Hardware Students Guide - Chapter 8 Mass Storage

7/30/2019 Hardware Students Guide - Chapter 8 Mass Storage

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Table of ContentsIntroduction......................................................................................................................................3Technologies....................................................................................................................................6

Introduction.............................................................................................................................6Magnetic..................................................................................................................................7

Background.....................................................................................................................7Magnetism.......................................................................................................................8Magnetic Materials..........................................................................................................9Magnetic Storage..........................................................................................................10Digital Magnetic Systems.............................................................................................11

Background...........................................................................................................11Saturation..............................................................................................................12Coercivity.............................................................................................................13Retentivity.............................................................................................................15

Magneto-Optical....................................................................................................................17Background...................................................................................................................17Write Operation.............................................................................................................18Read Operation..............................................................................................................21Media.............................................................................................................................21

Optical...................................................................................................................................22Data Organization..........................................................................................................................24

Introduction...........................................................................................................................24Sequential Media...................................................................................................................24Random Access Media..........................................................................................................26Combination Technologies....................................................................................................27

Data Coding...................................................................................................................................28Introduction...........................................................................................................................28Flux Transitions.....................................................................................................................28Single-Density Recording.....................................................................................................29Double-Density Recording....................................................................................................29Group Coded Recording........................................................................................................30Advanced RLL......................................................................................................................31Data Compression..................................................................................................................32

Background...................................................................................................................32Lossless Versus Lossy Compression.............................................................................33Compression Implementations......................................................................................34

Introduction..........................................................................................................34

DriveSpace............................................................................................................36No Compression Option................................................................................................37

Hardware Compression........................................................................................39File Compression and Archiving Systems....................................................................40

Control Electronics........................................................................................................................41Primeval Controllers..............................................................................................................41

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Combined Host Adapter and Controller................................................................................42Embedded Controllers...........................................................................................................42Integrated Hard Disk Cards...................................................................................................43

Caching..........................................................................................................................................44Background............................................................................................................................44Cache Operation....................................................................................................................45

Read Buffering..............................................................................................................45Write Buffering.............................................................................................................47

Memory Usage......................................................................................................................48Software Caches............................................................................................................48

Windows 95 Hard Disk Read-ahead Optimization .............................................49Hardware Caches..........................................................................................................49

Drive Arrays..................................................................................................................................51Introduction...........................................................................................................................51Technologies..........................................................................................................................51

Background...................................................................................................................51Data Striping.................................................................................................................52Redundancy and Reliability..........................................................................................52

Implementations....................................................................................................................53Background...................................................................................................................53RAID Level 0................................................................................................................54RAID Level 1................................................................................................................54RAID Level 2................................................................................................................55RAID Level 3................................................................................................................56RAID Level 4................................................................................................................56RAID Level 5................................................................................................................57

RAID Level 6................................................................................................................57RAID Level 10..............................................................................................................58RAID Level 53..............................................................................................................58

Parallel Access Arrays...........................................................................................................59RAID Advisory Board..................................................................................................59

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MASSSTORAGE

Mass storage is where you put the data that you need to keep at hand but which will not fit into memory.

Designed to hold and retrieve megabytes at a moment's notice, mass storage traditionally has been the

realm of magnetic disks, but other technologies and formats now serve specialized purposes and await

their chances to move into the mainstream.

1. Introduction

1. The difference between genius and mere intelligence is storage. The quick-witted react fast, but

the true genius can call upon memories, experiences, and knowledge to find real answers. PCs

are no different. Putting a fast microprocessor in your PC would be meaningless without a

means to store programs and data for current and future use. Mass storage is the key to giving

your PC the long-term memory that it needs.2. Essentially an electronic closet, mass storage is where you put information that you don't want

to constantly hold in your hands but that you don't want to throw away, either. As with the

straw hats, squash rackets, wallpaper tailings, and all the rest of your dimly remembered

possessions that pile up out of sight behind the closet door, retrieving a particular item from

mass storage can take longer than when you have what you want at hand.

3. Mass storage can be online storage, instantly accessible by your microprocessor's commands,

or offline storage, requiring some extra intervention (such as you sliding a cartridge into a

drive) for your system to get the bytes that it needs. Sometimes, the term near-line storage is

used to refer to systems in which information isn't instantly available but can be put into instant

reach by microprocessor command. The jukebox CCan automatic mechanism that selects CD-

ROM cartridges (sometimes tape cartridges) CC is the most common example.

4. Moving bytes from mass storage to memory determines how quickly stored information can be

accessed. In practical online systems, the time required for this access range from less than 0.01

second in the fastest hard disks to 1000 seconds in some tape systems, spanning a range of

100,000 or five orders of magnitude.

