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