William Stallings Computer Organization and Architecture 7 th Edition

1. William Stallings Computer Organization and Architecture7th Edition Chapter 6 External Memory Presented By: Tiffany Brown Kurt Cameron Raul Villanueva

2. Types of External Memory Magnetic Disk Removable Fixed Optical CD-ROM CD-Recordable (CD-R) CD-R/W DVD USB Flash Drive

3. History of Magnetic Disk • Foundation of external memory in practically all computer systems • 1st one built in 1955 by IBM – RAMAC 350 • Stored 5 Mb of data • Cost $3200 to rent per month

4. Magnetic Disk Circular platter of non-magnetic material (substrate) coated with a magnetizable material (iron oxide…rust) Substrate use to be aluminium Now glass is used due to many benefits: Improved surface uniformity Increases reliability Reduction in surface defects Reduced read/write errors Lower flight heights Better stiffness Better shock/damage resistance

5. Read and Write Mechanisms Recording & retrieval of data 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 write head Magnetic patterns recorded on surface below Read (traditional) Magnetic field moving relative to coil produces current Coil is the same for read and write Current of the same polarity recorded is produced Single heads were used especially in floppy disk systems

6. Read and Write Mechanisms continued • Read (contemporary) • Separate read head positioned close to write head • Partially shielded magneto resistive (MR) sensor • Electrical resistance depends on direction of magnetic field • Current passes through sensor producing resistance which is recorded as voltage signals • High frequency operation • Higher storage density and speed

7. Inductive Write MR Read head

8. Data Organization and Formatting Data organized in concentric rings or tracks Each track same width as head Adjacent tracks separated by gaps (minimizes misalignment errors or interference from fields) Data transferred in sectors Adjacent sectors separated by intersector or intertrack gaps Hundreds of sectors per track (can be fixed or non-fixed).

9. Disk Data Layout

10. Disk Velocity Bit on outside of rotating disk passes fixed point faster than bit near centre of disk Increase spacing between bits in different tracks allows all bits to be read at the same time Rotate disk at constant angular velocity (CAV) Gives pie shaped sectors and concentric tracks Individual blocks of data addressable by tracks and sectors Move head to particular track and wait for given sector to spin under head Waste of space on outer tracks Lower data density Multiple zone recording increases capacity Each zone has fixed bits per track More complex circuitry

11. Disk Layout Methods Diagram

12. Finding Sectors Must be able to identify start of track and sector Format disk Additional information used by disk drive only Marks tracks by track number and sectors are identified by ID fields

13. Winchester Disk FormatSeagate ST506

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

15. Fixed/Movable Head Disk Fixed head One read write head per track Heads mounted on fixed ridged arm that extends across all tracks Movable head One read write head Head mounted on a movable arm

16. Removable or Not Removable disk Can be removed from drive and replaced with another disk Provides unlimited storage capacity Easy data transfer between systems Example: Zip disk and floppy disk Non-removable disk Permanently mounted in the drive Example: hard drive in personal computers

17. Double Sided or Single Sided Double sided Magnetic disk usually coated on both sides with magnetizable substance Single sidedLess expensive disks have only one side coated

18. Multiple Platter Platters stacked vertically a fraction of an inch part One read write head per platter surface Moveable heads are joined and aligned Aligned tracks on each platter form cylinders Data is striped by cylinder reduces head movement Increases speed (transfer rate)

19. Multiple Platters

20. Tracks and Cylinders

21. Head Mechanisms • Contact • Head physically touches platter during read write operation • Fixed Gap • Head positioned a certain distance above platter • Aerodynamic Gap • Head made of aerodynamic foil rests on platter but when disk spins, air pushes foil up

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

23. Winchester Hard Disk (1) Developed by IBM in Winchester (USA) Sealed unit free of contaminants One or more platters (disks) Heads caused to rise by air pressure as disk spins Very small head to disk gap Greater data density Getting more robust

24. Winchester Hard Disk (2) Universal Cheap Fastest external storage Getting larger all the time 1 Terabyte can be obtained

25. Speed Seek time Time taken to position head to correct track Typically under 10 ms Rotational delay/latency Time taken for beginning of sector to reach head Average delay for floppy – 100 to 150 ms Average delay for other disks – 2ms Access time = Seek + LatencyTime taken to be positioned to read or write Transfer timeTime taken for sector to move under head

26. Timing of Disk I/O Transfer

27. Optical Memory

28. Compact Disk CD-ROM • Originally for audio • 650Mbytes giving over 70 minutes audio • Polycarbonate coated with highly reflective coat, usually aluminium • Data stored as pits • Read by reflecting laser • Constant packing density • Constant linear velocity

29. CD Fabrication The CD fabrication machine uses a high-powered laser to etch the bump pattern into photoresist material coated onto a glass plate. Through an elaborate imprinting process, this pattern is pressed onto acrylic discs. The discs are then coated with aluminum (or another metal) to create the readable reflective surface. Finally, the disc is coated with a transparent plastic layer that protects the reflective metal from nicks, scratches and debris.

30. CD Operation CDs store music and other files in digital form -- that is, the information on the disc is represented by a series of 1s(when the laser reflects off the land towards the sensor) and of 0s (when reflecteed off the bump or pit )

31. Reading Procedure

32. Reading a CD • In conventional CDs, these 1s and 0s are represented by millions of tiny bumps and flat areas on the disc's reflective surface. The bumps and flats are arranged in a continuous track that measures about 0.5 microns (millionths of a meter) across and 3.5 miles (5 km) long. • To read this information, the CD player passes a laser beam over the track. When the laser passes over a flat area in the track, the beam is reflected directly to an optical sensor on the laser assembly. The CD player interprets this as a 1. When the beam passes over a bump, the light is bounced away from the optical sensor. The CD player recognizes this as a 0.