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5. By definition, the best offline storage systems have substantially longer access times than the

quickest online systems. Even with fast-access disk cartridges, the minimum access time for

offline data is measured in seconds because of the need to find and load a particular cartridge.

The slowest online and the fastest offline storage system speeds, however, may overlap because

the time to ready an offline cartridge can be substantially shorter than the period required to

locate needed information written on a long online tape.

6. Various mass storage systems span other ranges as well as speeds. Storage capacity reaches

from as little as the 160 kilobytes of the single-sided floppy disk to the multiple gigabytes

accommodated by helical tape systems. Costs run from less than $100 to more than $10,000.

7. Personal computers use several varieties of mass storage. You can classify mass storage in

several ways: the technology and material the storage system uses for its memory, the way (and

often, speed) your PC accesses the data, and whether you can exchange the storage medium to

increase storage, to exchange information, or provide security.

8. Another way of dividing mass storageCCprobably the most familiarCCis by device type. Mass

storage devices common among PCs include hard disks, floppy disks, PC Cards, magneto-

optical drives, CD ROM drives (players and recorders), and tape drives. Although each of these

devices gives your PC a unique kind of storage, they share technologies and media. For

example, magnetic storage serves as the foundation for both hard disks and tape drives. The

devices differ, however, in how they put magnetic technology to work. Hard disks give your PC

nearly instant access to megabytes and gigabytes of data while tape drives offer slower, evenlaggardly, access in exchange for an inexpensive cartridge medium that gives you a safe backup

system.

9. All mass storage systems have four essential qualities: capacity, speed, convenience, and cost.

The practical differences between mass storage devices are the trade-offs they make in these

qualities.

10. Today=s mass storage systems use three basic technologies: magnetic, optical, and solid state

memory. Hard disks, floppy disks, and tape systems use magnetic storage. CD drives use

optical storage. PC Cards use solid-state memory. (Of course, hard disk drives also come in PC

Card format). Magneto-optical drives combine magnetic and optical technologies.

11. Mass storage systems use one of two means of accessing data: random access and sequential

access. Tape drives are the only sequential media devices in common use with PCs. New

technologies, however, are blurring the distinction between random and sequential storage. MO



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disks and CD ROMs began life as sequential devices with enhanced random-access capabilities.

Special hard disks, called AV drives, are random access devices that have been specially

designed to enhance their sequential storage abilities.

12. Most mass storage systems put their storage media in interchangeable cartridges. Only one kind

of mass storage does not permit you to interchange cartridges, the hard disk drive. This

inflexible technology is the most popular today chiefly because it scores highest in all other

mass storage qualities: capacity, speed, and cost.

13. All of these media share the defining characteristics of mass storage. They deal with data en

masse in that they store thousands and millions of bytes at a time. They also store that

information online. To earn their huge capacities, the mass storage system moves the data out

of the direct control of your PC's microprocessor. Instead of being held in your computer's

memory where each byte can be accessed directly by your system's microprocessor, mass

storage data requires two steps to use. First, the information must be moved from the mass

storage device into your system's memory. Then that information can be accessed by the

microprocessor.

14. The best way to put these huge ranges into perspective is to examine the technologies that

underlie them. All mass storage systems are unified by a singular principalCCthey use some

kind of mechanical motion to separate and organize each bit of information they store. To retain

each bit, these systems make some kind of physical change to the storage mediumCCburning

holes in it, blasting bits into oblivion, changing its color, or altering a magnetic field.



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2. Technologies

1. Introduction

1. The key to mass storage is the medium. Mass storage relies on having a medium that can

be readily changed from one state to anotherandretains those changes for a substantialperiod, usually measured in years, without the need for maintenance such as an external

power source. Paper and ink have long been a successful storage medium for human

thoughtsCCthe ink readily changes the paper from white to black, and those changes can

last centuries, providing the printer doesn=t skimp on the quality of the paper or ink.

2. In fact, paper and ink have been used successfully for computer storage. Bar codes and

even the Optical Character Recognition (OCR) of printed text allow PCs to work with

this time-proven storage system. But paper and ink come up short as a computer storage

system. It lacks the speed, capacity, and convenience required for a truly effective PC

mass storage system. You can=t avoid the comparisonsCCwhatever latest computer

storage system some benighted manufacturer introduces has the capacity of several

Libraries of Congress full of printed text and speed that makes Evelyn Woods look

dyslexic. Perhaps a reverse metaphor is more apt. A single VGA screen image, if printed

in its hexadecimal code as text characters, would fill an average book. Text characters of

the code of a single Windows program would fill an encyclopedia. Your computer needs

to read the entire VGA-image book in less than a blink of an eye and load the

encyclopedic program in a few seconds.

3. Compared to what paper and ink delivers, the needs of a PC for mass storage capacity are

prodigious indeed. The storage system must also allow the PC to sort through its storage

and find what it wants faster than the speed of frustration, which typically runs neck and

neck with light. And the medium must be convenient to work with, for you and your PC.

