1. Backing Storage, Memory and Data Representation
Chapter 2
Backing Storage, Memory and Data Representation

2. 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
Backing Storage, Memory and Data Representation

3. 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.
Backing Storage, Memory and Data Representation

4. A byte can store one character of data Kbyte -> 1024 bytes
Mbyte -> 1024 ____
Gbyte -> 1024 Mbytes
Terabyte -> 1024 _____
Backing Storage, Memory and Data Representation

5. 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
Backing Storage, Memory and Data Representation

6. Main storage area in a desktop computer is the hard _____
Backing Storage, Memory and Data Representation

7. Floppy disks Round, flexible magnetic disk Access to the data: slow
Storage capacity: 1.44Mbytes
Backing Storage, Memory and Data Representation

8. 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)
Backing Storage, Memory and Data Representation

9. Optical disks
Backing Storage, Memory and Data Representation

10. Compact disks (CDs) Storage capacity: 700 Mbytes
Access time: faster than a fl____, but slower than hard disk
Backing Storage, Memory and Data Representation

11. Compact disks (CDs)
The CD drive has a laser beam in the read/write head, which can read the information on the d___
Backing Storage, Memory and Data Representation

12. Compact disks (CDs)
CD-ROMs -> data can be r___ but not written to them
CD-RW -> data can be read from and w____ to
Backing Storage, Memory and Data Representation

13. 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.
Backing Storage, Memory and Data Representation

14. 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
Backing Storage, Memory and Data Representation

15. Zip drives and disks Single hard disk that stores data magnetically
Storage capacity: Mbytes
Internal and external zip disks
Backing Storage, Memory and Data Representation

16. Uses of zip disks
Uses: -> for transferring large graphics files between computers
-> backing up personal computers
Backing Storage, Memory and Data Representation

17. 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
Backing Storage, Memory and Data Representation

18. Magnetic tapes Uses: backing up data
One type of magnetic tape is the magnetic tape cartridge
Backing Storage, Memory and Data Representation

19. Backup
Backup file: a file is copied in case the original file is lost or destroyed.
E.g?
Backing Storage, Memory and Data Representation

20. 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
Backing Storage, Memory and Data Representation

21. 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? 
Backing Storage, Memory and Data Representation

22. HW - answers
How many 250 mbyte zip discs would be needed to store 1 gb?
One gb= 1024 megabytes
5 zip discs
What are the differences between a floppy disc and a hard disc?
Floppy disc: smaller, stores less amount of data (1.44), slow access
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.
Describe one use for tape.
Backing up data
Why do we need to back up data?
In case the original file is lost, damages or corrupted.
Backing Storage, Memory and Data Representation

23. 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 Storage, Memory and Data Representation

24. 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 Storage, Memory and Data Representation

25. 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 Storage, Memory and Data Representation

26. 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 Storage, Memory and Data Representation

27. 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
Backing Storage, Memory and Data Representation

28. 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
Backing Storage, Memory and Data Representation

29. Main mem (cont)
RAM
Can be upgraded, thus more memory modules can be installed in the system unit
Backing Storage, Memory and Data Representation

30. 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
Backing Storage, Memory and Data Representation

31. 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
Backing Storage, Memory and Data Representation

32. 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
Backing Storage, Memory and Data Representation

33. 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.
Backing Storage, Memory and Data Representation

34. Flash memory Example applications PDAs laptop, digital audio players,
digital cameras and
mobile phones.
Backing Storage, Memory and Data Representation

35. 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
Backing Storage, Memory and Data Representation

36. 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
Backing Storage, Memory and Data Representation

37. 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
Backing Storage, Memory and Data Representation

38. 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 Disk
Send one word
At a time
e.g. number 8
Backing Storage, Memory and Data Representation

39. 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
Backing Storage, Memory and Data Representation

40. Backing Storage, Memory and Data Representation

41. 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?
Backing Storage, Memory and Data Representation

42. Data representation How data is represented inside the computer
Backing Storage, Memory and Data Representation

43. Data representation – Representing numbers
Base ten: 1,2,3,4,5,6,7,8,9
Backing Storage, Memory and Data Representation

44. 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.
Backing Storage, Memory and Data Representation

45. bytes
8 digits make a byte, and one byte can represent a character (see next slide)
See figure 2.9
Backing Storage, Memory and Data Representation

46. Representing numbers in binary system
Backing Storage, Memory and Data Representation

47. 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.
Backing Storage, Memory and Data Representation

48. How do we measure memory
Backing Storage, Memory and Data Representation

49. Data representation – Representing characters
A character is a symbol that can be represented by a computer (e.g: 1, !, A)
Backing Storage, Memory and Data Representation

50. Questions?
Backing Storage, Memory and Data Representation