALSE). What is the difference between Internal and External

Fragmentation? Interrupt change the state of process from _________ to

_______ state?


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Self StudySelf Study

Memory Managements Techniques.

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SummarySummary

Operating System Memory Management Memory management Schemes

Paging Segmentation Paged Segmentation



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Thank You

The contents of this document are proprietary and confidential to Infosys Technologies Ltd. and may not be Disklosed in whole or in part at any time, to any third party without the prior written consent of

Infosys Technologies Ltd.

2008 Infosys Technologies Ltd. All rights reserved. Copyright in the whole and any part of this document belongs to Infosys Technologies Ltd. This work may not be used, sold, transferred, adapted, abridged, copied or

reproduced in whole or in part, in any manner or form, or in any media, without the prior written consent of Infosys Technologies Ltd.

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Page Replacement TechniquesPage Replacement TechniquesAppendix 1Appendix 1


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Page Replacement TechniquesPage Replacement Techniques

In demand paging whenever page fault occurs and if free slot is also not available, we have to replace an existing page with the required page.

For selecting the page which will be moved out we different techniques. Some of them are FIFO (First in First Out) (Discussed in slide) LFU (Least Frequently Used) (Discussed in slide) LRU (Least Recently Used) (Discussed in slide) NRU (Not Recently Used) MRU(Most Recently Used) MFU (Most Frequently Used)

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Not Recently Used (NRU)Not Recently Used (NRU) NRU page replacement algorithm allow the pages to reside in

the memory that have been recently used

When replacement required it select the not recently used page for the replacement

When a page is referenced by the CPU, a referenced bit is set for that page

After fixed time interval, the referenced bit of all the pages are cleared, so only pages reffered within the current clock interval are marked with a referenced bit.

When a page needs to be replaced, One page is selected randomly from the pages for which reference bit is not set.

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Most Recently Used (MRU)Most Recently Used (MRU) The page which has been used recently will be replaced by the

incoming page.

Example Consider a dynamic paging scheme, where the page numbers and request sequence is as follows:

1 1 3 1 1 6 4 2 3 7 2 5 6 4 3 4 2 7 6Given that the total number of frames in the memory is 4 and that the first two frames are loaded with pages 1 and 3, calculate the number of page faults that occur when the MRU algorithm is used.

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SolutionSolution

1 1 3 1 1 6 4 2 3 7 2 5 6 4 3 4 2 7 6

1

3

1

3

1

3

1

3

1

3

1

3

6

1

3

6

4

1

3

6

2

1

3

6

2

1

7

6

2

1

7

6

2

1

7

6

5

1

7

6

5

1

7

4

5

1

7

3

5

1

7

4

5

1

7

2

5

1

7

2

5

1

6

2

5

No. of page faults : 10No. of page replacements : 8 (For first two page faults page will not be replaced)

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Most Frequently Used (MFU)Most Frequently Used (MFU)This technique replaces the page which is used most frequentlyWith each page we can attach one page reference count and when ever page

replacement is required we will select the page in memory havingmaximum count

To resolve any tie we use FIFO Most of the time it does not give good performanceExample

Consider a dynamic paging scheme, where the page numbers and request sequence is as follows:

1 1 3 1 1 6 4 2 3 7 2 5 6 4 3 4 2 7 6Given that the total number of frames in the memory is 4 and that the first two frames are loaded with pages 1 and 3, calculate the number of page faults that occur when the MFU algorithm is used (Use FIFO for resolving any tie and do not reset the counter whenever a page is replaced).

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SolutionSolution

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