Storage DevicesMain/Internal Memory

All computers have main/internal memory to store programs and data while the computer is running. This memory is in the form of memory chips and the contents are lost when the computer is switched off (it is also know as 'volatile' memory).

Comparing internal memory with backing storage:It is always faster to access data from internal memory than from backing storage. Data stored in internal memory is lost when the computer is turned off but data stored in backing storage is retained. When programs are run or data files are loaded the contents are copied from the backing storage to the internal memory of the computer. Internal memory is much smaller than backing storage. It is far too small to hold all the data/programs that would be on the backing storage of a typical computer.

How memory works:Programs and data files are stored as binary numbers.  This is made up of just 0's or 1's unlike decimal numbers (0-9).To store the 0's and 1's while the computer is running you need a memory chip. This is made up of millions of tiny switches called transistors. They can store a 0 or a 1 by the 'switch' being either open or closed. This 0 or 1 is the simplest unit of memory and is called a ‘bit’ (Binary Digit). Bits are arranged in units of eight to make a byte. A byte can therefore store eight 0's or 1's in 256 different combinations.  (00101011 and 01110110 would be just 2 possible combinations for example).

One byte is a very small amount of storage and it is more usual to refer to kilobytes (KB), megabytes (MB) and gigabytes (GB).

1 KB = 1024 bytes (approximately 1 thousand bytes) 1 MB = 1024 KB (approximately 1 million bytes) 1 GB = 1024 MB (approximately 1 thousand million bytes)

 4 KB of memory could store roughly one full A4 page of text. 600 MB (on a CD-ROM) could store roughly the text contents of a 10 volume encyclopedia.

Encoding Data:Memory chips can only store binary numbers so other data such as sounds, images or text for example have to be encoded into binary (digitised). If you want to store a character from the keyboard the computer gives it a number code made up of eight bits (1 byte). These text codes are the same internationally and are called the ASCII code (American Standard Code for Information Interchange). The code for the letter ‘a’ is 01100001 and one byte of memory is therefore used to store the letter 'a' (in code) on a memory chip.

Remember - since computers can only store binary numbers, all computer data has to be in this digital format. Images, sounds, video etc. all have to be digitised before they can be processed by a computer.

Backing storage:

Backing storage (also known as secondary storage) means data storage that retains its contents when the computer is switched off.  It can be used to hold both programs and data.

It is always faster to access data from internal memory than from backing storage. Data stored in main/internal memory is lost when the computer is turned off but data stored in backing storage is retained. When the user wants to access the data from backing storage it is copied to the main/internal memory of the computer. Common backing storage media:

Hard disks:The hard disk is usually the usual main backing storage media for a typical computer or server. The operating system (e.g. Microsoft®

Windows), applications software (e.g. word-processor, database, spreadsheet, etc.), and any program data would all be stored on a hard disk. A typical home/school microcomputer would have a disk capacity of 20 or 30 gigabytes.

Floppy disk drives:These can be found on most microcomputers and accept the usual 3.5 inch floppy disks. High density disks for a PC hold 1.44 MB of data (enough to store about 350 pages of A4 text). A floppy disk needs to be formatted before it can be used but most disks are now sold already formatted for PC's. Floppy disks are useful for transferring data between computers and for keeping a back-up of small files.

Tape: Magnetic tape can be used for permanent storage. Tapes are often used to make a copy of hard disks for back-up (security) reasons. This is automatically done overnight on the Whitefield network and the tapes are kept in a safe place away from the computer. 

CD-ROM (Compact disk): CD-ROM's hold large quantities of data (650 MB) in the form of programs, text, sound, still pictures and video clips. They are also known as optical

disks because the data is read by a laser beam reflecting or not reflecting from the disk surface.

CD-ROM's - ROM means Read Only Memory and this means you can only read from the disc, not write or store data onto it. This is the most common sort of CD available and is the way most software programs are now sold. Like a floppy disk, a CD-ROM only starts spinning when requested and it has to spin up to the correct speed each time it is accessed. It is much faster to access than a floppy but it is currently slower than a hard disk (a modern 40x speed CD-ROM drive is 40 times faster than the 15 KB per second of the original single-speed CD-ROM's).

DVDs - "Digital Versatile Disk." (Formerly Digital Video Disk.) Same size as a CD but stores seven times CD capacity on a single side. DVDs can also be double-sided or dual layer. Today most DVDs are used to display full-length commercial motion pictures, plus additional material such as outtakes, director's notes, movie trailers, etc. Single sided DVDs can store 4.7GB for single layer and 8.5GB for dual-layer disks. Double sided DVDs can store 9.4GB for single layer and 17GB for dual-layer disks.

Keydrive: -A keydrive is a small removable data storage device that uses flash memory and a USB connector. Keydrives are also known as keychain drive, micro hard drive, pen drive, pocket drive, thumb drive, jump drive, USB flash drive, USB flash memory drive, USB key, USB memory key, USB stick, They range in size from 56Mb to 1GBUSB ports are much more common than CD and especially DVD burners, so your USB flash drive will work with more computers than a CD-R or DVD-R. Also, the process of uploading files from your computer to a USB flash drive is much faster and simpler than burning a CD or DVD. Additionally, USB flash drives act like portable hard drives to which you can add and delete files as many times as you like. With CD-Rs and DVD-Rs, you can only add data once, and you can't remove data once it's burned onto the disc.

TASK http://www.waingels.wokingham.sch.uk/Departments/IT/Resources/StorageMedia2001.doc

1 Give two ways in which a CD-ROM disc is different from a floppy disc. [2]

2 A pupil is researching material for a school project. The pupil has a computer which has a CD-ROM drive and access to the Internet.[2]

Give one advantage of using the Internet rather than a CD-ROM.

Give one advantage of using a CD-ROM rather than the Internet.

3 Describe three steps a student will take to find and obtain a picture from the Internet. [3]

4 A company is developing a piece of multimedia language software for use in a school. Give two reasons why the company decides to issue the software on CD-ROM and not on floppy discs. [2]

5 A newspaper needs some pictures to insert onto the front page. Give three ways you can use Information Technology to obtain the pictures in electronic form. [3]

6 Give one reason why a digital camera is suitable for obtaining pictures for a multimedia presentation. [2]

7 Give one reason why you would not edit a large movie file on a CD-ROM.

8 A company is developing a piece of multimedia language software for use in a school. [2]

Give two reasons why the company decides to issue the software on CD-ROM and not on floppy discs.