Chapter 3-Memory Management

7/29/2019 Chapter 3-Memory Management

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CHAPTER 3

MEMORY MANAGEMENT

CLO1: Analyzed the step process performed by operating systembased on management of memory, resource and file to ensure thecomputer system operates at optimum performance. (C4)

CLO2: Install and configure correctly workstation and domainserver using MS Windows server or Open Sources server operatingsystem.(P4)


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Memory Management

Memory management Resident and transient routines Related terminologies Dynamic address translation Virtual memory

Processor management Control blocks and interrupts Types of scheduling processes

Scheduling algorithms Queuing routine and scheduler Multiprogramming and time sharing Deadlock


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Scenario that evokes memorymanagement

Picture a computer with a single program in memory. Theprogram cannot process data it does not yet have, andsuccess cannot be assumed until an output operation is

finished, so the program waits for input or output. Since theprogram controls the computer, the computer waits, too.

Typically, given the speed disparity between the processorand its peripheral devices, a program spends far more timewaiting for I/O than processing data.

Why not put two programs in memory? Then, when program Ais waiting for data, the processor can turn its attention toprogram B. And why stop at two programs? With three,

even more otherwise wasted time is utilized .Generally,the more programs in memory, the greater the utilization of the

processor.


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Memory Management

Memory managementis the act ofmanaging computer memory.

In its simpler forms, this involvesproviding ways to allocate portionsof memory to programs at theirrequest, and freeing it for reuse when

no longer needed.

The management of main memory iscritical to the computer system.


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Memory Management

Memory management is done by memorymanager.

Main tasks are to:

Keep track of which parts of memory are in useand which parts are not in use

Allocate memory to processes when theyneed it and deallocate it when they are done

Manage swapping between main memoryand disk when main memory is too small tohold all the processes


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Memory Management

The allocation of computer storage in amultiprogramming system so as tomaximize processing efficiency.

The collection of routines for placing,fetching, and removing pages orsegments into or out of the main memoryof a computer system.


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Memory Management

Memory management systems onmulti-tasking operating systemsusually deal with the following issues:

Relocation

Protection

Sharing

Logical organization

Physical Organization


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Relocation

In systems with virtual memory, programs in memory must beable to reside in different parts of the memory at differenttimes.

The program is swapped back into memory after beingswapped out for a while it can not always be placed in the

same location. Memory management in the operating system should

therefore be able to relocate programs in memory andhandle memory references and addresses in the code of theprogram so that they always point to the right location in

memory.


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Protection

Processes should not be able toreference the memory for anotherprocess without permission.

This is called memory protection, andprevents malicious or malfunctioningcode in one program from interfering

with the operation of other runningprograms


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Sharing

Even though the memory for differentprocesses is normally protected fromeach other, different processes

sometimes need to be able to shareinformation and therefore access thesame part of memory.

Shared memory is one of the fastesttechniques for Inter-processcommunication.


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Logical organization

Programs are often organized in modules.

Some of these modules could be shared

between different programs, some are

read only and some contain data thatcan be modified.

The memory management is responsible

for handling this logical organization that is

different from the physical linear addressspace. One way to arrange this

organization is segmentation.


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Physical Organization

Memory is usually divided into fastprimary storage and slow secondarystorage.

Memory management in theoperating system handles movinginformation between these two levels

of memory.


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Memory Allocation

in a PC


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Resident Routines

OS is a collection of software routines Resident routines (Rutin Tetap)

The part of a program that must remain in memory at all times.Instructions and data that remain in memory can be accessedinstantly.

A routine that stays in the memory eg routines that controlphysical I/O and directly support application programs as theyrun

In computing, a resident module orresident program stays inmemory throughout the lifetime of a computing session.

One such program might be an anti-virus program. This hasgiven rise to the term resident protection.


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Transient Routines

Transient routines (Rutin Sementara)

A routine that is stored on diskloaded as needed

routines that format disks


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Resident and Transient routines

The operating system occupieslow memory.

Key control information comes

first followed by the various

resident operating system

routines.

The transient area, is where

application programs andtransient operating system

routines are loaded.

