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Chapter 3

Data Storage

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Learning outcomes

By the end of this Chapter you will know the difference between

• Electronic memory

• Magnetic memory

• Optical memory

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Additional Reading

Essential Reading• Stalling (2003): Chapters 5 and 6

Further Reading• Burrell (2004): Chapters 3 and 7

• Schneider and Gersting (2004): Chapters 4 and 5

• White (2002): Parts 3 and 4.

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Hardware Components

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Introduction

Information can be stored in different ways:• Books,

• Films

Information is stored to be re-used.

Information in computers must be able to able to be processed by computers:• Information must be stored in appropriate places

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Introduction (Cont)

Computers use the binary system to store and process information.

Different type of media storage

• Electronic memory (main memory)

• Magnetic memory (hard disc)

• optical memory (CD-ROM)

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CPUMain

memory

Add. bus

Data bus

Control bus

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Main memory

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Main Memory (Electronic Memory)

What is the role of the main memory?• Main memory stores data which are currently been processed or

ready to be processed

Electronic memory (fast)• Based on electronic principles.• Formed with logic gates

Main Memory is volatile

It is divided into cells• Each cell is a sequence of one-bit memories• Each cell has a unique address

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Arrangement of Memory Cells

value = 01101101

Each cell has a unique address Longer strings stored by using

consecutive cells

RAM (random access memory)

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Media storage•Magnetic Storage

•Tapes •Hard drives (not always removable)•Floppy•Zips•Jaz

•,Optical Storage

•CDs•DVDs

•Solid State Storage•Compact flash•Smart Media

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What is Magnetic Media?

The most common and enduring form of removable-

storage technology is magnetic storage.

The magnetic medium can be easily erased and rewritten, and it will "remember" things stored onto the medium for many years.

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Magnetic Memory

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Magnetic tape (2)

• Serial access (slow)• Good choice for off-line data storage (archives)

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Magnetic Tape (1)

Serial access Slow Very cheap High capacity Backup

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Magnetic Memory

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Magnetic disc

• Each track contains same number of sectors • Each sector contains a number of

• magnetized and demagnetized spots

spots

sector

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Stalling (2003) pages:167-168

Magnetic Disk Terminology Platter:

• rigid metal or glass platter Coated with magnetic material.• rotating at constant angular velocity

Arm:• With movable magnetic read/write heads

Track: • A complete ring of data• The disk surface is divided into tracks

Sectors:• Each track is subdivided into sectors

Cylinder:• A vertical collection of tracks at the same radial position

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Magnetic Disks

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Magnetic disc density

Same number of bits per track Tracks near the centre are more dense The further is the tracks the less dense it is. Constant Angular Velocity (CAV)

The disc rotation speed is constant to maintain a constant transfer.

Stalling (2003) pages:167-168

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Thus as the platter rotates under the head, a stream of bits can be written and later read back.

Magnetic Disks

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Read and Write Mechanism (1)

Recording and retrieval via conductive coil called a head May be single read/write head or separate ones During read/write, head is stationary, platter rotates Write

• Electric Current through coil of wire produces magnetic field• Magnetic Pulses sent to the head• Magnetic pattern recorded on surface below

Read• Magnetised bit pattern

• Magnetic field induces an electrical current in the coil• The bit pattern contains 1

• Demagnetised bit pattern • No Magnetic field induced, hence, no electrical current in the coil• The bit pattern contains 0

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Fixed/Movable Head Disk

Fixed head• One read/write head per track

• Heads mounted on fixed ridged arm

Movable head• One read/write head per side

• Mounted on a movable arm

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Access Information on a Floppy disk

To access information on a floppy:

• Track number, and

• Sector number.

Head moves to the target track. waits for the desired sector to spin underneath it read/write begins.

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Maximum data transfer rate

It is the rate at which data passes under the read/write head (bytes/sec).• Number of bytes / track * Number of rev / sec

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Multiple Platter (hard disk)

Permanent storage that is inside of the computer, and NOT portable.

Consists of several platters which spin very fast

Heads are joined and aligned Aligned tracks on each platter form

cylinders Data is striped by cylinder

• reduces head movement

• Increases speed (transfer rate)

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Multiple Platters (2)

• Disk platters speed (3600 to 10 000 rpm (rev/min).

•floppy (360rpm).

•The read data we need to specify cylinder, head, and sector numbers. Each cylinder represents a track number.

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Cylinders

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Magnetic Tape (1)

Serial access Slow Very cheap High capacity Backup

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Features of Magnetic Memory Memory capacity: Floppy can hold 700KB – 120MB. Hard disk can hold dozen of GB, 10, 20,.. Tapes can hold 100MB- 80GB. Access method

• Floppy and hard disks is random as the main memory• Tape is serial

Access time:• It is the average time taken to position the R/W head over

the data to be read• For disk drives is about 10-3 sec when in MM 10-9 sec.

Transfer rate: is slower. It is the transfer of data between MM and Mag/M. Floppy (500kB-2MB) and hard disc (4-12MB).

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Optical Storage – CD-ROM Is a disc with highly reflective surface. Tiny areas flat and depressed:

• Flat (land) strong reflection.• Depressed (pits) low reflection.

Laser landstrong reflectionphoto-sensor generates electrical voltagestore 1s.• Laser: (light Amplification stimulated emission of

radiation). Lightpitslow reflection no electrical voltage

stores 0s.

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CD-ROM Operation

• Data stored by creating variations in the reflective surface• Data retrieved by means of a laser beam• Data stored uniformly (so CD rotation speed varies)• Random access much slower than for magnetic disks

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The pits and lands are written in a single continuous spiral starting near the hole and working out a distance of 32 mm toward the edge. The spiral makes 22,188 revolutions around the disk (about 600 per mm). If unwound, it would be 5.6 km long.  

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Optical disc– Random access

Difficult Move head to the right position Set correct speed Read address Adjust to required location

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Constant linear velocity

centre edges

rev/m

sector

Constantedensity

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Flash memory

A very popular type of removable storage for small devices, such as digital cameras and PDAs, is Flash memory.

Flash memory stores information in an array of memory cells made from floating-gate transistors. In traditional single-level cell (SLC) devices, each cell stores only one bit of information. Some newer flash memory, known as multi-level cell (MLC) devices, can store more than one bit per cell by choosing

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Summary Main memory

• RAM• Low storage capacity• Fast (electrical signals)• Volatile.

Magnetic memory• Floppy disk • Hard disk• Magnetic tape

Optical memory• CD_ROM disk• DVD

Solid state storage• Flash memory