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Chapter 3

Chapter 3. Data Storage. Learning outcomes. By the end of this Chapter you will know the difference between Electronic memory Magnetic memory Optical memory. Additional Reading. Essential Reading Stalling (2003): Chapters 5 and 6 Further Reading Burrell (2004): Chapters 3 and 7

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Chapter 3

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  1. Chapter 3 Data Storage IS50004A

  2. Learning outcomes • By the end of this Chapter you will know the difference between • Electronic memory • Magnetic memory • Optical memory IS50004A

  3. Additional Reading • Essential Reading • Stalling (2003): Chapters 5 and 6 • Further Reading • Burrell (2004): Chapters 3 and 7 • Schneider and Gersting (2004): Chapters 4 and 5 • White (2002): Parts 3 and 4. IS50004A

  4. Hardware Components IS50004A

  5. Introduction • Information can be stored in different ways: • Books, • Films • Information is stored to be re-used. • Information in computers must be able to able to be processed by computers: • Information must be stored in appropriate places IS50004A

  6. Introduction (Cont) • Computers use the binary system to store and process information. • Different type of media storage • Electronic memory (main memory) • Magnetic memory (hard disc) • optical memory (CD-ROM) IS50004A

  7. Main memory CPU Add. bus Data bus Control bus IS50004A

  8. Main memory IS50004A

  9. Main Memory (Electronic Memory) • What is the role of the main memory? • Main memory stores data which are currently been processed or ready to be processed • Electronic memory (fast) • Based on electronic principles. • Formed with logic gates • Main Memory is volatile • It is divided into cells • Each cell is a sequence of one-bit memories • Each cell has a unique address IS50004A

  10. Arrangement of Memory Cells • Each cell has a unique address • Longer strings stored by using consecutive cells value = 01101101 • RAM (random access memory) IS50004A

  11. Media storage • Magnetic Storage • Tapes • Hard drives (not always removable) • Floppy • Zips • Jaz • ,Optical Storage • CDs • DVDs • Solid State Storage • Compact flash • Smart Media IS50004A

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  13. What is Magnetic Media? • The most common and enduring form of removable-storage technology is magnetic storage. • The magnetic medium can be easily erased and rewritten, and it will "remember" things stored onto the medium for many years. IS50004A

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  15. Magnetic Memory IS50004A

  16. Magnetic tape (2) • Serial access (slow) • Good choice for off-line data storage (archives) IS50004A

  17. Magnetic Tape (1) • Serial access • Slow • Very cheap • High capacity • Backup IS50004A

  18. Magnetic Memory IS50004A

  19. spots sector Magnetic disc • Each track contains same number of sectors • Each sector contains a number of • magnetized and demagnetized spots IS50004A

  20. Magnetic Disk Terminology • Platter: • rigid metal or glass platter Coated with magnetic material. • rotating at constant angular velocity • Arm: • With movable magnetic read/write heads • Track: • A complete ring of data • The disk surface is divided into tracks • Sectors: • Each track is subdivided into sectors • Cylinder: • A vertical collection of tracks at the same radial position IS50004A

  21. Magnetic Disks IS50004A

  22. Magnetic disc density • Same number of bits per track • Tracks near the centre are more dense • The further is the tracks the less dense it is. • Constant Angular Velocity (CAV) • The disc rotation speed is constant to maintain a constant transfer. Stalling (2003) pages:167-168 IS50004A

  23. Magnetic Disks Thus as the platter rotates under the head, a stream of bits can be written and later read back. IS50004A

  24. Read and Write Mechanism (1) • Recording and retrieval via conductive coil called a head • May be single read/write head or separate ones • During read/write, head is stationary, platter rotates • Write • Electric Current through coil of wire produces magnetic field • Magnetic Pulses sent to the head • Magnetic pattern recorded on surface below • Read • Magnetised bit pattern • Magnetic field induces an electrical current in the coil • The bit pattern contains 1 • Demagnetised bit pattern • No Magnetic field induced, hence, no electrical current in the coil • The bit pattern contains 0 IS50004A

  25. Fixed/Movable Head Disk • Fixed head • One read/write head per track • Heads mounted on fixed ridged arm • Movable head • One read/write head per side • Mounted on a movable arm IS50004A

  26. Access Information on a Floppy disk • To access information on a floppy: • Track number, and • Sector number. • Head moves to the target track. • waits for the desired sector to spin underneath it • read/write begins. IS50004A

  27. Maximum data transfer rate • It is the rate at which data passes under the read/write head (bytes/sec). • Number of bytes / track * Number of rev / sec IS50004A

  28. Multiple Platter (hard disk) • Permanent storage that is inside of the computer, and NOT portable. • Consists of several platters which spin very fast • Heads are joined and aligned • Aligned tracks on each platter form cylinders • Data is striped by cylinder • reduces head movement • Increases speed (transfer rate) IS50004A

  29. Multiple Platters (2) • Disk platters speed (3600 to 10 000 rpm (rev/min). • floppy (360rpm). • The read data we need to specify cylinder, head, and sector numbers. • Each cylinder represents a track number. IS50004A

  30. Cylinders IS50004A

  31. Magnetic Tape (1) • Serial access • Slow • Very cheap • High capacity • Backup IS50004A

  32. Features of Magnetic Memory • Memory capacity: • Floppy can hold 700KB – 120MB. • Hard disk can hold dozen of GB, 10, 20,.. • Tapes can hold 100MB- 80GB. • Access method • Floppy and hard disks is random as the main memory • Tape is serial • Access time: • It is the average time taken to position the R/W head over the data to be read • For disk drives is about 10-3 sec when in MM 10-9sec. • Transfer rate: is slower. It is the transfer of data between MM and Mag/M. Floppy (500kB-2MB) and hard disc (4-12MB). IS50004A

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  36. Optical Storage – CD-ROM • Is a disc with highly reflective surface. • Tiny areas flat and depressed: • Flat (land)  strong reflection. • Depressed (pits)  low reflection. • Laser  landstrong reflectionphoto-sensor generates electrical voltagestore 1s. • Laser: (light Amplification stimulated emission of radiation). • Lightpitslow reflection no electrical voltage  stores 0s. IS50004A

  37. CD-ROM Operation • Data stored by creating variations in the reflective surface • Data retrieved by means of a laser beam • Data stored uniformly (so CD rotation speed varies) • Random access much slower than for magnetic disks IS50004A

  38. The pits and lands are written in a single continuous spiral starting near the hole and working out a distance of 32 mm toward the edge. The spiral makes 22,188 revolutions around the disk (about 600 per mm). If unwound, it would be 5.6 km long.   IS50004A

  39. Optical disc– Random access • Difficult • Move head to the right position • Set correct speed • Read address • Adjust to required location IS50004A

  40. rev/m edges Constant linear velocity sector Constante density centre IS50004A

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