SystemControl

Information

ResidentOperating

System

TransientArea


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Terminologies related to memorymanagement :

Fixed partition memorymanagement

Dynamic memory management Segmentation

Paging

Memory management

Real memory Virtual memory

Fixed

Partition

Dynamic Segmentat

ion

Paging


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Memory is divided into n (possiblyunequal and fixed length) partitioneach of which holds one program

Partition sizes are set when the systemis initially started

So the memory allocation decision ismade before the actual amount ofspace needed by a given program isknown


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When a job arrives, it can be put intothe input queue for the smallestpartition large enough to hold it.

Since a partition is fixed in thisscheme, any space in a partition notused by a job is lost (internal

fragmentation)


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Major advantage is simplicity

Disadvantage of sorting the incoming jobs intoseparate queues is when the queue for a largepartition is empty but the queue of a smallpartition is full(Refer figure a)

Small jobs have to wait to get into memory, eventhough plenty of memory is free

Alternative of separate queues is to maintain asingle queue(Refer figure b)

i i i


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Whenever a partition becomes free,the job closest to the front of thequeue that fits in could be loaded

into the empty partition and run

Small partition is usually used to allowsmall jobs to run without having to

allocate large partition for themwhich will cause waste of partitionspace

i d titi


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Fixed partition memorymanagement: Figure a

Partition 4

Partition 3

Partition 2

Partition 1

Operating

System

Multiple input queues800K

700K

400K

200K

100K

0K

Fi d titi


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Fixed partition memorymanagement: Figure b

Partition 4

Partition 3

Partition 2

Partition 1

Operating

System

Single input queue

800K

700K

400K

200K

100K

0K


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Dynamic memory management

Transient area is treated as a pool ofunstructured free space.

When the system loads a particularprogram, a region of memory justsufficient to hold the program isallocated from the pool.

Because a program gets only thespace it needs, relatively little spaceis wasted


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Dynamic memory management

Disadvantage :

does not completely solve the wasted

space problems. (Refer figure c)

D i t


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Dynamic memory management:Figure c

Operating

System

Other Regions

250K Region

300K Region

Unused 90K

Fragment

Other Regions

Example, a 640K program hasjust finished executing. If thereare no 640K programs

available, the system mightload a 250K and a 300Kprograms, but note that 90Kremains unallocated.

If there are no 90K or smallerprograms available, the spacewill not be used. This will lead toexternalfragmentation .


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Segmentation: Figure d

Programs aredivided into

independentlyaddressedsegments andstored in

noncontiguousmemory

(Refer Figure d)

Operating Systems

Other Programs

Program A, segment 0

Other Programs


Other Programs


Other Programs


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Segmentation:

Different segments may and usually do

have different lengths

Segments length may changed during

execution


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Segmentation:

requires adding a step to the addresstranslation process.

When a program is loaded intomemory, the operating system buildsa segment table listing the (absolute)entry point address of each of the

programs segments (Refer figure e)


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Segmentation:

When the operating system starts a given

program, it loads the address of that

programs segment table into a special

register. As the program runs, addresses must be

translated from relative to absolute form

because programmers still write the same

code and compilers still generate base-plus-displacement addresses.


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Segmentation

After fetching an instruction, theinstruction control unit expands eachoperand address by adding the base

register and the displacement.

Traditionally, the expanded addresswas an absolute address.


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Segmentation

The expanded address consists oftwo parts:

a segment number

a displacement

Dynamic Address Translation willconvert the segment/displacement

address to an absolute address.


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Absolute Address.

The term absolute distinguishes it from arelativeaddress, which indicates a location byspecifying a distance from another location.Absolute addresses are also calledreal

addresses and machine addresses.


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Segmentation: Figure e

Segme

nt

Start

addre

ss0 Aaa

1 Bbb

2 Ccc

Segment 0

Segment 1

Segment 2

Segment Displaceme

nt

Base Displacement

Segment start address

plus displacement equals

absolute segment

address

Expand


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Paging

a program is broken into fixed-lengthpages

Page size is generally small (perhaps2K to 4K)

Like segments, a programs pagesare loaded into noncontiguous

memory.


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Paging

Addresses consist of two parts :

a page number in the high-order

positions

a displacement in the low-order bits.

Addresses are dynamically translatedas the program runs.


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Paging

When an instruction is fetched, itsbase-plus displacement addressesare expanded to absolute addresses

by hardware.

Then the pages base address islooked up in a program page table

(like the segment table, maintainedby the operating system) and addedto the displacement.


