paging in os

7/24/2019 Paging In OS

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Paging

To, By

Dr. Varsha Garg Sanjeeb Nayak


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1. Basic defnition o paging.

2. Address mapping.

3. Page translation.

4. Paging hardware.

5. Paging example. 6. Implementing paging.

. Associati!e memor".

#. Ad!antages and disad!antages o paging.

$opics to %e co!ered


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Physical memory is divided into fed si!e"

blocks called FRAMES #ogical memory is divided into blocks o$

the same si!e called PAGES

% $rame has the same si!e as a &age is a &lace 'here a (logical) &age can be

(&hysically) &laced

Basic &efnition o Paging

Memory-management technique that

permits thephysicaladdress space of aprocess to be non-contiguous


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Address 'apping


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*very address generated by the +P is

divided into t'o &arts- Page n(m%er )p*and Page o+set )d*

Paging ,ardware


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The &age nmber is sed as an inde into a

Page $a%leThe &age si!e is defned by the hard'areThe si!e o$ a &age is ty&ically a &o'er o$ /,

varying bet'een 01/ bytes and 123B &er

&age 4eason- 5$ the si!e o$ logical address is

/6m and &age si!e is /6n, then the high"

orderm"n bits o$ a logical address designate the&age nmber

Paging ,ardware


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Paging ,ardware


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Implementing Page $a%le

To im&lement &aging, the sim&lest methodis to im&lement the &age table as a set o$registers

7o'ever, the si!e o$ register is limited andthe si!e o$ &age table is sally large

There$ore, the &age table is ke&t in mainmemory


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8hen 'e se a &aging scheme, 'e have noeternal $ragmentation- %N9 $ree $rame canbe allocated to a &rocess that needs it.

7o'ever, 'e may have internal

$ragmentation 5$ the &rocess re:ires n &ages, at least n$rames are re:ired

The frst &age o$ the &rocess is loaded into the

frst $rame listed on $ree"$rame list, and the$rame nmber is &t into &age table

Paging -xample


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'' Address $ranslation


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$/B Assisted $ranslation

5$ 'e 'ant to access any location, 'emst frst inde into the &age table

This re:ires at least one memory

accessThe standard soltion is to se a

s&ecial, small, $ast cache, called

$ranslation loo0aside %(+er )$/B*or associati!e memor"


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$/B Assisted $ranslation


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$/B

5$ the &age nmber is not in the T#B (T#B

miss) a memory re$erence to the &age tablemst be made.

5n addition, 'e add the &age nmber and$rame nmber into T#B

5$ the T#B already $ll, the ;S have to mstselect one $or re&lacement

Some T#Bs allo' entries to be wire down,meaning that they cannot be removed $romthe T#B, $or eam&le kernel codes

The &ercentage o$ times that a &articlar&age nmber is $ond in the T#N is calledhit ratio

5$ it takes /< nanosecond to search the T#Band 1


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%DV%NT%G*S

Noeternal =ragmentation

Sim&le memory management algorithm S'a&&ing is easy (*:al si!ed Pages and

Page =rames) Share common code es&ecially in a time"sharing

environment

D5S%DV%NT%G*S

5nternal $ragmentation

Page tables may consme more memory. 3lti level &aging leads to memory

re$erence overhead.


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Thank You

paging in os

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