The list of suitable technologies is amazingly short: magnetic and optical. All PC mass

storage media are based on those two basic technologies or a combination of them.


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2. Magnetic

1. Background

1. Magnetic storage media have long been the favored choice for computer mass

storage. The primary attraction of magnetic storage is non-volatility. That is, unlikemost electronic or solid-state storage systems, magnetic fields require no periodic

addition of energy to maintain their state once it is set. Over decades of

development, the capacities of magnetic storage systems have increased by a factor

in the thousands and their speed of access has shrunk similarly. Despite these

differences, today=s magnetic storage system relies on exactly the same principles

as the first devices.

2. The original electronic mass storage system was magnetic tapeCCthat thin strip of

paper (in the United States) upon which a thin layer of refined rust had been glued.

Later, the paper gave way to plastic, and the iron oxide coating gave way to a

number of improved magnetic particles based on iron, chrome dioxide, and various

mixtures of similar compounds.

3. The machine that recorded upon these rust-covered ribbons was the Magnetophon,

the first practical tape recorder, created by the German division of the General

Electric Company, Allgemeine Elektricitaets Gesellschaft (AEG) in 1934.

Continually improved but essentially secret through the years of World War II

despite its use at German radio stations, the Magnetophon was the first device to

record and play back sound indistinguishable from live performances. After its

introduction to the United States (in a demonstration by John T. Mullin to the

Institute of Radio Engineers in San Francisco on May 16, 1946, tape recording

quickly became the premiere recording medium and within a decade gained the

ability to record video and digital data. Today, both data cassettes and streaming

tape system are based on the direct offspring of the first Magnetophon.

4. The principle is simple. Some materials become magnetized under the influence of a

magnetic field. Once the material becomes magnetized, it retains its magnetic field.

The magnetic field turns a suitable mixture or compound based on one of themagnetic materials into a permanent magnet with its own magnetic field. A

galvanometer or similar device can later detect the resulting magnetic field and

determine that the material has been magnetized. The magnet material remembers.


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2. Magnetism

1. Key to the memory of magnetism is permanence. Magnetic fields have the

wonderful property of being static and semi- permanent. On their own, they don't

move or change. The electricity used by electronic circuits is just the opposite. It isconstantly on the go and seeks to dissipate itself as quickly as possible. The

difference is fundamental. Magnetic fields are set up by the spins of atoms

physically locked in place. Electric charges are carried by mobile particlesCCmostly

electronsCCthat not only refuse to stay in place but also are individually resistant to

predictions of where they are or are going.

2. Given the right force in the right amount, however, magnetic spins can be upset,

twisted from one orientation to another. Because magnetic fields are amenable to

change rather than being entirely permanent, magnetism is useful for data storage.

After all, if a magnetic field were permanent and unchangeable, it would present no

means of recording information. If it couldn't be changed, nothing about it could be

altered to reflect the addition of information.

3. At the elemental particle level, magnetic spins are eternal, but taken collectively,

they can be made to come and go. A single spin can be oriented in only one

direction, but in virtually any direction. If two adjacent particles spin in opposite

directions, they cancel one another out when viewed from a larger, macroscopic

perspective.

4. Altering those spin orientations takes a force of some kind, and that's the key to

making magnetic storage work. That force can make an alteration to a magnetic

field, and after the field has changed, it will keep its new state until some other force

acts upon it.

5. The force that most readily changes one magnetic field is another magnetic field.

(Yes, some permanent magnets can be demagnetized just by heating them

sufficiently, but the demagnetization is actually an effect of the interaction of the

many minute magnetic fields of the magnetic material.)

6. Despite their different behavior in electronics and storage systems, magnetism andelectricity are manifestations of the same underlying elemental force. Both are

electromagnetic phenomena. One result of that commonalty makes magnetic storage

particularly desirable to electronics designersCCmagnetic fields can be created by

the flow of electrical energy. Consequently, evanescent electricity can be used to

create and alter semi-permanent magnetic fields.


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7. When set up, magnetic fields are essentially self-sustaining. They require no energy

to maintain, because they are fundamentally a characteristic displayed by the minute

particles that make up the entire universe (at least according to current physical

theories). On the sub-microscopic scale of elemental particles, the spins that form

magnetic fields are, for the most part, unchangeable and unchanging. Nothing is

normally subtracted from themCCthey don't give up energy even when they are put

to work. They can affect other electromagnetic phenomena, such as that used in

mass to divert the flow of electricity. In such a case, however, all the energy in the

system comes from the electrical flowCCthe magnetism is a gate, but the cattle that

escape from the corral are solely electrons.

