Chapter 2 Backing Storage, Memory and Data Representation.
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Transcript of Chapter 2 Backing Storage, Memory and Data Representation.
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Chapter 2
Backing Storage, Memory and
Data Representation
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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
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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.
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A byte can store one character of data
Kbyte -> 1024 bytes
Mbyte -> 1024 ____
Gbyte -> 1024 Mbytes
Terabyte -> 1024 _____
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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
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Main storage area in a desktop computer is the
hard _____
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Floppy disks
Round, flexible magnetic disk
Access to the data: slow
Storage capacity: 1.44Mbytes
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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)
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Optical disks
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Compact disks (CDs)
Storage capacity: 700 Mbytes
Access time: faster than a fl____, but slower than hard
disk
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Compact disks (CDs)
The CD drive has a laser beam in the read/write
head, which can read the information on the
d___
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Compact disks (CDs)
CD-ROMs -> data can be r___ but not written to
them
CD-RW -> data can be read from and w____ to
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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.
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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
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Zip drives and disks
Single hard disk that stores data magnetically
Storage capacity: 100-250 Mbytes
Internal and external zip disks
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Uses of zip disks
Uses: -> for transferring large graphics files
between computers
-> backing up personal computers
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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
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Magnetic tapes
Uses: backing up data
One type of magnetic tape is the magnetic tape
cartridge
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Backup
Backup file: a file is copied in case the original file is lost
or destroyed. E.g?
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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
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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?
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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.
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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
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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.
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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.
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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
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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
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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
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Main mem (cont)
RAM Can be upgraded, thus more memory modules
can be installed in the system unit
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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
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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
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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
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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.
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Flash memory
Example applications PDAs laptop, digital audio players, digital cameras and mobile phones.
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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
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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
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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
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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
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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
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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?
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Data representation
How data is represented inside the computer
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Data representation – Representing numbers
Base ten: 1,2,3,4,5,6,7,8,9
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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.
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bytes
8 digits make a byte, and one byte can represent
a character (see next slide)
See figure 2.9
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Representing numbers in binary system
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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.
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How do we measure memory
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Data representation – Representing characters
A character is a symbol that can be represented by a
computer (e.g: 1, !, A)
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Questions?