SECONDARY STORAGE DEVICES

Y.Colette LeMard

The ability to store data and programs so that they can be used/reused in the future is one of the most important features of a computer.

?However, where to store this data is the question.

We can not change what’s in ROM so that is out of the question

RAM is volatile so it will not preserve anything we place there when the computer is off

In addition both ROM and RAM are much too small anyway.

Thus the need for :

secondary storage

a.k.a. auxiliary storage

a.k.a. storage

Secondary Storage

Sometimes secondary storage is also called external storage.

However, it is important to bear in mind that one of our most important secondary storage media is the hard disk which is not external.

Medium vs Device

There is also the tendency when we discuss secondary storage to mix up medium and device.

Medium – the actual physical material on which the bits are stored

Device – the mechanism used to access the data on the medium

Medium

Device

Floppy disk

Floppy drive

Distinguishing Characteristics of Secondary Storage

AS we examine some secondary storage media, look out for the following :

their capacity their access speed their access method and their portability.

Definitions

Storage Capacity

Maximum amount of data which a data storage

medium can hold

Definitions

Access Speed

How quickly the required data is retrieved by the

storage device

Definitions

Portability

Readily and easily transported from one

computer system to another.

Definitions

Access Method

What particular mechanics/algorithm is used to

locate specific data at a particular place on a storage

medium.

SASD : Sequential Access Storage Device

DASD : Direct (Random) Access Storage Device

MAGNETIC MEDIA

The Floppy Disk

The Floppy Disk

There was the 8” floppy,and the 5¼” floppy, and then the 3½“ floppy.

Floppies have come a long way

The Floppy Disk

The present floppy is a 3.5 inch magnetic disk of flexible material which typically stores 1.44 MB of data maximum

The Floppy Disk

The thin plastic circle is enclosed in rigid plastic to protect it.

A metal sliding access shutter opens when the disk is in the drive so that the read/write head can access the disk itself.

The Floppy Disk

Data can be written to and read from a floppy disk.

A write protect tab can be used to prevent accidental overwriting of data

The Floppy DiskBefore data can be written to a disk, it must be formatted. This prepares the surface for storing bits by creating a map of sectors and tracks.

The Floppy Disk

The address of a particular file is determined by the track and sector it occupies.

Most floppies now come preformatted, so many users are not aware of the requirement for formatting

The Floppy Disk preformatted

Micro-floppy disk

2 -sided

The Floppy Disk

Formatting also creates a File Allocation Table (FAT) - a directory which stores the names of the files on the disk and their address.

A FAT is necessary for management of the storage space on the disk.

Floppies are highly portable

and are great for transporting

small files between machines

They are direct access but

rather slow

Most desktops still have a

floppy drive but newer laptops

no longer contain a floppy disk

drive

The Zip Disk

The Zip Disk

The zip disk comes in different capacities ranging from 100MB to 750MB.

This means that each can store the contents of many floppies

The Zip Disk

The zip disk is portable and the disk itself operates similarly to a hard disk.

The Zip Disk

Zip disks require a zip drive. These are not automatically attached to new systems however, they have to be specially ordered.

A zip drive can be external or internal.

The Zip Disk

Zip disks are relatively inexpensive. The 750MB version can store the same amount of data as 520 floppy disks.Physically the zip disk is about 1½ times the size of a floppy.

The Zip Disk

Zip disks are popular for backing up hard disks and for transferring large files from one computer to another.

The Hard Disk

The Hard Disk

The hard disk is made from rigid aluminum coated with a magnetic material.

It is normally supplied as a sealed unit and will often consists of a number of disks (platters) on a common spindle

The Hard Disk

The hard disk and the hard drive are packaged together

The Hard Disk

Each platter has a pair of read/write heads which are used for writing to and reading from that particular surface of the disk.

The Hard Disk

Each disk is divided into tracks and sectors. Each sector can store between 256 and 512 bytes.

The tracks are immediately above and below each other. A particular set of such tracks is called a cylinder.

The Hard Disk As the spindle turns the platters, the read/write heads move together in and out between the platters. At any point in time they are all positioned over one particular cylinder

The Hard Disk

It is now fairly standard for a pc to have a hard disk of at least 60GB. When PCs were first used in the 1980s, the hard disk’s capacity was measured in MB. But modern data and programs take up much more space especially as they are now GUI based.

The Hard Disk

Hard disks are pretty reliable

However as a standard they are fixed inside the systems unit/computer case.

