Chap1.4 Memory

8/12/2019 Chap1.4 Memory

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MEMORY

Main Memory :It is a temporary storage area that is built into the computer

hardware and in which instructions and data of a program reside mainly

when the program is executed by the CPU. Physically, this memory consistsof some chips either on the motherboard or on a small circuit board attached

to the motherboard of the computer system. This built in memory allow the

CPU to store and retriee data ery !uic"ly. #ence the rate of data fetching

from main memory is about $%% times faster than that from a high speed

secondary storage li"e dis".

&torage ealuation criteria

'ny storage unit of a computer system has the following characterstitcs

$. &torage capacity( It is the amount of data that can be stored in the

storage unit. ' large capacity is desired. ' primary storage unit has

less capacity.

). 'ccess time * Time re!uired to locate and retriee stored data from

storage unit in response to a program instruction. ' fast access time is

preferred. Primary storage units hae faster access time.

+. Cost per bit of storage * cost of storage unit for a gien storage

capacity. ow cost is desirable. Primary unit hae higher cost per bit

of storage.-. olatile( If the storage unit can retain the data stored in it een when

the power is turned off or interrupted , it is called non(olatile storage.

If the data stored are lost when the power is turned off or interrupted it

is called olatile storage. Primary * olatile, secondary * non (olatile

/. 0andom access( If the time ta"en to access a piece of data from

storage unit is independent of the location of the data in the storage

unit, it is called random access storage or random access memory

10'M2.

&e!uential access is the access time that directly depends on the

location of the data.


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The primary storage or main memory is made of seeral small storage

areas called locations or cells.

4ach of these cells can store fixed number of its called word length .

7ien memory is diided into 3 words, where 3 is some power of )

4ach word or location has a built in and uni!ue number assigned to it.

This number is called address of the location .

4ach location can hold either a data item or an instruction and its

address remains same regardless of its contents.

The address normally starts a % and the highest address e!uals the

number of words that can be stored in the memory minus $.

4x. If the memory has $%)- locations, the address range is % to $%)+

The act of entering data into a storage location is calledmemory writeoperation.

The act of retrieing data from a storage location is called a memory

read operation

9hen we say (bit,$; bit or +) bit computer ,it refers to the word length

of the memory of a particular computer in terms of total number of bits

per memory word. 9ord length is an important architectural factor.

M'I3 M4M506 C'P'CIT6

The capacity is defined in the terms of the number of bytes a computer can

store.

Memory capacity is normally stated in terms of "ilobytes 1 $%)-

1)4$%2 or in terms of megabytes 1M=2 >)%-, gigabytes > )e+% , tetrabyte>

$%e$)

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$ 7= > $%4 ? =6T4&

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0'M

9hen people tal" about computer , they usually mean the olatile 0'M

memory. This memory consists of some integrated circuit chips either on the

motherboard or on a small circuit board attached to the mother board.0'M chips are of two types:

$. @0'M * @ynamic 0'M uses a external circuitry to refresh the

storage charge to retain the stored data.

). &0'M * &tatic 0'M * does not need any special regenerator circuit

to retain stored data.

&0'M is faster ,costlier and consume more power than @0'M. arge

memories use @0'M and &0'M is mostly used for specialiAed

applications.

05M

05M 10ead 5nly Memory2 is a non olatile memory chip in which the data

is stored permanently and cannot be altered by the programmer. &toring data

permanently on this "ind of memory is called B burning in the data because

data is such memory is stored by using fuse lin"s.

05M is used to store programs and data which do not change and arefre!uently used.

P05M( programmable read only memory * because a user can program it.

P05M is non(olatile storage,i.e. the stored information remains intact een

if the power is switched off.

4P05M * erasable programmable read only memory * it is possible to erase

information stored in the 4P05M chip and the chip can be reprogrammed to

store new information.

U4P05M * erasing the stored information by exposing to ultraiolet rays.

44P05M * erasing information by exposing to high oltage electric pulse.

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The primary storage deices hae the following limitations:

$. limited capacity * the storage capacity of the primary memory is

limited.

). olatile * the primary storage is limited and data stored in it is lost

when the electric power is turned off.

Therefore , in addition to the primary memory we hae auxillary memory

or secondary memory which is non olatile and has low cost per bit

stored but operating speed is far slower than that of primary memory.

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small , low capacity dis" to seeral thousands on a large , high

capacity dis".

4ach trac" is further diided into &4CT50&. In addition to the

concentric circles , the dis" surface is also diided into inisible pie

shaped segments. Thus if there are such pie shaped segments ,eachtrac" will get diided into eight parts, and each of these eight portions

of a trac" is called a &4CT50.