8. The magnetic fields that are useful in storage systems are those large enough to

measure and effect changes on things that we can see. This magnetism is the

macroscopic result of the sum of many microscopic magnetic fields, many elemental

spins. Magnetism is a characteristic of sub-microscopic particles. (Strictly

speaking, in modern science magnetism is made from particles itself, but we don't

have to be quite so particular for the purpose of understanding magnetic computer

storage.)

3. Magnetic Materials

1. Three chemical elements are magneticCCiron, nickel, and cobalt. The macroscopic

strength as well as other properties of these magnetic materials can be improved by

alloying them, together and with non-magnetic materials, particularly rare earths

like samarium.

2. Many particles at the molecular level have their own intrinsic magnetic fields. At the

observable (macroscopic) level, they do not behave like magnets because their

constituent particles are organizedCCor disorganizedCCrandomly so that in bulk,

the cumulative effects of all their magnetic fields tend to cancel out. In contrast, the

majority of the minute magnetic particles of a permanent magnet are oriented in the

same direction. The majority prevails, and the material has a net magnetic field.

3. Some materials can be magnetized. That is, their constituent microscopic magneticfields can be realigned so that they reveal a net macroscopic magnetic field. For

instance, by subjecting a piece of soft iron to a strong magnetic field, the iron will

become magnetized.


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4. Magnetic Storage

1. If that strong magnetic field is produced by an electromagnet, all the constituents of

a magnetic storage system become available. Electrical energy can be used to alter a

magnetic field, which can be later detected. Put a lump of soft iron within theconfines of an electromagnet that has not been energized. Any time you return, you

can determine whether the electromagnet has been energized in your absence by

checking for the presence of a magnetic field in the iron. In effect, you have stored

exactly one bit of information.

2. To store more, you need to be able to organize the information. You need to know

the order of the bits. In magnetic storage systems, information is arranged

physically by the way data travel serially in time. Instead of being electronic blips

that flicker on and off as the milliseconds tick off, magnetic pulses are stored like a

row of dots on a piece of paperCCa long chain with beginning and end. This

physical arrangement can be directly translated to the temporal arrangement of data

used in a serial transmission system just by scanning the dots across the paper. The

first dot becomes the first pulse in the serial stream, and each subsequent dot

follows neatly in the data stream as the paper is scanned.

3. Instead of paper, magnetic storage systems use one or another form of

mediaCCgenerally a disk or long ribbon of plastic tapeCCcovered with a

magnetically reactive mixture. The form of medium directly influences the speed at

which information can be retrieved from the system.

4. No matter whether tape or disk, when a magnetic storage medium is blank from the

factory, it contains no information. The various magnetic domains on it are

randomly oriented. Recording on the medium reorients the magnetic domains into a

pattern that represents the stored information

5. After you record on a magnetic medium, you can erase it by overwriting it with a

strong magnetic field. In practice, you cannot reproduce the true random orientation

of magnetic domains of the unused medium. However, by recording a pattern with a

frequency out of the range of the reading or playback systemCCa very high or lowfrequencyCCyou can obscure previously recorded data and make the medium act as

if it were blank.


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5. Digital Magnetic Systems

1. Background

(1) Computer mass storage systems differ in principle and operation from tape

systems used for audio and video recording. Whereas audio and videocassettes record analog signals on tape, computers use digital signals.

(2) In the next few years, this situation will likely change as digital audio and

video tape recorders become increasingly available. Eventually, the analog

audio and video tape will become historical footnotes, much as the analog

vinyl phonograph record was replaced by the all-digital compact disc.

(3) In analog systems, the strength of the magnetic field written on a tape varies in

correspondence with the signal being recorded. The intensity of the recorded

field can span a range of more than six orders of magnitude. Digital systems

generally use a code that relies on patterns of pulses, and all the pulses have

exactly the same intensity.

(4) The technological shift from analog to digital is rooted in some of the

characteristics of digital storage that make it the top choice where accuracy is

concerned. Digital storage resists the intrusion of noise that inevitably pollutes

and degrades analog storage. Every time a copy is made of an analog

recording, the noise that accompanies the desired signal essentially doubles

because the background noise of the original source is added to the

background noise of the new recording medium; however, the desired signal

does not change. This addition of noise is necessary to preserve the nuances of

the analog recordingCCevery twitch in the analog signal adds information to

the whole. The analog system cannot distinguish between noise and nuance. In

digital recording, however, there's a sharp line between noise and signal. Noise

below the digital threshold can be ignored without losing the nuances of the

signal. Consequently, a digital recording system can eliminate the noise built

up in making copies. Moreover, noise can creep into analog recordings as the

storage medium deteriorates; whereas the digital system can ignore most of thenoise added by age. In fact, properly designed digital systems can even correct

minor errors that get added to their signals.


