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EEL 4709C
Group 6
Priscila MaldonadoFreddy Figueroa
Chapter 6External Memory
Types of External Memory
Magnetic DiskRemovable
OpticalCD-ROMCD-Recordable (CD-R)CD-R/WDVDBlu-Ray DVD
USB Memory
Magnetic Disk
Disk substrate coated with magnetizable material (iron oxide…rust)
Substrate used to be aluminiumNow glass
Improved surface uniformity Increases reliability
Reduction in surface defects Reduced read/write errors
Lower flight heights (See later)Better stiffness to reduce disk dynamicsBetter shock/damage resistance
Read and Write Mechanisms
Recording & retrieval via conductive coil called a headMay be single read/write head or separate onesDuring read/write, head is stationary, platter rotatesWrite
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
Inductive Write MR Read
Data Organization and Formatting
Concentric rings or tracksGaps between tracksReduce gap to increase capacitySame number of bits per track (variable packing
density)Constant angular velocity
Tracks divided into sectorsMinimum block size is one sectorMay have more than one sector per block
Disk Data Layout
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
Disk Layout Methods Diagram
Finding Sectors
Must be able to identify start of track and sector
Format diskAdditional information not available to userMarks tracks and sectors
Winchester Disk FormatSeagate ST506
Characteristics
Fixed (rare) or movable headRemovable or fixedSingle or double (usually) sidedSingle or multiple platterHead mechanism
Contact (Floppy)Fixed gapFlying (Winchester)
Fixed/Movable Head Disk
Fixed headOne read write head per trackHeads mounted on fixed ridged arm
Movable headOne read write head per sideMounted on a movable arm
Removable or Not
Removable diskCan be removed from drive and replaced with another
diskProvides unlimited storage capacityEasy data transfer between systems
Nonremovable diskPermanently mounted in the drive
Multiple Platter
One head per sideHeads are joined and alignedAligned tracks on each platter form cylindersData is striped by cylinder
reduces head movementIncreases speed (transfer rate)
Multiple Platters
Tracks and Cylinders
Floppy Disk
8”, 5.25”, 3.5”Small capacity
Up to 1.44Mbyte (2.88M never popular)
SlowUniversalCheapObsolete?
Winchester Hard Disk (1)
Developed by IBM in Winchester (USA)Sealed unitOne or more platters (disks)Heads fly on boundary layer of air as disk
spinsVery small head to disk gapGetting more robust
Winchester Hard Disk (2)
UniversalCheapFastest external storageGetting larger all the time
250 Gigabyte now easily available
Speed
Seek timeMoving head to correct track
(Rotational) latencyWaiting for data to rotate under head
Access time = Seek + LatencyTransfer rate
Timing of Disk I/O Transfer
Optical Storage CD-ROM
Originally for audio680Mbytes giving over 70 minutes audioFormed 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 surfacesRead by reflecting laser (low-powered)Constant packing densityConstant linear velocity (rotate slower at
outer edge)
CD Operation
CD-ROM Drive Speeds
Audio is single speedConstant linear velocity1.2 ms-1
Track (spiral) is 5.27km longGives 4391 seconds = 73.2 minutes
Other speeds are quoted as multiplese.g. 24xQuoted figure is maximum drive can achieve
CD-ROM Format
Mode 0=blank data fieldMode 1=2048 byte data+error correctionMode 2=2336 byte data
Random Access on CD-ROM
DifficultMove head to rough positionSet correct speedRead addressAdjust to required location(Yawn!)
CD-ROM Advantages & Disadvantages
Relatively Large capacity compared to past methods
Easy to mass produce, once the master copy is made
RemovableRobust
Expensive for individual useSlow (takes up to half a second longer than
magnetic disk)Read only
Other Optical Storage
CD-Recordable (CD-R)WORM Very affordable, better for individual use (archival
storage)Compatible with CD-ROM drives
CD-RWErasable (with some limitations)Getting cheaperMostly CD-ROM drive compatiblePhase change
Material has two different reflectivities in different phase states
Laser changes between an amorphous state and a crystalline state.
DVD
Digital Versatile DiskReplaced VHS video tapeBetter than CD's
Bits are closer together, takes about half the space
DVD-ROM can be two-sided unlike CD-ROM
DVD - technology
Multi-layerVery high capacity (4.7G per layer)Full length movie on single disk
Using MPEG compression
Movies carry regional codingPlayers only play correct region films
DVD – Writable
Loads of trouble with standardsFirst generation DVD drives may not read
first generation DVD-W disksFirst generation DVD drives may not read
CD-RW disks
CD and DVD
High Definition Optical Disks
Designed for high definition videosMuch higher capacity than DVD
Shorter wavelength laser Blue-violet range
Smaller pitsHD-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)
Optical Memory Characteristics
USB Memory
Integrated with a USB (Universal Serial Bus)Removable, rewritable.Much better/faster than floppy disks.Most weigh less than an ounceStorage capacities can range
from 64 MB to 256 GB
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.
USES
Personal data transportSecure storage of dataSystem administrationComputer repairApplication carriersBooting operating systemsWindows vista readyBoostAudio PlayersMusic storage and
marketingBackup
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.
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.
Review Questions
1. What are the advantages of using glass substrate for a magnetic disk?
2. How are data written onto a magnetic disk?3. How are data read from a magnetic disk?4. Explain the difference between a simple CAV system and a
multiple zoned recording system?5. What is the typical sector size?6. What is the benefit from using Blue-Violet Laser than Red
Laser?7. How long is the spiral track of a CD-ROM?8. What are the essential components of a USB flash drive?9. What is the data transfer speed specification for USB 2.0?10. How many writes does it take for a USB flash drive to fail?
Answers to Review Questions
1. 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 themagnetization of the medium moving under it. By passing a current through theMR 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, in which the surface is divided into a number of zones, with zones farther from the center containing more bits than zones closer to the center.
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 Miles8. Male type-A USB connector, USB mass storage controller,
NAND flash memory chip, and crystal oscillator.9. 480 Mb/s.10. 90 million.
References
http://en.wikipedia.org/wiki/USB_flash_drive
http://answers.com/topic/keydrive
http://computer.howstuffworks.com/usb.htm
Computer Organization & Architecture