William Stallings Computer Organization

and Architecture7th Edition

Chapter 6

External Memory

Presented By:Tiffany BrownKurt Cameron

Raul Villanueva

Types of External Memory

• Magnetic Disk– Removable– Fixed

• Optical– CD-ROM– CD-Recordable (CD-R)– CD-R/W– DVD

• USB Flash Drive

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

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

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

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

Inductive Write MR Read

head

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).

Disk Data Layout

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

Disk Layout Methods Diagram

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

Winchester Disk FormatSeagate ST506

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)

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

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

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

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)

Multiple Platters

Tracks and Cylinders

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

Floppy Disk

• 8”, 5.25”, 3.5”

• Small capacity– Up to 1.44Mbyte (2.88M never popular)

• Slow

• Universal

• Cheap

• Obsolete?

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

Winchester Hard Disk (2)• Universal

• Cheap

• Fastest external storage

• Getting larger all the time– 1 Terabyte can be obtained

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 + Latency Time taken to be positioned to read or write

• Transfer time Time taken for sector to move under head

Timing of Disk I/O Transfer

Optical Memory

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

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.

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 )

Reading Procedure

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.

Reading a Cd

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

CD-ROM Format

• Mode 0=blank data field• Mode 1=2048 byte data+error correction• Mode 2=2336 byte data

Fields of the Block

Random Access on CD-ROM

• Difficult

• Move head to rough position

• Set correct speed

• Read address

• Adjust to required location

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

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

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.

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.

CD-RW (REWRITEABLE)

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.

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.

CD-RW STRUCTURE

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.

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

Digital Versatile Disk (DVD)

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.

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

DVD Characteristics

DVD - technology

• Multi-layer

• Very high capacity (4.7G per layer)

• Full length movie on single disk– Using MPEG compression

DVD Tracks

• Each writable layer of a DVD has a spiral track of data. On single-layer DVDs, the track always circles from the inside of the disc to the outside.

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

CD and DVD

CD vs. DVD

USB Flash Drive

• Also known as…– USB Key– Jump Drive– Thumb Drive– Pen Drive

• Popular Manufacturers– PNY– Kingston– San Disk– Lexar

USB Flash Drive:A Brief History

• Developed in 1998 by IBM and Trek Technology.

• Seen as a replacement for the popular portable storage device at the time, the floppy disk.

• M-Systems manufactured the first USB Flash Drives to be sold commercially.

• Storage Capacity of Original Drives: 8MB, 16MB, 32MB & 64MB sizes.

USB Flash Drive:At a Glance

• Primary Components– USB Connector– NAND-type Flash Memory Chip

• Function– Removable portable storage device (able to read/write).

• Primary Advantages (vs. Floppy-Disk)– Greater Capacity

• Floppy Disk’s Max Capacity: 32MB• USB Flash Drive’s Max Capacity: 32GB

– Increased Portability– Greater Durability & Reliability

• Lack of moving parts increases reliability.– Faster read/write speeds

• Miscellaneous– Supported by all major Operations Systems

USB Flash Drive:Under The Hood

1. USB Connector• Male type-A Connector.

2. USB Mass Storage Controller• Newer drives implement USB 2.0.

3. Test points• Used fort testing during

manufacturing of drive. 4. Flash memory chip

• NAND Flash Memory Device5. Crystal Oscillator

• Produces Clock Signal6. LED

• Flashes to show activity7. Write-Protect Switch

• Indicates if write-protect mode is on/off.

8. Space for second flash memory chip

USB Flash Drive:The USB Factor

• The USB male type-A connector serves two primary purposes.– A means to interface with just about any

computer sporting a USB input port.– Provide power to the microcontroller onboard

all USB Flash Drives.

• USB Flash Drives must adhere to the USB Mass Storage Device standard– Defined by USB Implementers Forum

USB Flash Drive:The Flash Memory Factor

• Flash Memory is…– a much improved version of EEPROM that can be erased in a

“Flash”. The increased speed is attributed to flash memory’s ability to erase blocks of memory instead of bytes of memory due to the arrangement of it’s memory cells.

– non-Volatile, meaning, flash memory retains stored information even in the absence of power.

• NAND Flash Memory– Created by Toshiba in 1989– All USB Flash Drives use NAND Flash Memory– NAND Memory consist of blocks. Data must be written

sequential within a block. Each block is made-up of pages typically in sizes of 512, 2048, or 4096 bytes. Data can be written page by page but erasing must be done block by block.

USB Flash Drive:Why Choose NAND?

USB Flash Drive:Advantages Over…

• Zip Drives– USB FDs are smaller, more durable & reliable, has a

much larger capacity, and can be used on just about all modern computers compared to Zip Drives.

• Optical Media– USB FDs have faster read/write times, higher rated

erase/write cycles, more compact, and more durable compared to Optical Media.

• External Hard Drives– USB FDs are less fragile, more shock resistance, less

vulnerable to electromagnetic fields, and immune to file fragmentation compared to External Hard Drives.

Internet Resources• Magnetic Disk

– http://www.ibm.com/ibm/history/exhibits/storage/storage_PH11-52.html

– http://www.pcmag.com/encyclopedia_term/0,2542,t=magnetic+disk&i=46490,00.asp

• Optical Storage– www.osta.org/

• Good source of information about optical storage technology and vendors• Extensive list of relevant links

– http://computer.howstuffworks.com/removable-storage7.htm

• USB Flash Drive– http://en.wikipedia.org/wiki/Usb_flash_drive

– http://en.wikipedia.org/wiki/Flash_memory

Chapter 6 External Memory:Review Questions

1. What are the three main types of external memory?2. What are the two disk layout methods for magnetic disks?3. How are the sectors and tracks laid out in these two disk layouts?4. Why is the head mechanism of the Winchester hard disk

sometimes referred to as flying?5. What differences between a CD and DVD account for the larger

capacity of the latter?6. What is the difference between CAV and CLV?7. Data is stored as what on CDs?8. What type of USB connector is used in a USB Flash Drive?9. What type of Flash Memory is used in a USB Flash Drive?10. Are USB Flash Drives erased bit by bit, page by page, or block by

block?