2006is112Ch05

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Chapter 5

Data Storage Technology

2005 IS112

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Chapter goals

Describe the distinguishing characteristics of

primary and secondary storage

Describe the devices used to implement primary

storage Describe the memory allocation schemes

Compare and contrast secondary storage

technology alternatives

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Goals cont.

Describe factors that determine storage device performance

Choose appropriate secondary storage technologies and devices

Eplore storage devices and their technologies

!utlines characteristics common to all storage devices

Eplains the technology strengths and "ea#nesses of primary

storage and secondary storage

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Storage types

$rimary storage % memory or &'( )olds instructions and data for currently eecuting

programs

*olatile % re+uires electricity to maintain data Secondary storage % electromagnetic or

optical devices ,on-volatile storage devices "ith large capacities

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Storage device components

Storage devices are comprised of Storage medium

&ead."rite mechanism

Device controller % interface bet"een the storage

device and the system bus /discussed in chapter

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Storage device

characteristics

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Speed

Speed of primary storage /&'( directly impactsperformance of entire system

&'( etends the limited capacity of C$ registers

The C$ continually moves data and instructionsbet"een registers and &'( If a read."rite to &'( ta#es more than one C$

cycle3 then C$ must "ait for information

&'( is faster than secondary storage by a factor of105 or more

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Speed cont.

Speed is also an issue for secondary storage

Called 4access time or 4see# time

'ccess time is defined as time to complete one read

or "rite operation 'ccess time for dis# or tape storage can vary

depending on location of information3 therefore

access time is epressed as an average

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Access times

$rimary storage % epressed in nanoseconds

/billionths of a second

Secondary storage % epressed in

milliseconds /thousandths of a second

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Data transfer rate

Complete measure of data access speed

consists of access time and the unit of data

transfer to.from the storage device 'ccess time plus ho" much data is transferred

Data transfer unit for primary storage is

based on "ord si6e /usually 72 bit

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Data transfer unit

Data transfer unit /amount of data moved at atime for secondary storage varies dependingon the device

nit is called a 4bloc# 8loc# si6e is stated in bytes 4Sector is data transfer unit for magnetic and

optical devices Common sector.bloc# si6e is 512 bytes

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Data transfer rate

Epressed in terms of bytes.second 'ccess time combined "ith data transfer unit

Data transfer rate describes ho" much data

can be transferred bet"een devices over a

period of time

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Volatility

*olatile % storage device is volatile if it cannot

reliable hold data for long periods of time

,on%volatile % storage device is non-volatile if it can

reliably store data for long periods of time Computer systems need a combination of volatile

and non-volatile storage devices

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Access method

$hysical structure of storage device9s

read."rite mechanism determines the "ay/s

data can be accessed Serial access &andom access

$arallel access

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Serial access

Stores and retrieves data items in a linear or

se+uential order

Slo"est access method

Tape typically used for bac#up purposes

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Random access

'lso called a direct access device Can directly access data stored on the device 'll primary storage and dis# storage devices

are direct access $arallel access % "ith multiple read."rite

heads3 can simultaneously access more than

one storage location

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Cost and capacity

'n increase in speed3 permanence or portability

generally comes at increased cost if all other factors

are held constant:

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Storage DeviceCharacteristics

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Primary storage devices

&andom access memory /&'( is a generic

term for storage device that (icrochip implementation using semiconductors

'bility to read and "rite "ith e+ual speed &andom access to stored bytes3 "ords3 or larger

data units

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Primary Storage Devices

Critical performance characteristics 'ccess speed

Data transfer unit si6e

(ust closely match C$ speed and "ord

si6e to avoid "ait states

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Storing lectrical Signals

Directly 8y devices such as batteries and capacitors

Trade off bet"een access speed and volatility

Indirectly ses energy to alter the state of a device; inverse

process regenerates e+uivalent electrical signal

(odern computers use memory implemented"ith semiconductors /&'( and ,*(

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Random Access !emory

Characteristics (icrochip implementation using

semiconductors

'bility to read and "rite "ith e+ual speed

&andom access to stored bytes3 "ords3 or

larger data units

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SRA! vs. DRA!

