AS Computing Storage devices. Primary storage A computer’s main memory (RAM) is known as primary memory. Primary memory is volatile; when the computer.

AS ComputingAS Computing

Storage devices

Primary storage Primary storage

A computer’s main memory (RAM) is known as A computer’s main memory (RAM) is known as primary memory. primary memory.

Primary memory is volatile; when the computer is Primary memory is volatile; when the computer is switched off, all the contents of memory is lost.switched off, all the contents of memory is lost.

Secondary storage Secondary storage

A more permanent, non-volatile form of storage A more permanent, non-volatile form of storage is required by all computer system to save is required by all computer system to save software and data files. software and data files.

Magnetic tape, magnetic disks, CD-ROM Magnetic tape, magnetic disks, CD-ROM (Compact Disk Read Only Memory) and microfilm (Compact Disk Read Only Memory) and microfilm are all example of what is known as secondary are all example of what is known as secondary storage.storage.

Types of Secondary StorageTypes of Secondary Storage

Magnetic disks and tapesMagnetic disks and tapes

e.g. floppy, hard disk and zip drivese.g. floppy, hard disk and zip drives

Optical disksOptical disks

e.g.CD and DVD diskse.g.CD and DVD disks

Solid state devicesSolid state devices

e.g USB diskse.g USB disks

Magnetic diskMagnetic disk

The magnetic disk is covered in a magnetic The magnetic disk is covered in a magnetic material that can be used to store data in the form of material that can be used to store data in the form of binary digits. binary digits.

The data is written and read by a moving read/write The data is written and read by a moving read/write head over the diskhead over the disk

The disk spins and when data is read or written The disk spins and when data is read or written to a disk the head remains stationary and data is to a disk the head remains stationary and data is read from a circular track on the diskread from a circular track on the disk

Moving the head in to the centre and out locates Moving the head in to the centre and out locates other tracks on the diskother tracks on the disk

Track on a magnetic diskTrack on a magnetic disk

DiskDisk

TrackTrack

Read write headRead write head

Blocks of DataBlocks of Data

SectorSector

Block of dataBlock of data

Magnetic devices read data in blocks ~ 100s of KbytesMagnetic devices read data in blocks ~ 100s of Kbytes

A block of data is the data that passes under the head in one A block of data is the data that passes under the head in one sector of the disk sector of the disk

Floppy disks Floppy disks

The standard Floppy disks is 3 The standard Floppy disks is 3

Floppy disks are thin, flexible plastic disks coated Floppy disks are thin, flexible plastic disks coated in metal oxide enclosed in a rigid plastic casting.in metal oxide enclosed in a rigid plastic casting.

Floppy disks are slow because the disk rotates Floppy disks are slow because the disk rotates slowlyslowly

This also limits the amount of data they can store This also limits the amount of data they can store to 1.44MByteto 1.44MByte

1

2''

The use of Floppy DiskThe use of Floppy Disk

Advantages:It does have the advantage of being very portable.

Disadvantages:It only can be used to pass a small amount of data from one system to another.

Hard DisksHard Disks

Hard disks often contain more than one disk but each Hard disks often contain more than one disk but each has its own read/write head which move togetherhas its own read/write head which move together

The disks are rigid and made from a lightweight The disks are rigid and made from a lightweight material such as aluminium and covered on both sides material such as aluminium and covered on both sides with metal oxidewith metal oxide

The discs rotate very fast between 5000 and 7000 The discs rotate very fast between 5000 and 7000 revolutions per minute (RPM)revolutions per minute (RPM)

This means more data storage and faster data This means more data storage and faster data access than a floppy diskaccess than a floppy disk

A modern hard disk will store 80GBytes of dataA modern hard disk will store 80GBytes of data

The use of hard disksThe use of hard disks

The hard disk has become the main storage device The hard disk has become the main storage device for storing the software of a computer system.for storing the software of a computer system.

It is also the preferred device for storing databases It is also the preferred device for storing databases and other company data that are required and other company data that are required immediately.immediately.

Optical DisksOptical Disks

An optical disc is a hard plastic disk with a mirror like An optical disc is a hard plastic disk with a mirror like surfacesurface

Data is written on it by pulsing a laser which burns off the Data is written on it by pulsing a laser which burns off the mirror at a point on the trackmirror at a point on the trackData is read by shining another laser onto the diskData is read by shining another laser onto the disk

where data has been written there is no mirror and the where data has been written there is no mirror and the light is not reflectedlight is not reflectedWhere there is no data light is reflected.Where there is no data light is reflected.

By looking at the light from the reading laser that is By looking at the light from the reading laser that is reflected off the disk the data that was “burnt” into the disc reflected off the disk the data that was “burnt” into the disc can be read again many timescan be read again many times

Optical DisksOptical Disks

Optical disks come in different types Read only (CD ROM,DVD ROM)

These disks are factory made and can only be read Read/Write (CD R/W, DVD R/W)

WORM(Write Once Read Many times) disks can normally be written once only

CD-RW (ReWritable disk)

CD-ROMCD-ROM

CD-ROM is a direct access media.CD-ROM is a direct access media.

It normally holds 650 Mbytes data or moreIt normally holds 650 Mbytes data or more

It is used for distributing softwareIt is used for distributing software

DVDDVD

Digital Versatile Disc (DVD) is a direct access media. Digital Versatile Disc (DVD) is a direct access media.

It can hold approximately 17 Gbytes.It can hold approximately 17 Gbytes.

It is used for distributing multimedia, especially high It is used for distributing multimedia, especially high quality video.quality video.

The use of optical disksThe use of optical disks

Optical disks are used:To distribute software To distribute information, e.g. directoriesBackup – keeping copies of important data.and for storing large amounts of data

Magnetic TapeMagnetic Tape

Magnetic tape stores data on a reel of tape just like a video tape

It is used for recording analogue or digital data. Large amounts of data can be stored on small tape

cartridges ~ 200 Gbytes It is slow to access a given block of data as this requires

rewinding the tape but transfer once data is found is very fast

Because of this it is called a sequential device

The use of magnetic tapesThe use of magnetic tapes

The main use for magnetic tapes is as a back-up The main use for magnetic tapes is as a back-up medium. medium.

It is also used for other purpose:It is also used for other purpose:

Archiving – keeping large amount of data that might Archiving – keeping large amount of data that might be needed in the futurebe needed in the future

Transporting data from one system to anotherTransporting data from one system to another

Distributing softwareDistributing software

Magnetic tape – Jaz driveMagnetic tape – Jaz drive

A JAZA JAZTMTM drive with cartridge - one of the many types of drive with cartridge - one of the many types of magnetic storage devices availablemagnetic storage devices available

Solid StateSolid State

Solid state memory devices can be found in cameras and MP3 players

Solid state means the storage is in a silicon chip but it is organised in blocks like a disk

Nowadays people use USB “disks” to transfer data between computers

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