Chapter 2 Backing Storage, Memory and Data Representation

Chapter 2

Backing Storage, Memory and

Data Representation

Backing Storage, Memory and Data Representation 2/34

What you will learn

Storage devices and media Floppy disk, Hard disk, Raid, CDs, DVD, Zip drives

and disks, Magnetic tape Backup Memory

Main memory – RAM or ROM, Flash memory Data representation

Representing numbers, characters, graphics, sound


Storage devices and media

You need backing storage (or secondary

memory) to save d___ after the computer is

turned off.

Your work is stored on the computer as a file,

measured in bytes.


A byte can store one character of data

Kbyte -> 1024 bytes

Mbyte -> 1024 ____

Gbyte -> 1024 Mbytes

Terabyte -> 1024 _____


Storage devices and media

Storage medium: actual material object e.g. floppy ___

Storage device: item of equipment that handles the

use of the medium e.g. floppy disk drive


Main storage area in a desktop computer is the

hard _____


Floppy disks

Round, flexible magnetic disk

Access to the data: slow

Storage capacity: 1.44Mbytes


Hard disks

Storage capacity: 300 Gbytes and more

Access to the data: very fast

Total storage space: (number of readable sides) x

(number of cylinders) x (sectors per track) x (bytes per

sector)


Optical disks


Compact disks (CDs)

Storage capacity: 700 Mbytes

Access time: faster than a fl____, but slower than hard

disk


Compact disks (CDs)

The CD drive has a laser beam in the read/write

head, which can read the information on the

d___


Compact disks (CDs)

CD-ROMs -> data can be r___ but not written to

them

CD-RW -> data can be read from and w____ to


DVD

Digital Versatile Disk Storage capacity: 4.7 Gbytes, but it can carry two

layers of data on each of its two sides, thus it can hold up to 18 Gbytes.


DVD

DVD-Video: DVD format for full length movies

DVD-Audio: DVD format for carrying audio

information

DVD-RAM: can write to a special kind of DVD

rewritable


Zip drives and disks

Single hard disk that stores data magnetically

Storage capacity: 100-250 Mbytes

Internal and external zip disks


Uses of zip disks

Uses: -> for transferring large graphics files

between computers

-> backing up personal computers


Magnetic tapes

Storage capacity: very large due to large amount of

surface area

Serial access: e.g. if you want to access data at the

middle of the tape you need to start at the beginning,

therefore slow access


Magnetic tapes

Uses: backing up data

One type of magnetic tape is the magnetic tape

cartridge


Backup

Backup file: a file is copied in case the original file is lost

or destroyed. E.g?


When to back up data?

Back up your files regularly (after some

modifications, after the creation of important

files, daily or weekly or monthly) and keep it

separately from the computer


Questions?

A murderer is condemned to death. He has to

choose between three rooms. The first is full of

raging fires, the second is full of assassins with

loaded guns, and the third is full of lions that

haven't eaten in 3 years. Which room is safest

for him?


HW - answers

1. How many 250 mbyte zip discs would be needed to store 1 gb?

One gb= 1024 megabytes

5 zip discs

1. What are the differences between a floppy disc and a hard disc?

Floppy disc: smaller, stores less amount of data (1.44), slow access

1. A magnetic tape provides serial access. Explain why this can be a disadvantage.

You are not able to go to a specific piece of data without reading the data before it.

1. Describe one use for tape.

Backing up data

1. Why do we need to back up data?

In case the original file is lost, damages or corrupted.


Backup

Automatic backup of all the files

More than one copy should be kept

Keep a backup on hand in a fire-proof safe

Keep at least one backup in another location


Backing up a home computer

Firstly back up your data on your hard disk

Daily you can back up the data on a memory

stick

At set intervals back up these files to a CD-RW.

Every month use a different CD-RW.

If the capacity of your data is too large then

compress it before store it somewhere else.


Backing up a home computer

Therefore, if a file is lost or corrupted, you can

recover it by looking for a copy on floppy disk or

zip disk, and then on the most recent CD, or the

previous ones.


Backing up a network

Automatically backed up at least one a day

Backup copies are often put onto other hard

disks, as this speeds up the copying

If speed is not important, magnetic cartridges

are used which are cheaper


Backing up a network

Software used to take automatic backups keeps

track of when and where the backups were

taken

A systematic method of backups is the ‘ancestral

backup system’. This is described in detail in

chapter 12


Memory – Main memory

May be RAM or ROM. These are referred to as IAS (Immediate-access store)

RAM Can be written to, read from and edited Data can be accessed in any order (random) Volatile memory – is cleared when the computer is

switched off


Main mem (cont)

RAM Can be upgraded, thus more memory modules

can be installed in the system unit


RAM (cont.)

Uses: stores data and programs while the computer is running – that is why Ram is referred to as main memory (or

primary memory) whereas floppy and hard drives are called backing

storage or secondary memory


Memory - ROM

Data in ROM can be read and written to but not

changed

Data can be accessed in any order

Non-volatile memory – it retains what it is stored

in it even when the computer is switched off


Types of ROM

PROM[programmable read-only memory ]: at first there is no data, company can put its own information and then lock the data for no future alterations

EPROM:[erasable programmable read-only memory] same as PROM but it can erase the data


Flash memory

computer memory

primarily used in memory cards and

USB flash drives

for general storage and transfer of data between

computers and other digital products.


Flash memory

Example applications PDAs laptop, digital audio players, digital cameras and mobile phones.


Memory – Flash Memory

Is a type of EPROM

It is often removable A common use of flash memory is the BIOS which makes

sure the computer starts up as it should and that all the parts of the computer work together


Types of flash memory

Compact flash: removable, used in digital cameras, PDAs, etc, can store up to 2Gbytes

Flash pen devices: are lightweight, fit into your pocket – thus it’s easy to move them around. Used for storing presentations and files


Addresses and locations

Memory is divided into many different locations, each of

these can store one byte

The computer can find a specific location in memory

using its address – unique number


Words, bits and bytes

A word is the amount of data that a computer can

directly access at one time

It is the natural unit of data used by a particular

computer design.

A word is simply a fixed-sized group of bits that

are handled together by the system.

cpu Hard DiskSend one wordAt a timee.g. number 8


Words, bits and bytes

Different computers can access one, two or

more bytes of data at one time

Byte -> 8 signals which can be either on (0) or

off (1)

A signal is a Binary digIT (bit)

Word length: always is stated in terms of bits



Use this presentation + webopedia to answer the questions

What is a byte? Why do we have different types of storage devices? Why don’t they sell video games on memory sticks?

Cd roms are cheaper

Not all pcs have a memory stick port

Cd roms cannot be edited.

Very easy to copy data on a memory stick How can you represent number 57 in binary (ones and zeros?) What is a back up file? What storage device will

a student use to back up his data on a pc? A school use to back up its day to day transactions?

Why?


Data representation

How data is represented inside the computer


Data representation – Representing numbers

Base ten: 1,2,3,4,5,6,7,8,9


Data representation – Representing numbers cont

Base two: the computer uses only two digits

{0,1} to represent alphabet letters and other

items of information. Since it can take only two

values it is a binary digit.


bytes

8 digits make a byte, and one byte can represent

a character (see next slide)

See figure 2.9


Representing numbers in binary system


Experiment

in Notepad type

, "Four score and seven years ago".

Save the file

The file has a size of 30 bytes on disk: 1 byte for

each character.

Each character consumes a byte.


How do we measure memory


Data representation – Representing characters

A character is a symbol that can be represented by a

computer (e.g: 1, !, A)


Questions?

Documents

Chapter 2 Backing Storage, Memory and Data Representation