EEL 4709C Group 6 Priscila Maldonado Freddy Figueroa

1. Chapter 6External Memory EEL 4709C Group 6 Priscila Maldonado Freddy Figueroa

2. Types of External Memory • Magnetic Disk • Removable • Optical • CD-ROM • CD-Recordable (CD-R) • CD-R/W • DVD • Blu-Ray DVD • USB Memory

3. Magnetic Disk • Disk substrate coated with magnetizable material (iron oxide…rust) • Substrate used to be aluminium • Now glass • Improved surface uniformity • Increases reliability • Reduction in surface defects • Reduced read/write errors • Lower flight heights (See later) • Better stiffness to reduce disk dynamics • Better shock/damage resistance

4. Read and Write Mechanisms • Recording & 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 • Current through coil produces magnetic field • Pulses sent to head • Magnetic pattern recorded on surface below • Read (traditional) • Magnetic field moving relative to coil produces current • Coil is the same for read and write • Read (contemporary) • Separate read head, close to write head • Partially shielded magneto resistive (MR) sensor • Electrical resistance depends on direction of magnetic field • High frequency operation • Higher storage density and speed

5. Inductive Write MR Read

6. Data Organization and Formatting • Concentric rings or tracks • Gaps between tracks • Reduce gap to increase capacity • Same number of bits per track (variable packing density) • Constant angular velocity • Tracks divided into sectors • Minimum block size is one sector • May have more than one sector per block

7. Disk Data Layout

8. Disk Velocity • Bit near centre of rotating disk passes fixed point slower than bit on outside of disk • Increase spacing between bits in different tracks • Rotate disk at constant angular velocity (CAV) • Gives pie shaped sectors and concentric tracks • Individual tracks and sectors addressable • Move head to given track and wait for given sector • Waste of space on outer tracks • Lower data density • Can use zones to increase capacity • Each zone has fixed bits per track • More complex circuitry

9. Disk Layout Methods Diagram

10. Finding Sectors • Must be able to identify start of track and sector • Format disk • Additional information not available to user • Marks tracks and sectors

11. Winchester Disk FormatSeagate ST506

12. Characteristics • Fixed (rare) or movable head • Removable or fixed • Single or double (usually) sided • Single or multiple platter • Head mechanism • Contact (Floppy) • Fixed gap • Flying (Winchester)

13. 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

14. Removable or Not • Removable disk • Can be removed from drive and replaced with another disk • Provides unlimited storage capacity • Easy data transfer between systems • Nonremovable disk • Permanently mounted in the drive

15. Multiple Platter • One head per side • Heads are joined and aligned • Aligned tracks on each platter form cylinders • Data is striped by cylinder • reduces head movement • Increases speed (transfer rate)

16. Multiple Platters

17. Tracks and Cylinders

18. Floppy Disk • 8”, 5.25”, 3.5” • Small capacity • Up to 1.44Mbyte (2.88M never popular) • Slow • Universal • Cheap • Obsolete?

19. Winchester Hard Disk (1) • Developed by IBM in Winchester (USA) • Sealed unit • One or more platters (disks) • Heads fly on boundary layer of air as disk spins • Very small head to disk gap • Getting more robust

20. Winchester Hard Disk (2) • Universal • Cheap • Fastest external storage • Getting larger all the time • 250 Gigabyte now easily available

21. Speed • Seek time • Moving head to correct track • (Rotational) latency • Waiting for data to rotate under head • Access time = Seek + Latency • Transfer rate

22. Timing of Disk I/O Transfer

23. Optical Storage CD-ROM • Originally for audio • 680Mbytes giving over 70 minutes audio • Formed from a resin, like polycarbonate and covered by highly reflective coat, usually aluminium • Data stored as pits; areas between pits are lands • Lands are smooth surfaces • Read by reflecting laser (low-powered) • Constant packing density • Constant linear velocity (rotate slower at outer edge)

24. CD Operation

25. CD-ROM Drive Speeds • Audio is single speed • Constant linear velocity • 1.2 ms-1 • Track (spiral) is 5.27km long • Gives 4391 seconds = 73.2 minutes • Other speeds are quoted as multiples • e.g. 24x • Quoted figure is maximum drive can achieve

26. CD-ROM Format • Mode 0=blank data field • Mode 1=2048 byte data+error correction • Mode 2=2336 byte data

27. Random Access on CD-ROM • Difficult • Move head to rough position • Set correct speed • Read address • Adjust to required location • (Yawn!)

28. CD-ROM Advantages & Disadvantages • Relatively Large capacity compared to past methods • Easy to mass produce, once the master copy is made • Removable • Robust • Expensive for individual use • Slow (takes up to half a second longer than magnetic disk) • Read only

29. Other Optical Storage • CD-Recordable (CD-R) • WORM • Very affordable, better for individual use (archival storage) • Compatible with CD-ROM drives • CD-RW • Erasable (with some limitations) • Getting cheaper • Mostly CD-ROM drive compatible • Phase change • Material has two different reflectivities in different phase states • Laser changes between an amorphous state and a crystalline state.