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2. Saturation

(1) Digital recordings avoid noise because they ignore all strength variations of

the magnetic field except the most dramatic. They just look for the

unambiguous "it's either there or not" style of digital pulses of information.Analog systems achieve their varying strengths of field by aligning the tiny

molecular magnets in the medium. A stronger electromagnetic field causes a

greater percentage of the fields of these molecules to line up with the field,

almost in direct proportion to the field strength, to produce an analog

recording. Because digital systems need not worry about intermediate levels of

signal, they can lay down the strongest possible field that the tape can hold.

This level of signal is called saturation because, much as a saturated sponge

can suck up no more water, the particles on the tape cannot produce a stronger

magnetic field.

(2) Although going from no magnetic field to a saturated field would seem to be

the widest discrepancy possible in magnetic recordingCCand therefore the

least ambiguous and most suitable for digital informationCCthis contrast is

not the greatest possible nor is it easy to achieve. Magnetic systems attempt to

store information as densely as possible, trying to cram the information in so

that every magnetic particle holds one data bit. Magnetic particles are

extremely difficult to demagnetizeCCbut the polarity of their magnetic

orientation is relatively easy to change. Digital magnetic systems exploit this

capability to change polarity and record data as shifts between the orientations

of the magnetic fields of the particles on the tape. The difference between the

tape being saturated with a field in one direction and the tape being saturated

with a field in the opposite direction is the greatest contrast possible in a

magnetic system and is exploited by nearly all of today's digital magnetic

storage systems.


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3. Coercivity

(1) One word that you may encounter in the description of a magnetic medium is

coercivity, a term that describes how strongly a magnetic field resists change,

which translates into how strong of a magnetic field a particular medium canstore. Stronger stored fields are better because the more intense field stands

out better against the random background noise that is present in any storage

medium. Because a higher coercivity medium resists change better than a low

coercivity material, it also is less likely to change or degrade because of the

effects of external influences. Of course, a higher coercivity and its greater

resistance to change means that a recording system requires a more powerful

magnetic field to maximally magnetize the medium. Equipment must be

particularly designed to take advantage of high coercivity materials.

(2) With hard disks, which characteristically mate the medium with the

mechanisms for life, matching the coercivity of a medium with the recording

equipment is permanently handled by the manufacturer. The two are matched

permanently when a drive is made. Removable media devicesCCfloppy disks,

tape cartridges, cassettes, etc.CCpose more of a problem. If media are

interchangeable and have different coercivities, you face the possibility of

using the wrong media in a particular drive. Such problems often occur with

floppy disks, particularly when you want to skimp and use cheaper double-

density media in high density or extra density drives.

(3) Moreover, the need for matching drive and medium makes upgrading a less-

than-simple mater. Obtaining optimum performance requires that changes in

media be matched by hardware upgrades. Even when better media are

developed, they may not deliver better results with existing equipment.


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(4) The unit of measurement for coercivity is the Oersted. As storage media have

been miniaturized, the coercivity of the magnetic materials, as measured in

Oersteds, has generally increased. The greater intrinsic field strength makes

up for the smaller area upon which data are recorded. With higher

coercivities, more information can be squeezed into the tighter confines of the

newer storage formats. For example, old 5.25-inch floppy disks had a

coercivity of 300 Oersteds. Today's high density 3.5-inch floppies have

coercivities of 750 Oersteds. Similarly, the coercivities of the tapes used in

today's high capacity quarter-inch cartridges is greater than that of the last

generation. Older standards used 550 Oersted media; data cartridges with

capacities in excess of 1.5 gigabytes and mini-cartridges with capacities

beyond 128 megabytes require 900-Oersted tape. Although invisible to you,

the coercivities of tiny modern hard disk drives are much higher than big old

drives.


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(5) Coercivity is a temperature-dependent property. As the temperature of a

medium increases, its resistance to magnetic change declines. That's one

reason you can demagnetize an otherwise permanent magnet by heating it red

hot. Magnetic media dramatically shift from being unchangeable to

changeableCCmeaning a drop in coercivityCCat a material-dependent

temperature called the Curie temperature. Magneto-optical recording systems

take advantage of this coercivity shift by using a laser beam to heat a small

area of magnetic medium that is under the influence of a magnetic field

otherwise not strong enough to affect the medium. At room temperature, the

media used by magneto-optical systems have coercivities on the order of 6,000

Oersteds; when heated by a laser, that coercivity falls to a few hundred

Oersteds. Because of this dramatic change in coercivity, the magnetic field

applied to the magneto-optical medium changes only the area heated by the

laser above its Curie temperature (rather than the whole area under the

magnetic influence). Because a laser can be tightly focused to a much smaller

spot than is possible with traditional disk read/write heads, using such a laser-

boosted system allows data to be defined by tinier areas of recording medium.

