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Chapter 3

Storage Principles

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Storage

• Magnetic– Floppy disks ,Hard disk ,Zip drive

• Optic– CD-ROM, DVD

• Magneto optic– High end drives

• Data on any drive are digitized; they are expressed as myriads of 0s and 1s.

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Interface

• Individual drives are connected to other PC components through an Interface.

• EIDE or eSATAHard disks – CD-ROM

• SCSI– Hard disks (all sizes) and CD-ROM

• ISA (internal)– Floppy drives ,CDROM, others connected through

parallel port

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Floppy Disk

• a data storage medium

• made of a disk of thin, flexible magnetic storage medium encased in a square plastic shell.

• read and write

• Invented by IBM, floppy disks in 8", 5.25" and 3.5" formats

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Floppy Controller

• governed by a controller. The controller has to be programmed at each start up.

• reads data from the diskette media in serial mode (one bit at a time)

• data are delivered in parallel mode (16 bits at a time) to RAM via a DMA channel.

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Floppy Drive

• Read/Write Heads

• Drive Motor

• Stepper Motor

• Mechanical Frame

• Circuit Board

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Hard Disk• is a non-volatile storage device which stores

digitally encoded data

• is a 3.5” diameter rigid-disk drive.

• consist of thin platters with a magnetic coating.

• Write mode:-– If a current is applied to the coil, the head will become

magnetic.

• Read mode:-– If the head moves along the track without current

applied to the coil, it will sense the micro magnets.

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Hard Disk

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Domains

• The bits that store in microscopic magnets (called domains) on the disk.

• Before recording data, the drive uses the read/write heads to orient the domains in a small region so that the magnetic poles all point in the same direction.

• Then, a reversal of polarity is interpreted as a digit one. Unchanged polarity is interpreted as a digit zero.

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Hard Disk Speeds • disk access time

– is the time required for a computer to process data from a storage device.

– Access Time = Seek Time + Latency time + Data Transfer time

• Seek time – is the time for the access arm to reach the desired disk track.

• Latency time – the time for the rotation of the disk to bring the required disk

sector under the read-write mechanism.

• Data Transfer Rate – time during which data is actually read or written to medium,

with a certain throughput.

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Other Disks

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Optical Storage

• CD-ROM and DVD are optic readable media.

• read with a very thin and precisely aimed laser beam.

• the disks are removable.

• the CD-ROM is an optic media.

• data storage consists of millions of indentations burnt into the lacquer coated, light reflecting silver surface.

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Tracks

• data consist of bits

• bits are arranged in a pattern along the track.

• has only one track, a spiral winding its way from the center to the outer edge

• 5km long spiral track holds up to 650Mb data in about 5.5 billions dot (each is one bit).

CD-ROM

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Types of Optical Media

Drive type

Name The drive can

CD-ROMCompact Disk Read Only Memory

Read CD-ROM and CD-R

CD-RCompact Disk Recordable

Read CD-ROM and CD-R. Write once on special disks named CD R

CD-RWCompact Disk ReWritable

Read CD-ROMs and CD-R. Write and re-write on special disks (CD-RW).

DVD RAMDigital Versatile Disk Random Access Memory

Reads all CD formats. Reads DVD ROM. Reads and writes DVD disks

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Optical media

• Data can be packed much more densely in optic media than in magnetic media.

• They have much longer life span.

• There are different types of optical disk:

– Compact Disk– CD-ROM– DVD

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Compact Disk

• was introduced by Philips and Sony in 1980.

• a small plastic disk with a reflecting metal coating.

• is organized in tracks.

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CD-ROM

• The CD-ROM (Read Only Memory) came in 1984.

• are the same as the CD; the difference is in the data storage organization.

• In a CD-ROM, the data are stored in sectors. The CD-ROM can hold 700 MB of data, and it is very inexpensive to produce.

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DVD

• is a high-capacity optic media and was developed in the mid 1990s. DVD stands for Digital Versatile Disk.

• The DVD is the same size as a CD but the tracks are narrower

• are read by a laser beam of shorter wave-length than used by the CD-ROM drives. This allows for smaller indentations and increased storage capacity.

• A single layer DVD-5 disk holds 4.7 GB. A dual-layered DVD-9 disk holds 8.5 GB. The dual-sided DVDs are named DVD-10 (9.4 GB) and DVD-18 (17 GB).