If a dis" has )%% trac"s and sectors per trac", then we say that there

are )%%F > $;%% sectors and not sectors

4ach sector is assigned a uni!ue number. The dis" address is the

physical location of the record on the dis" and it is comprised of

$. sector number

). trac" number

+. surface number 1when double sided dis" are used2

@5& operating system also combines two or more sectors to form a

cluster. In this case, the smallest unit of data access from a dis"

becomes a cluster and not a sector. Cluster based dis"

organiAation ,on a aerage, half of the cluster siAe is wasted for

each file stored on the dis". #oweer, cluster based organiAation

leads to less management oerhead for mapping data records to


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physical dis" locations.

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&T50'74 C'P'CIT6

The storage capacity of the dis" system depends on

$. number of recording surfaces

). number of trac"s per surface

+. number of sectors per trac"

-. number of bytes per sector

therefore the

&T50'74 C'P'CIT6 5D ' @I&< &6&T4M > number of recording

surfaces F number of trac"s per surface F number of sectors per trac" F

number of bytes per sector

4G'MP4: ' dis" pac" has $% dis" plates each haing );// trac"s.

&uppose there are $)/ sectors per trac" and each sector can store /$) bytes.

9hat is the storage capacityH

$ recording surface 1 as we do not include uppermost and lowermost2

capacity > $ F );// F$)/ F/$) > +.%/,/,;%,%%% > + F $%e? > +7=

1gigabytes2

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@ata are recorded on the trac"s of a spinning dis" surface and read

from the surface by one or more readEwrite heads.

Most dis" dries hae a single readEwrite head for each dis" surface.

The access arms can be moed in and out in the direction so that the

readEwrite head can be moed horiAontally across the surface of the

dis"s.

'CC4&& TIM4

@is" access time is the interal between the instant a computer ma"es a

re!uest for transfer of data from dis" system to primary storage. The dis"

access time depends on three parameters


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$. &44< TIM4 : The time re!uired to position the readEwrite head oer

the desired trac" is called the see" time.The aerage see" time is of

the order of $% to$%% milliseconds.

). 'T43C6 * The rotational waiting time ,ie that is the time re!uired

to spin the desired sector under the head is called latency. The latency

is also "nown as rotational delay time.

+. T0'3&D40 0'T4 * Transfer rate refers to the rate at which data are

read from or written to the dis".

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&ince we hae dis" of arious siAes, the dis" formatting concept was

introduced. =efore a dis" can be used with a computer system, it must first

be prepared by the means of a process called @I&< D5M'TTI37. 0aw 1unformatted2 dis" is inserted in the dis" drie so that it can lay

down a magnetic pattern on the dis" surface. This pattern enables the

dis" drie to organiAe and store data in the data organiAation defined

for the dis" drie.

Most computer maintain a dis" table with sector and trac" location of

the data. This table is called D'T1fie allocation table2, which helps the

computer to locate the data easily.

@is" formatting is handled by the operating system

If the formatting done by the dis" drie of two different computersystem is exactly the same , the computer are said to hae compatible

disk drives.

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handling protection to the dis" surface. They are also called floppies or

diskettes.

Floppy disk drive: is a deice that is used to readEwrite data fromEto

floppy dis". The drie has a spindle that rotates the dis", and readEwrite head

that can moe in and out to position the readEwrite head on any trac" of the

dis" surface. It is of the interchangeable dis" type. That is, it allows the

loading and unloading of magnetic dis"s and when they are needed for

readingEwriting data on to them.

Types of floppy disks: two types are used ery popularly today :

1. The ! inch diskette: it is a floopy dis" with of / J inch diameter.

The ac"et has on oal cut 1aperture2 through which thereadEwrite head ma"es direct contact with the dis" surface.

The dis" has a hole in the center in which the drie spindle fits so

that the dis" can be rotated at high speed during operation.

4arly data could be recorded on one side of the dis" and were

"now as single sided 1&&2 dis"ettes.

Today data is recorded on both the sides and is "nown as double

sided dis"ettes 1@@2.

@ouble sided dis"ettes today come in two capacities * double

density1@@2 and high density 1#@2, dis" density is the number ofbits that can be stored per s!uare inch area on the surface of the

dis"

.


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". The # $ inch diskettes: It has the diameter of + K inches. Unli"e the /

J inch dis"ette , the aperture of + K inch dis"ette is coered with a

sliding metal piece. 9hen the dis"ette is inserted into the drie, this

coer slides bac" to expose the dis" surface to the readEwrite head.

%&R' '()*)

#ard dis"s are primary on(line secondary storage deices for most computer

systems today. #ard dis" are made up of rigid metal 1 fre!uently

aluminium2. The hard dis" platters come in many siAes ranging from $ to $-

inch diameter .


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5ptical dis" are relatiely new secondary storage medium.

'n optical dis" storage consists of a rotating dis" which is coated with a

thin metal or some material that is highly reflectie. aser beam technology

is used for recordingEreading of the data on the dis". @ue to the use of laser

beam technology , optical dis" are also "nown as laser dis"s or optical laser

dis"s.