A disk of a given size thus can store more data when its magnetic storage is

optically assisted. Such media are resistant to the effects of stray magnetic

fields (which may change low coercivity fields) as long as they are kept at

room temperature.

4. Retentivity

(1) Another term that appears in the descriptions of magnetic media is retentivity,

which measures how well a particular medium retains or remembers the field

that it is subjected to. Although magnetic media are sometimes depended upon

to last foreverCCthink of the master tapes of phonograph recordsCCthe stored

magnetic fields begin to degrade as soon as they have been recorded. A higher

retentivity ensures a longer life for the signals recorded on the medium.

(2) No practical magnetic material has perfect retentivity, however; the randomelement of modern physical theories ensures that. Even the best hard disks

slowly deteriorate with age, showing an increasing number of errors as time

passes after data has been written. To avoid such deterioration of so-called

permanent records, many computer professionals believe that magnetically

stored recordings should be periodically refreshed. For example, they exercise


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tapes stored in mainframe computer libraries periodically (in intervals from

several months to several years depending on the personal philosophy and

paranoia of the person managing the storage). Although noticeable

degradation may require several years (perhaps a decade or more), these tape

caretakers do not want to stake their dataCCand their jobsCCon media written

long ago.

(3) If you're worried about the impermanence of magnetic recording, you can do

the same thing the professionals doCCrefresh your storage. You can back up

your data, restore your hard disk, copy floppy disks, or simply make new

backup tapes.

(4) Hard disks, the storage medium that most people depend on and worry about,

take an extra step to completely refresh. After you back up your hard disk,

you should low level format it, if you have a drive that allows low level

formatting. Only older drives with device-level interfaces, such as ST506 and

ESDI, benefit from (and even permit) low level reformatting. These drives

also are the ones most likely to benefit from reformatting because of the lower

coercivities and retentivities of their old-technology coatings.

(5) Modern hard disks that use the system-level AT, EIDE, or SCSI interfaces

make the disk formatting inaccessible to PC software. They are permanently

low level formatted during manufacture and have retentivities high enough to

maintain their format integrity throughout their useful lives.

(6) If you have an older disk, you need to low level reformat your older drive

instead of issuing a simple DOS FORMAT command, because only low level

formatting writes to the complete disk surface. DOS formatting affects only

the data storage areas of a hard disk. The low level format process writes

address marks on the disk that also can deteriorate with age. When one of

these sector markings inadvertently changes, you should see a Sector Not

Found or similar error that makes files impossible to read. Rewriting one of

these drives with a low level format can rejuvenate a cantankerous old disk.(7) After you low level format your older disk, partition it using DOS's FDISK

command. Finally, restore all of your files.


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(8) Several hard disk utility packages have special nondestructive low level

formatting procedures directly aimed at hard disk rejuvenation. These work by

copying all the data stored on a disk track to another location on the disk,

which low level reformats the original sector to refresh it (including its sector

identification markings) and then rewrites the original data back to the track.

The advantage of these systems is convenience. Although they still recommend

that you make a full disk backup before running the disk rejuvenation routine,

they relieve you of the need to make a full restoration of the backup (except in

the rare case that something goes wrong with the rejuvenation process). As

with the manual rejuvenation procedure, these programs usually work only

with hard disk drives that use device-level interfaces.

3. Magneto-Optical

1. Background

1. Magneto-optical technology uses an optical laser to enhance the capabilities of a

conventional magnetic storage system. In an MO system, the recording medium is

fundamentally a magnetic material (but one unlike anything you'll find on hard disks

and floppies) that relies on magnetic fields to store information. The optical part is

used only to assist the magnetic mechanism, to refine it perceptions. A tightly-

focused laser beam points out where the magnetic mechanism is to write data onto

the disk and prepares the medium to make it recordable. In reading, however, MO

drives are purely optical. The laser by itself reads the magnetically stored data from

the disk.

2. The combination of magnetic and laser technologies allows MO drives to achieve

high data densities. Several factors limit the data density that hard disks can

achieve, for example, the flying height of the read/write head. The underlying

problem is that the magnetic fields from the read/write head inevitably spread out.

The lasers used by optical drives are readily focused and can shoot great distances

from the optical head to the substrate without spreading out. The Figure below

illustrates this difference.


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3. In typical applications the laser of an optical drive reaches its sharpest focus below

the surface of an optical disc. Newer systems vary the focus of the laser to allow

storage to be arranged in independent layers, multiplying the capacity of the drive.

4. Certainly lasers can be used alone for purely optical drives. Compact Disc

technology, a purely optical system, has been used commercially for more than a

decade. But purely optical technology has a distinct disadvantage in re-writable

media. Pure optical system suffer from fatigue. Materials that change their

reflectivity in response to light wear out. With CD-Recordable systems, the wear is

fast and permanent. Other optical systems can withstand hundreds of thousands or

millions of write/read cycles but nevertheless decay with use.