30. DVD • Digital Versatile Disk • Replaced VHS video tape • Better than CD's • Bits are closer together, takes about half the space • DVD-ROM can be two-sided unlike CD-ROM

31. DVD - technology • Multi-layer • Very high capacity (4.7G per layer) • Full length movie on single disk • Using MPEG compression • Movies carry regional coding • Players only play correct region films

32. DVD – Writable • Loads of trouble with standards • First generation DVD drives may not read first generation DVD-W disks • First generation DVD drives may not read CD-RW disks

33. CD and DVD

34. High Definition Optical Disks • Designed for high definition videos • Much higher capacity than DVD • Shorter wavelength laser • Blue-violet range • Smaller pits • HD-DVD • 15GB single side single layer • Gone the way of the 8-track • Blu-ray • Data layer closer to laser • Tighter focus, less distortion, smaller pits • 25GB on single layer • Available read only (BD-ROM), Recordable once (BR-R) and re-recordable (BR-RE)

35. Optical Memory Characteristics

36. USB Memory • Integrated with a USB (Universal Serial Bus) • Removable, rewritable. • Much better/faster than floppy disks. • Most weigh less than an ounce • Storage capacities can range from 64 MB to 256 GB

37. Design and implementation • One end of the device is fitted with a single male type-A USB connector. Inside the plastic casing is a small printed circuit board. Mounted on this board is some simple power circuitry and a small number of surface-mounted integrated circuits (ICs). Typically, one of these ICs provides an interface to the USB port, another drives the onboard memory, and the other is the flash memory. • Drives typically use the USB mass storage device class to communicate with the host. • Essential components • Male type-A USB connector. (1) • USB mass storage controller. (2) (OTi-2168) • NAND flash memory chip. (4) • Crystal oscillator. (5) • Additional components • Jumpers and test pins. (3) • LEDs. (6) • Write-protect switches. (7) • Unpopulated space. (8) • USB connector cover or cap. • Transport aid. • Expansion slot.

38. USES • Personal data transport • Secure storage of data • System administration • Computer repair • Application carriers • Booting operating systems • Windows vista readyBoost • Audio Players • Music storage and marketing • Backup

39. Advantages Impervious to scratches and dust, and mechanically very robust. Almost all personal computers support USB. Store data relatively densely compared to many removable media. Power efficient, have no fragile moving parts, small and light. Work without installing device drivers on the computer. The operating system can use any file system or block addressing scheme. Extremely resistant to tough conditions.

40. Disadvantages They can sustain only a limited number of write and erase cycles before failure (takes around 90 million writes for the drive to die). Most USB flash drives don’t include a write-protect mechanism. Write-protection is suitable for repairing virus-infected host computers without the risk of infecting the USB drive itself. They are easily misplaced because of their small size. As a consequence of this, some manufacturers have added encryption hardware to their drives for data protection. When they are compared to other portable storage device, for example external hard drives, USB drives have a high price per unit of storage and are only available in comparatively small capacities.

41. Review Questions What are the advantages of using glass substrate for a magnetic disk? How are data written onto a magnetic disk? How are data read from a magnetic disk? Explain the difference between a simple CAV system and a multiple zoned recording system? What is the typical sector size? What is the benefit from using Blue-Violet Laser than Red Laser? How long is the spiral track of a CD-ROM? What are the essential components of a USB flash drive? What is the data transfer speed specification for USB 2.0? How many writes does it take for a USB flash drive to fail?

42. Answers to Review Questions Improvement in the uniformity of the magnetic film surface to increase disk reliability. A significant reduction in overall surface defects to help reduce read/write errors. Ability to support lower fly heights (described subsequently). Better stiffness to reduce disk dynamics. Greater ability to withstand shock and damage. 2. The write mechanism is based on the fact that electricity flowing through a coil produces a magnetic field. Pulses are sent to the write head, and magnetic patterns are recorded on the surface below, with different patterns for positive and negative currents. 3. The read head consists of a partially shielded magnetoresistive (MR) sensor. The MR material has an electrical resistance that depends on the direction of the magnetization of the medium moving under it. By passing a current through the MR sensor, resistance changes are detected as voltage signals. 4. For the constant angular velocity (CAV) system, the number of bits per track is constant. An increase in density is achieved with multiple zoned recording, inwhich the surface is divided into a number of zones, with zones farther from the center containing more bits than zones closer to the center.

43. Answers to Review Questions 5. 512 bytes. 6. The benefit of using a blue-violet laser (405nm) is that it has a shorter wavelength than a red laser (650nm), which makes it possible to focus the laser spot with even greater precision. This allows data to be packed more tightly and stored in less space, so it's possible to fit more data on the disc even though it's the same size as a CD/DVD. This together with the change of numerical aperture to 0.85 is what enables Blu-ray Discs to hold 25GB/50GB 7. 3.59 Miles 8. Male type-A USB connector, USB mass storage controller, NAND flash memory chip, and crystal oscillator. 9. 480 Mb/s. 10. 90 million.

44. References http://en.wikipedia.org/wiki/USB_flash_drive http://answers.com/topic/keydrive http://computer.howstuffworks.com/usb.htm Computer Organization & Architecture