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5ptical dis" has one long trac" that starts at the outer edge and spiral

inward to the center. This spiral trac" is ideal for reading large bloc"

of se!uential data.

5ptical dis" trac" are also spilt into sectors but each sector has the

same length regardless of whether it is near the center or away from

the center.

It needs a more complicated drie mechanism because the rotation

speed of the dis" must ary inersely with the radius, the drie must

slow down the dis" rotation speed to read sectors towards the outside

of the dis" and speed it up to read sectors towards the center of the

dis".

&T50'74 C'P'CIT6

's optical dis" hae a single trac" , their storage capacity depends on the

following:

$. number of sectors

). number of bytes per sector

storage capacity of an optical dis" > number of sectors F number of bytes

per sector

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5ptical dis" uses laser technology for reading Erecording of data on the dis"

surface. 5ne laser beam1 of greater intensity2 is used to write to the

recording surface by etching microscopic pits on the dis" surface and

another laser beam 1of lesser intensity2 is used to read the data from the light

sensitie recording surface. The beam is strongly reflected by the coated

surface 1"now as land2 and wea"ly reflected by the burnt surface 1"nown as

pit2, producing patterns of on and off that are conerted into electronic

signals of binary $ and % by the sensor.

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etched on the surface of a 950M dis" , it becomes permanent ,

which can be read but not altered. 9riting on the surface of a 950M

dis" cannot be done in multiple sessions and all the data to be

recorded hae to be written on the dis" surface in a single recording

session.

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$. mirroring

). stripping

In , mirroring the system ma"es exact copies of the files on to hard dis"s.

That is, one is the mirror image of the other one and if one of the two

copies gets corrupted , the other copy is aailable for accessing.

In case of stripping, a file is partitioned into smaller parts and different

parts of the files are stored on different dis"s. 9hile the file is read, the data

on the spare dis" ensures that the data read from other dis" are correct.

0'I@ unit can hae $% dis"s. ' large 0'I@ unit can be

composed of more than one hundred dis"s and can hae

storage capacity of seeral tetrabytes.

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'n automated tape library is a set of magnetic tapes with a

controller mounted on a single unit. The unit has one or

more tape drie to read and write data on the tapes in the

tape library. In case of unit haing multiple tape dries, it is


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possible to readEwrite data fromEto multiple magnetic tapes

simultaneously resulting in faster data transfer rate. If

5ne drie fails, the unit can continue to function with other

dries.

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A thumb drive is portable memory storage. It is re-writeable and holdsits memory without a power supply, unlike RAM. Thumb drives will fitinto any U! port on a "omputer. They will also #hot swap,# whi"hmeans a user "an plug the drive into a "omputer and will not have torestart it to a""ess the thumb drive. The drives are small, about thesi$e of a human thumb - hen"e, their name - and are very stablememory storage devi"es.

At first, some users saw their la"k of "ompatibility with %indows&'as a drawba"k of thumb drives, sin"e so many users were stillrunningthat operating systemwhen the drives were introdu"ed.(owever, most thumb drives "ame e)uipped with softwarethatinstalled a %in&' driveron the "omputer, enabling the user to"onne"t a thumb drive. Installing the software took about *+ se"onds.

Thumb drives also pose se"urity threats, sin"e they are easily"on"ealed. Users "ould "opy proprietary information to them, oruploadha"kingsoftware from them, all undete"ted by the systemadministrator.

(owever, system administrators "an also upload anti-virus softwareto an infe"ted"omputer from a thumb drive, for instan"e, withoutrisking the system servers. Their write speeds and read speeds onlyreally "ome into play when users are running large appli"ations fromthem. %hen the user is saving tet do"umentsor photos, for

instan"e, these speeds are not nearly as important. Most thumbdrives also have millions of re-write "y"les and will store data for tenyears before they need repla"ing.

The thumb drive is available in storage si$es of up to * gigabytes.Most people, however, will find that si$es of /* megabytesto /M! will do ni"ely. If the person is storing mostly tet, with few images,then an even smaller thumb drive may meet the need.

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'isk partitioninis the act of diiding a hard dis" drieinto multiple

logical storage units referred to as partitions, to treat one physical dis" drie
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as if it were multiple dis"s. Partitions are also termed NslicesN for operating

systems based on =&@and &olaris. 'partition editorsoftware program can

be used to create, resiAe, delete, and manipulate these partitions on the hard

dis".
http://en.wikipedia.org/wiki/Slice_(disk)http://en.wikipedia.org/wiki/BSDhttp://en.wikipedia.org/wiki/Solaris_(operating_system)http://en.wikipedia.org/wiki/Partition_editorhttp://en.wikipedia.org/wiki/Slice_(disk)http://en.wikipedia.org/wiki/BSDhttp://en.wikipedia.org/wiki/Solaris_(operating_system)http://en.wikipedia.org/wiki/Partition_editor

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Chap1.4 Memory