33. Reading a Cd

34. CD-ROM Drive Speeds • Audio is single speed • Constant linear velocity CLV • 1.2 ms-1 • Track (spiral) is 5.27km long • Gives 4391 seconds = 73.2 minutes

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

36. Fields of the Block

37. Random Access on CD-ROM • Difficult • Move head to rough position • Set correct speed • Read address • Adjust to required location

38. CD-ROM for & against • FOR • Large capacity (compared to floppy disk) • Easy to mass produce • Removable • Robust • AGAINST • Expensive for small runs • Slow • Read only

39. Other Optical Storage • CD-Recordable (CD-R) • WORM • Now affordable • Compatible with CD-ROM drives • CD-RW • Erasable • Getting cheaper • Mostly CD-ROM drive compatible • Phase change • Material has two different reflectivities in different phase states

40. CD –R (RECORDABLE) • CD-Rs, don't have any bumps or flat areas at all. Instead, they have a smooth reflective metal layer, which rests on top of a layer of photosensitive dye. • When the disc is blank, the dye is translucent: Light can shine through and reflect off the metal surface. But when you heat the dye layer with concentrated light of a particular frequency and intensity, the dye turns opaque: It darkens to the point that light can't pass through. • By selectively darkening particular points along the CD track, and leaving other areas of dye translucent, you can create a digital pattern that a standard CD player can read. The light from the player's laser beam will only bounce back to the sensor when the dye is left translucent, in the same way that it will only bounce back from the flat areas of a conventional CD. So, even though the CD-R disc doesn't have any bumps pressed into it at all, it behaves just like a standard disc.

41. CD-R compared with CD A CD-R doesn't have the same bumps and lands as a conventional CD. Instead, the disc has a dye layer underneath a smooth, reflective surface. On a blank CD-R disc, the dye layer is completely translucent, so all light reflects. The write laser darkens the spots where the bumps would be in a conventional CD, forming non-reflecting areas.

42. CD-RW (REWRITEABLE)

43. CD-RW STRUCTURE • CD-RW discs have taken the idea of writable CDs a step further, building in an erase function so you can record over old data you don't need anymore. These discs are based on phase-change technology. In CD-RW discs, the phase-change element is a chemical compound of silver, antimony, tellurium and indium. As with any physical material, you can change this compound's form by heating it to certain temperatures. When the compound is heated above its melting temperature (around 600 degrees Celsius), it becomes a liquid; at its crystallization temperature (around 200 degrees Celsius), it turns into a solid.

44. CD-RW STRUCTURE In a CD-RW disc, the reflecting lands and non-reflecting bumps of a conventional CD are represented by phase shifts in a special compound. When the compound is in a crystalline state, it is translucent, so light can shine through to the metal layer above and reflect back to the laser assembly. When the compound is melted into an amorphous state, it becomes opaque, making the area non-reflective.

45. Phase-change Compounds • In phase-change compounds, these shifts in form can be "locked into place": They persist even after the material cools down again. If you heat the compound in CD-RW discs to the melting temperature and let it cool rapidly, it will remain in a fluid, amorphous state, even though it is below the crystallization temperature. In order to crystallize the compound, you have to keep it at the crystallization temperature for a certain length of time so that it turns into a solid before it cools down again. • In the compound used in CD-RW discs, the crystalline form is translucent while the amorphous fluid form will absorb most light. On a new, blank CD, all of the material in the writable area is in the crystalline form, so light will shine through this layer to the reflective metal above and bounce back to the light sensor. To encode information on the disc, the CD burner uses its write laser, which is powerful enough to heat the compound to its melting temperature. These "melted" spots serve the same purpose as the bumps on a conventional CD and the opaque spots on a CD-R: They block the "read" laser so it won't reflect off the metal layer. Each non-reflective area indicates a 0 in the digital code. Every spot that remains crystalline is still reflective, indicating a 1.

46. DVD • Digital Video Disk • Used to indicate a player for movies • Only plays video disks • Digital Versatile Disk • Used to indicate a computer drive • Will read computer disks and play video disks

47. Digital Versatile Disk (DVD)

48. DVD Composition • A DVD is composed of several layers of plastic, totaling about 1.2 millimeters thick. Each layer is created by injection molding polycarbonate plastic. This process forms a disc that has microscopic bumps arranged as a single, continuous and extremely long spiral track of data. More on the bumps later. • Once the clear pieces of polycarbonate are formed, a thin reflective layer is sputtered onto the disc, covering the bumps. Aluminum is used behind the inner layers, but a semi-reflective gold layer is used for the outer layers, allowing the laser to focus through the outer and onto the inner layers. After all of the layers are made, each one is coated with lacquer, squeezed together and cured under infrared light.

49. DVD Pit layout They appear as pits on the aluminum side, but on the side that the laser reads from, they are bumps. If a DVD stretches out into a straight line, it would be almost 7.5 miles long! That means that a double-sided, double-layer DVD would have 30 miles worth of data. Single-sided, single-layer DVDs can store about seven times more data than CDs

50. DVD Characteristics