5. Magnetic systems, on the other hand, suffer no such degradation. They can be

written and rewritten almost indefinitely (nothing is forever, remember). Even

though the fields of the particles in magnetic materials change, the particles

themselves don't change. Because MO drives are based on this recyclableCCand

well understoodCCmagnetic principle, they are generally considered to be capable

of an unlimited number of write-rewrite cycles. There's no worry about stress,

fatigue, failure, and data loss, yet they still can achieve the high storage densities of

optical systems.

2. Write Operation

1. The writing process for an MO system relies on the combined effects of magnetic

fields and laser-beam optics. The drives use a conventional magnetic field, called the

bias field, to write data onto the disk. Of course, the nature of the field is limited by

the same factors in traditional magnetic hard disksCCthe size of the magnetic

domains that are written is limited by the distance between the read-write head and

the medium and is, at any practical distance, much larger that the size of a spot

created by a focused laser.

2. To get the size of a magnetic domain down to truly minuscule size, MO drives use

the laser beam to assist magnetic writing. In effect, the laser illuminates a tiny area

within a larger magnetic field, and only this area is affected by the field.


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3. This optical-assist in magnetic recording works because of the particular magnetic

medium chosen for use in MO disks. This medium differs from that of ordinary

magnetic hard disk drives in having a higher coercivity, a resistance to changing its

magnetic orientation. In fact, the coercivity of an MO disk is about an order of

magnitude higher than the 600 or so Oersteds of coercivity of the typical magnetic

hard disk.

4. This high coercivity alone gives MO disks one of their biggest advantages over

traditional magnetic mediaCCthey are virtually immune to self-erasure. All

magnetic media tend to self-erase, that is, with passing time their magnetic fields

lose intensity because of the combined effects of all external and internal magnetic

fields upon them. The fields just get weaker. The higher the coercivity, the better a

medium resists self-erasure. Consequently, MO disks with their high coercivities are

able to maintain data more reliably over a longer period than purely magnetic hard

disks.

5. Along with such benefits, the higher coercivity of MO media brings another

challenge: obtaining a high enough magnetic flux to change the magnetic orientation

of the media while keeping the size of recorded domains small. Reducing this high

coercivity is how the laser assists the bias magnet in an MO drive.

6. The coercivity of the magnetic medium used by MO disks, as with virtually all

magnetic materials, decreases as its temperature increases and becomes zero at the

media-dependent Curie temperature. By warming the MO disk medium sufficiently

close to the Curie temperature, the necessary field strength to initiate a change can

be reduced to a practical level. The magnetic medium used by MO disks is

specifically engineered for a low Curie temperature, about 150 degrees Celsius.

7. The same laser that's used for reading the MO disk can simply be increased in

intensity to heat up the recording medium to its Curie temperature. This laser beam

can be tightly focused to achieve a tiny spot size. While the magnetic field acting on

the medium may cover a wide area, only the tiny spot heated by the laser actually

changes its magnetic orientation because only that tiny spot is heated high enough tohave a sufficiently low coercivity.

8. Practical mechanisms based on this design have one intrinsic drawback. The bias

magnetic field must remain oriented in a single direction during the process of

writing a large swath of a disk, a full sector or track. The field cannot change

quickly because the high inductance of the electromagnet that forms the field


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prevents the rapid switching of the magnet's polarities. The field must be far larger

and stronger than those of traditional magnetic disks because the magneto-optical

head is substantially farther from the diskCCit doesn't fly but rides on a track.

9. Because of the inability of the magnetic field to change rapidly, the bias magnet in

today's MO drives can align magnetic fields in a given area of a disk track only one

direction each time that portion of a track passes beneath the read/write head. For

example, when the bias field is polarized in the upward direction, it can change

downward-oriented fields on the disk to upward polarity but it cannot alter upward-

oriented field to the downward direction.

10. Practical MO systems today therefore require a two-step rewriting process. Before

an area can be rewritten on the disk, all fields in that area must be oriented in a

single direction. In other words, a given disk area must be separately erased before

it can be recorded. In conventional MO drive designs, this erasure process requires a

separate pass under the bias magnet with the polarity of the magnet temporarily

reversed. After one pass for erasing previously written material, the field of the

magnetic head is reversed again to the writing orientation. The actual information is

written to disk on a second pass under the head. The only areas that change

magnetic polarity are those struck with and heated by the laser beam.

11. The penalty for this two-step process is an apparent increase in the average access

time of MO drives when writing data. The extra time for a second pass is

substantial. Although speeds vary, many MO drives spin their disks at a leisurely

2400 revolutions per minute, roughly a third slower than even the oldest hard disk

drives (which typically operated at about 3600 RPM; current drives may spin up to

twice as fast). Each turn of such an MO disk therefore requires 25 milliseconds.

