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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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M'I3 M4M506 507'3I8'TI53
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>
$%e$)
$ $%4+ =6T4&
$ M= > $%4; =6T4&
$ 7= > $%4 ? =6T4&
$T= > $%4$) =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.
&4C53@'06 &T50'74 @4IC4&
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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.
@IDD4043T
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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.
@I&< P'C
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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 $%e? > +7=
1gigabytes2
'CC4&& M4C#'3I&M
@ata are recorded on the trac"s of a spinning dis" surface and read
from the surface by one or more readEwrite heads.
Most dis" dries hae a single readEwrite head for each dis" surface.
The access arms can be moed in and out in the direction so that the
readEwrite head can be moed horiAontally across the surface of the
dis"s.
'CC4&& TIM4
@is" access time is the interal between the instant a computer ma"es a
re!uest for transfer of data from dis" system to primary storage. The dis"
access time depends on three parameters
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$. &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".
@I&< D50M'TTI37
&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.
T6P4& 5D M'734TIC @I&
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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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T6P4& 5D #'0@ @I&
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#'0@ @I&< 5UTP40D50M D5PP6 @I&
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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.
='&IC P0I3CIP4 5D 5P40'TI53
&T50'74 507'3I8'TI53
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
'CC4&& M4C#'3I&M
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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.
T6P4& 5D 5PTIC' @I&
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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.
'@'3T'74& 5D 5PTIC' @I&
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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 $% dis"s. ' large 0'I@ unit can be
composed of more than one hundred dis"s and can hae
storage capacity of seeral tetrabytes.
'UT5M'T4@ T'P4 I=0'06
'n automated tape library is a set of magnetic tapes with a
controller mounted on a single unit. The unit has one or
more tape drie to read and write data on the tapes in the
tape library. In case of unit haing multiple tape dries, it is
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possible to readEwrite data fromEto multiple magnetic tapes
simultaneously resulting in faster data transfer rate. If
5ne drie fails, the unit can continue to function with other
dries.
C@ 05M U
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A thumb drive is portable memory storage. It is re-writeable and holdsits memory without a power supply, unlike RAM. Thumb drives will fitinto any U! port on a "omputer. They will also #hot swap,# whi"hmeans a user "an plug the drive into a "omputer and will not have torestart it to a""ess the thumb drive. The drives are small, about thesi$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.
@I&< P'0TITI53&
'isk partitioninis the act of diiding a hard dis" drieinto multiple
logical storage units referred to as partitions, to treat one physical dis" drie
http://www.wisegeek.com/topics/portable-memory-storage.htmhttp://www.wisegeek.com/what-is-a-power-supply.htmhttp://www.wisegeek.com/topics/computer.htmhttp://www.wisegeek.com/topics/windows.htmhttp://www.wisegeek.com/topics/running.htmhttp://www.wisegeek.com/topics/operating-system.htmhttp://www.wisegeek.com/topics/software.htmhttp://www.wisegeek.com/topics/driver.htmhttp://www.wisegeek.com/what-is-an-upload.htmhttp://www.wisegeek.com/what-is-a-hacker.htmhttp://www.wisegeek.com/topics/virus-software.htmhttp://www.wisegeek.com/topics/infected.htmhttp://www.wisegeek.com/topics/servers.htmhttp://www.wisegeek.com/topics/documents.htmhttp://www.wisegeek.com/what-are-megabytes.htmhttp://en.wikipedia.org/wiki/Hard_disk_drivehttp://www.wisegeek.com/what-is-a-power-supply.htmhttp://www.wisegeek.com/topics/computer.htmhttp://www.wisegeek.com/topics/windows.htmhttp://www.wisegeek.com/topics/running.htmhttp://www.wisegeek.com/topics/operating-system.htmhttp://www.wisegeek.com/topics/software.htmhttp://www.wisegeek.com/topics/driver.htmhttp://www.wisegeek.com/what-is-an-upload.htmhttp://www.wisegeek.com/what-is-a-hacker.htmhttp://www.wisegeek.com/topics/virus-software.htmhttp://www.wisegeek.com/topics/infected.htmhttp://www.wisegeek.com/topics/servers.htmhttp://www.wisegeek.com/topics/documents.htmhttp://www.wisegeek.com/what-are-megabytes.htmhttp://en.wikipedia.org/wiki/Hard_disk_drivehttp://www.wisegeek.com/topics/portable-memory-storage.htm8/12/2019 Chap1.4 Memory
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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