Static &'( % implemented "ith transistors &e+uires continuous supply of electricity to preserve data

Dynamic &'( % uses transistors and capacitors

&e+uire a fresh infusion of po"er thousands of times persecond:

Each refresh operation is called a refresh cycle

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Random Access !emory

To bridge performance gap bet"een memory

and microprocessors &ead-ahead memory access

Synchronous read operations !n-chip memory caches

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"onvolatile !emory

&andom access memory "ith long-term or

permanent data retention

sually relegated to speciali6ed roles and

secondary storage; slo"er "rite speeds andlimited number of re"rites

<enerations of devices /&!(3 E$&!(3 and

EE$&!(

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"onvolatile !emory

=lash &'( /most common ,*( Competitive "ith D&'( in capacity and read

performance

&elatively slo" "rite speed >imited number of "rite cycles

,*( technologies under development

=erroelectric &'( $olymer memory

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SRA! vs. DRA!

Static &'( % implemented "ith transistors &e+uires continuous supply of electricity to preserve data

Dynamic &'( % uses transistors and capacitors

&e+uire a fresh infusion of po"er thousands of times persecond:

Each refresh operation is called a refresh cycle

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Read only memory

&!( % random access memory device that

can store data permanently or semi-

permanently

Typically used to store 8I!S /basic inputoutput services

Instructions stored in &!( is called firm"are

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!emory pac#aging

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CP$ !emory Access

(anagement of &'( is critical to

performance of computer

!rgani6ation3 access3 and management or

&'( is done by the operating system )o" memory is accessed is large factor in

performance of &'(

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Physical memoryorgani%ation (ain memory of any computer is a se+uence of

contiguous memory cells 'ddressable memory % highest number storage byte

that can be represented Determined by the number of bits used to represent an

address If 72 bits used to represent and address3 highest address is

272 ? @32A@3AB32A3 or @ <8

$hysical memory % actual memory installed3 usuallyless than addressable memory

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!emory addressing &allocation (emory allocation is the assignment or reservation

of memory segments for system soft"are3

application programs3 and data

(emory allocation is the responsibility of theoperating system

Common scheme is to place !S in lo" memory and

applications in high memory

This can be demonstrated "ith C program

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!emory allocation

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Absolute vs. relativeaddressing Some programming languages /C3 C

allo" instructions that reference eplicitmemory locations

8&',C) to location ST! to location

'bsolute addressing describes memoryaddress operands that refer to actual physical

memory locations

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Relative addressing

Instructions that refer to memory use a combinationof registers to compute addresses

hen !S loads application into memory3 !S loadsstarting point of application into one register

Instruction in application that refers to memorylocation is using an offset /i:e: distance frombeginning of application

!S adds offset to starting point to calculate physical

memory location

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Segmented memory

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!agnetic storage

ses magnetism to store binary information onto astorage medium that can store magnetic information

>east epensive medium for secondary storage

Can be portable &etains data "ithout electricity !ver longer periods of time "ill eventually lose

information

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Read)'rite in magneticdevice

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!agnetic decay and lea#age

$rimary disadvantage is loss of data over

time Magnetic Decay % the tendency of magnetically

charges particles to lose their charge over time Magnetic Leakage % a decrease in the strength of

individual bit charges

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!agnetic storage

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*rgani%ation of trac#s andsectors

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*ptical mass storage devices

Advantages: )igher recording density

>onger data life

&etain data for decades

,ot subFect to problems of magnetic decay and lea#age

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*ptical storage

!ptical storage devices store bit values as

variations in light reflection:

Storage medium is a surface of highly reflectivematerial:

The read mechanism consists of a lo"-po"er laser

and a photoelectric cell:

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Storing binary information

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+amples of optical devices

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Chapter summary

' typical computer system has primary and

secondary storage devices

The critical performance characteristics of primarystorage devices are their access speed and the

number of bits that can be accessed in a single read

or "rite operation

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Summary cont.

$rograms generally are created as through theyoccupied contiguous primary storage locationsstarting at the first location

(agnetic storage storage devices store data bits asmagnetic charges

!ptical discs store data bits as variations in lightreflection

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2006is112Ch05