1. AS Computing Storage devices

2. Primary storage  A computer’s main memory (RAM) is known as primary memory.  Primary memory is volatile; when the computer is switched off, all the contents of memory is lost.

3. Secondary storage  A more permanent, non-volatile form of storage is required by all computer system to save software and data files.  Magnetic tape, magnetic disks, CD-ROM (Compact Disk Read Only Memory) and microfilm are all example of what is known as secondary storage.

4. Types of Secondary Storage  Magnetic disks and tapes  e.g. floppy, hard disk and zip drives  Optical disks  e.g.CD and DVD disks  Solid state devices  e.g USB disks

5. Magnetic disk  The magnetic disk is covered in a magnetic material that can be used to store data in the form of binary digits.  The data is written and read by a moving read/write head over the disk  The disk spins and when data is read or written to a disk the head remains stationary and data is read from a circular track on the disk  Moving the head in to the centre and out locates other tracks on the disk

6. Track on a magnetic disk Disk Track Read write head

7. Blocks of Data Sector Block of data Magnetic devices read data in blocks ~ 100s of Kbytes A block of data is the data that passes under the head in one sector of the disk

8. Floppy disks  The standard Floppy disks is 3  Floppy disks are thin, flexible plastic disks coated in metal oxide enclosed in a rigid plastic casting.  Floppy disks are slow because the disk rotates slowly  This also limits the amount of data they can store to 1.44MByte

9. The use of Floppy Disk   Advantages:   It does have the advantage of being very portable.   Disadvantages:   It only can be used to pass a small amount of data from one system to another.

10. Hard Disks  Hard disks often contain more than one disk but each has its own read/write head which move together  The disks are rigid and made from a lightweight material such as aluminium and covered on both sides with metal oxide  The discs rotate very fast between 5000 and 7000 revolutions per minute (RPM)  This means more data storage and faster data access than a floppy disk  A modern hard disk will store 80GBytes of data

11. The use of hard disks  The hard disk has become the main storage device for storing the software of a computer system.  It is also the preferred device for storing databases and other company data that are required immediately.

12. Optical Disks  An optical disc is a hard plastic disk with a mirror like surface  Data is written on it by pulsing a laser which burns off the mirror at a point on the track  Data is read by shining another laser onto the disk  where data has been written there is no mirror and the light is not reflected  Where there is no data light is reflected.  By looking at the light from the reading laser that is reflected off the disk the data that was “burnt” into the disc can be read again many times

13. Optical Disks  Optical disks come in different types  Read only (CD ROM,DVD ROM)  These disks are factory made and can only be read  Read/Write (CD R/W, DVD R/W)  WORM(Write Once Read Many times) disks can normally be written once only  CD-RW (ReWritable disk)

14. CD-ROM  CD-ROM is a direct access media.  It normally holds 650 Mbytes data or more  It is used for distributing software

15. DVD Digital Versatile Disc (DVD) is a direct access media. It can hold approximately 17 Gbytes. It is used for distributing multimedia, especially high quality video.

16. The use of optical disks Optical disks are used:   To distribute software   To distribute information, e.g. directories   Backup – keeping copies of important data.and for storing large amounts of data

17. Magnetic Tape  Magnetic tape stores data on a reel of tape just like a video tape  It is used for recording analogue or digital data.  Large amounts of data can be stored on small tape cartridges ~ 200 Gbytes  It is slow to access a given block of data as this requires rewinding the tape but transfer once data is found is very fast  Because of this it is called a sequential device

18. The use of magnetic tapes The main use for magnetic tapes is as a back-up medium. It is also used for other purpose:  Archiving – keeping large amount of data that might be needed in the future  Transporting data from one system to another  Distributing software

19. Magnetic tape – Jaz drive A JAZ TM drive with cartridge - one of the many types of magnetic storage devices available

20. Solid State  Solid state memory devices can be found in cameras and MP3 players  Solid state means the storage is in a silicon chip but it is organised in blocks like a disk  Nowadays people use USB “disks” to transfer data between computers

21. Exercise