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Paging: Figure f

Page Startaddress

0 aaa

1 bbb

2 ccc

Page 0

Page 1

Page 2

Page Displaceme

nt

Base Displaceme

nt

Segment start address

plus displacement

equals absolute pageaddress

Expand


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Dynamic Address Translation

If, while executing an instruction, a CPU fetches aninstruction located at a particular virtual address, or fetchesdata from a specific virtual address or stores data to aparticular virtual address, the virtual address must betranslated to the corresponding physical address. This isdone by a hardware component, sometimes called a

memory management unit, which looks up the realaddress (from the page table) corresponding to a virtualaddress and passes the real address to the parts of the CPUwhich execute instructions. If the page tables indicate thatthe virtual memory page is not currently in real memory,the hardware raises a page faultexception (specialinternal signal) which invokes the paging supervisor

component of the operating system.
http://en.wikipedia.org/wiki/CPUhttp://en.wikipedia.org/wiki/Memory_management_unithttp://en.wikipedia.org/wiki/Memory_management_unithttp://en.wikipedia.org/wiki/CPUhttp://en.wikipedia.org/wiki/CPUhttp://en.wikipedia.org/wiki/Page_faulthttp://en.wikipedia.org/wiki/Exceptionhttp://en.wikipedia.org/wiki/Page_faulthttp://en.wikipedia.org/wiki/Exceptionhttp://en.wikipedia.org/wiki/Operating_systemhttp://en.wikipedia.org/wiki/Operating_systemhttp://en.wikipedia.org/wiki/Operating_systemhttp://en.wikipedia.org/wiki/Exceptionhttp://en.wikipedia.org/wiki/Page_faulthttp://en.wikipedia.org/wiki/CPUhttp://en.wikipedia.org/wiki/Memory_management_unithttp://en.wikipedia.org/wiki/CPU


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Steps to convert the

segment/displacement address to an

absolute address, hardware:

1. checks the special register to find the

programs segment table,

2. extracts the segment number from the

expanded address,3. uses the segment number to search the

programs segment table,


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4. finds the segments absolute entry point

address, and

5. adds the displacement to the entry point

address to get an absolute address


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Virtual Memory

splits up a program into small modular

pieces

a technique that allows the execution of

processes that may not be completely in

memory.

ie, the process can start executing with only

part of the process image in the memory.


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Virtual Memory

Advantage :

the program can be larger than the physical

memory.

You can run more applications at once.

You can run larger applications with less real

RAM.

Applications may launch faster because ofFile Mapping.

You don't have to buy more memory(RAM).


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Virtual Memory

Applications run slower.

It takes more time to switch between

applications. Less hard drive space for your use.

Reduced system stability.


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Virtual Memory

It shows three levels of storagevirtual

memory, the external paging device, and

real memory.

Real memory is good, old-fashioned

main memory, directly addressable by

the processor.


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Virtual Memory

The external paging device is usually

disk, model that simplifies address

translation.

Virtual memory is a model that simplifies

address translation. It contains the

operating system and all the application

programs, but it does not physically existanywhere


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Virtual Memory

A computer system

technique, which

gives an application

program theimpression that it

has contiguous

working memory(an address space),

while in fact it may

be physically


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Virtual Memory

Consists of 3 main parts :

Main memory (RAM)

Secondary memory(disk)

Page Table


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Virtual MemoryMain Memory(RAM)

Store the more recently used pages.

Each page is stored into subdivisions ofmemory called "frames." A frame size is

the same as page size. If we have a 4096 byte page size, then each

frame will hold 4096 bytes to accommodatethe pages.

The first frame will take up actualaddresses 0x00000000 to 0x00000FFF.The second frame will take up address

0x00001000 to 0x00001FFF. Etc. If


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Virtual MemoryDisk

The disk memory is a repository for

pages not currently in use. When a page

needs to be brought to memory, the

appropriate page is found andtransferred to main memory. Whenever a

page that has been modified during its

time in main memory, it is written to disk.Think of the pages on the disk as being

in long term storage.


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Virtual MemoryPage Table

keeps track of where pages are, and whattheir properties are. The system updatesthe page table as changes in the state ofthe system warrant. It makes sense that

there are as many entries in the page tableas there are pages in our virtual addressspace.

Therefore, in a virtual address space

addressed by 32 bits, at 4K per page that's220 pages, and hence 220 entries in thepage table. The first entry in the page tablecontains information about the first page.The second entry contains information

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Chapter 3-Memory Management