Secondary Storage:Architecture 2


Why is Secondary Storage needed?

It is backing storage that is external to the CPU, it stores data and programs and is non-volatile (i.e. when the computer is switched off, the backing storage will not lose its memory.

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What is a storage device?

Storage is the act of storing something. A storage device (SD) is a device for storing data. It can hold data on a permanent or semi-permanent basis

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What are the three forms of media?

Flash (e.g. USB)

Magnetic (e.g. hard disk or tape)

Optical (Shiny) (e.g. CD, DVD, HD or Blu Ray)

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What is a Magnetic Disk?

Tapes or disks are coated with a magnetic surface used for storing electronic data.

Data is stored using magnetisied spots.

6GB hard disk drive contains over six billion spots.

Direct across medium.

Two types- Floppy disk and hard disk.

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Features Hard Disks (Fixed)

Series of metal disks (platters).

Each surface has read/write heads.

Data from one  file is stored on different surfaces.

Read / write heads work independently.

Fast data transfer speed.

It is a form of magnetic disk.

They are standard in most computers.

They have a capacity of 40-300GB+

Cost; £50 for 120GB Hard disk.

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Uses of Hard Disks (Fixed)

Holds the application and operating system software

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PPQ: Explain how data is written to and read from

The hard disk contains a stack of platters/disks

...and a set of read/write heads

Data is written magnetically sectors tracks clusters

[1] for each of five points.

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Zip Disk Features

More tracks and sectors than a floppy disk

Portable and faster data rate than floppy.

Capacity 100-200MB

Cost £9 (cheap)

JAZ Disk similar with 2GB

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Zip Disk Uses

Hold a larger application (e.g. a database)

Backs up files on a personal computer.

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DAT Tape (Magnetic Tape)

Digital Audio Tape- used for back-ups

Light and removable but can be slow.

Capacity can be 20-200GB - Cost £14.

A tape streamer uses a magnetic tape cartridge to store data.

A tape streamer is often attached to a server in order to make periodic data backups.

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Uses of DAT Tape

Backup a server or network information.

Storage for large programs or files.

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Optical Disks

Optical: high-capacity stroage medium that is read/write by a laser light.

Binary computer data are represented by the way the disk reflects light.

Optical diska are portable.

Direct acced medium.

Two-types: CD & DVD

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Compact Disk (CD)

Optical media, can be CD-ROM; CD-R; CD-RW

Data is burnt onto surface using a laser.

Capacity is 700MB

Cost £1

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Uses of Compact Disk

Holds a program or large files e.g. a database.

Holds music and large image files.

Backup personal files on a computer.

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PPQ: Explain how data is read from and written to

A laser is used; heat areas of organic dye layer permanently

...change the reflectively of those areas

A lower powered laser reads the data detecting alternating regions of heated and unaltered dye.

[1] for each of the three points.

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PPQ: Explain how data is written to and read from

A laser is used; temporarily

...modify the phase change properties of a dye

...between crystal phase (erased) and amorphous phase (recorded)

A laser detects the difference between resulting light and dark areas.

[1] for each of three points.

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PPQ; Explain how data is written to and read from

A laser beam is used to read the data,

On a read-only DVD, pits are pressed onto the surface,

These are detected by changes in the intenisity of the reflected laser beam.

On a DVD-R or DVD+R, the laser burns/heats the dye on the surface of hte DVD.

...changing its reflectivity.

On a non-rewriteable DVD, the writing laser is more powerful than the reading laser.

[1] for each of four points.

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Dual-Layer Disks

Dual layer disks are DVD's that have multiple layers

They have a semi-transparent extra layer over the same side of a disc.

The player acknowledges the shift from one layer to the next with a momentary pause.

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Features of Digital Versatile Disk (DVD)

Optical media; DVD; DVD-R; DVD-RW

Excellent quality and fast data speed.

Capacity 4.7-17GB

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Uses of a DVD

Can hold a program or large files e.g.a database.

Holds music or large image files

Holds a feature length film.

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HD DVD (High-Density DVD)

A high capacity storage medium. A single-layer HD-DVD provides up to 15GBs (GB) of storage capacity.

Triple layer HD-DVD provides 45GB of storage capacity.

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Can store even more data than a DVD

-A single layer disk can hold 25GB

-A double layer can store up to 50GB.

Only a dedicated Blu-Ray player can play them.

They are mainly used for storing high quality commercial movies. But you can buy a Blu-Ray burner for a pesonal computer and write-able blank Blu-Ray disks.

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PPQ: Blu-Ray Advantages

The format was developed for high-definition video (HD) and for storing large amounts of data.

...permitting high definition audio and video/providing clearer pictures with improved colour and vividness.

[1] for each of two points.

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PPQ: Blu-Ray Disadvantages

Blu-ray is a new/emerging technology.

...and players and discs are still relatively expensive.

...the range of films is restricted.

[1] for each of two points

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Flash Memory Stick

Not optical or magnetic- connects to USB.

Works like RAM - solid chips.

Data is retained when switched off.

Capacity 128MB - 2GB+ (growing)

Cost: £20+

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Flash Memory Stick Uses

-Hold a range of large files e.g. database

-Moving from one computer to another (presentations).

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