Portable (removable) units are available though, and tend to be used mostly by laptop owners.

The Hard Disk

Hard disks are direct access devices. The data transfer rate is also much higher than for floppy disks

They also have a FAT which store the addresses of the files stored thereon.

Magnetic Tapes

Magnetic Tape

Tape is flexible reels of plastic with a magnetic coating. It may range from ¼” cartridges to large reels

Magnetic Tape

The data is stored in tracks that run along the entire length of the tape. Usually there are 9 tracks.

Magnetic Tape

Magnetic tape requires a special drive of course and access is slow as it is a sequential access devices.

It is an ideal medium for backup however, because it is fast, inexpensive and has a high storage capacity.

OPTICAL MEDIA

Optical Disks

These have a hard plastic disk with a mirrored surface. The data is stored by removing the mirroring from a particular point through the use of a laser. The laser creates a pit on the surface. These pits are usually on the underside of the disk and cannot be seen with the naked eye.

Optical Disks

pits

Optical Disks

Optical disks have a larger

storage capacity than floppies

but less than hard disks or

tapes. They also fall in the

middle in terms of access speed.

They are portable but must be

transported with care because

scratching the recording surface

will damage the data.

Optical Disks

The earliest optical

disks could only be

written to once so they

became known as write

once read many

(WORM)disks.

The CD-ROM

The most common WORM disk is

the

Compact Disc Read Only Memory

[CD-ROM].

A typical CD-ROM holds 650MB

of data

The CD-ROM

CD-ROMs are great for

distributing programs and

other material that is not to

be changed.

CD-ROM drives became a

standard for desktops in the

1990s

The CD-R

CD-R (CD-recordable) allows end

users to record their own data

on CDs. Data can be burned onto

these once and thereafter

cannot be changed.

This makes CD-Rs great for

permanent backup

The CD-R

The CD-R is write once, read

many.

The dye layer allows writing of

data to this disk

The CD-RW

CD-RW (CD

rewriteable)

These became

available in

1997

The CD-RW

While a prerecorded CD has

its information permanently

stamped into its

polycarbonate plastic

substrate layer, a CD-RW disc

contains a phase-change alloy

recording layer.

The CD-RW

An infra-red laser beam is used to

heat (at 400oC) and melt, the

crystallized recording layer into a

fluid state or to freeze it at a lower

temperature back to its crystalline

state.

In this way previous data is ‘erased’

and new data can be written (burned)

The DVDDigital versatile disk

Digital video disk

These look similar to CDs but are

capable of storing up to 6 times

more data principally because the

tracks are closer together and

because some store data on both

surfaces

The DVD

DVDs are used mainly for movies as the

quality of the sound and the video is

superior to that of videotapes.

They are also used for software, and data

archiving.

The DVD

The DVD is a direct access device and is

fairly fast.

Types

DVD-ROM

DVD-R

DVD-RAM (DVD-RW)

Optical Disks

The CD-ROM drive is a standard on PCs,

but now CD-R/W drives, DVD-ROM drives,

and combo drives are also becoming very

popular.

DVD writers can also now be seen on sale

in computer stores

Any Questions ?

My Questions

1. Magnetic tape has sequential-access capabilities (T/F)?

2. What is the most appropriate application of magnetic tape storage?

3. Why are floppy drives no longer included on many newer machines?

4. What happens during formatting? Why must hard disks and diskettes be formatted?

My Questions

5. Describe the potential impact of optical disk technology on public and university libraries

6. Describe at least two applications where CD-RW would be preferred over hard disk for storage.

7. How many diskettes would you need to backup your Computer Science notes which is 2.8MB large.

8. The DVD is compatible with the CD-ROM. Currently the DVD drive is more expensive than the CD-ROM, but prices are converging. Will the DVD replace the CD?

ASSIGNMENT

The ‘THUMB DRIVE’ is now very popular for personal computing.

What is a ‘thumb drive’?

How does a thumb drive work/what mechanism does it use to store data?

What are its advantages?

What are the other names for this ‘device’?

FLASH MEMORY

You may recall that EEPROM can be erased by using high voltages.

There is a type of memory which can be erased with low voltages and then rewritten even while still inside the computer. This type of memory is called flash memory.

The thumb drive is a type of flash memory

Reading Assignment

Computers – Information Technology in Perspective : Long and Long 12th edition Pages 192 to 206

Understanding Computer Science for Advanced Level : Ray Bradley

Chapter 12

A’Level Computing : Heathcote and LangfieldPages 144 to 148

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