Even discounting head movement, the average access time for writing to an MO

drive cannot possibly be faster than 37.5 milliseconds. (On the average, the data to

be erased will be half a spin away from the read-write head, 12.5 milliseconds, and

a second spin to write the data will take an additional 25 milliseconds.)

Understandably, most manufacturers are working on "one-pass" MO drives and arespeeding the spins of their disks. Some drives now spin faster, at the same 3600

RPM rate as hard disks. The 3.5-inch MO drives typically operate at 3000 RPM.


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3. Read Operation

1. At the field strengths common to electronic gear, light beams are generally

unreactive with magnetic fields. Fiber optic cables, for example, are impervious to

the effects of normal electromagnetic noise that would pollute ordinary wires.Consequently, getting a laser to read the minuscule magnetic fields on an MO disk

is a challenge.

2. The trick used in MO technology is polarization. The MO disk is read by a laser

beam that is reflected from the disk surface as in other technologies, but in the MO

drive the laser beam is polarized. That is, the plane of orientation of its photons in

the laser beam are all aligned in one direction.

3. When the polarized beam strikes the magnetically aligned particles of the disk, the

magnetic field of the media particles causes the plane of polarization of the light

beam to rotate slightly, a phenomenon called theKerr effect. While small, as little

as a 1 percent shift in early MO media but now reportedly up to 7 percent, this

change in polarization can be detected as reliably as the direct magnetic reading of a

conventional magnetic hard disk. A polarized beam passing through a second

polarizing material diminishes in intensity depending on how closely the polarity of

the beam is aligned with that of the second material. In effect, the polarity change

becomes a readily detected intensity change.

4. Media

1. The media used by magneto-optical disks differs substantially from their magnetic

siblings. Moreover, all magneto-optical systems use cartridges. The MO medium is

suited to cartridge design because it is relatively invulnerable to the environmental

dangers that can damage magnetic media. Moreover, the storage densities of MO

allow a single platter cartridge to hold useful amounts of data.

2. MO cartridge media come in two size, roughly corresponding to those of hard disks:

5.25-inch and 3.5-inch. Despite their common name, 5.25-inch MO cartridges are

filled with optical disk platters that actually measure 130 millimeters (5.12 inches)

in diameter. The cartridges themselves measure 0.43 by 5.31 by 6.02 inches (HWD)and somewhat resemble 3.5-inch floppy disks in that the disk itself is protected by a

sliding metal shutter. So-called 3.5-inch magneto-optical disks have platters that are

actually 90 millimeters across in a cartridge shell, about the same size and

appearance as a 3.5-inch floppy diskCConly the MO disks are thicker.


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3. The magnetic medium on an MO disk is constructed from several layers. First, the

plastic substrate of the disk is isolated with a dielectric coating. The actual

magneto-optical compoundCCan alloy of terbium (a rare-earth element), iron, and

cobaltCCcomes next, protected by another dielectric coating. A layer of aluminum

atop this provides a reflective surface for the tracking mechanism. This sandwich is

then covered by 0.30 millimeters inches of transparent plastic. Disks are made

single-sided, then two are glued together back to back to produce two-sided media.

4. Unlike conventional magnetic hard disks that store data on a number of concentric

tracks or cylinders, under the ISO standard, MO drives use a single, continuous

spiral track much like the groove on an old vinyl phonograph record. The spiral

optimizes the data transfer of the drive because the read/write head does not need to

be moved between tracks during extended data transfers. It instead smoothly scans

across the disk.

4. Optical

1. Optical mass storage systems fall into three classes, read-only, write-once, and erasable.

The basic CD and DVD formats through which softwareCCbe it computer programs or

moviesCCis distributed are classic read-only media. WORM drives, used in archiving

systems, and CD-R drives that let you make your own CDs at home are write-once media.

Three erasable systems are or soon will be available, PCD, PD, and CD-E. These stand

for Phase Change Disc, Phase Disc, and Compact Disc-Erasable. All erasable optical

media use the same technology with minor variations.

2. The erasable optical technology is calledphase change. It is based on exotic compounds

that have two desirable properties. The materials have two markedly different

reflectivities in two different phase states. They have an amorphous state in which their

molecules are jumbled in such a way they form a rough surface that does not reflect light

well. They also have a crystalline state in which their molecules form the perfect array of

a crystal that has a smooth surface that reflects light well.

3. The other desirable quality is that a beam of light can change the material from one phase

to another. The material starts off in its amorphous state. Upon heating it with a laserbeam, the molecules align to the crystalline state. Further, stronger heating destroys the

crystal and returns the material to the amorphous state.

4. The primary drawback of phase-change media is that the material eventually wears out

and cannot reliably change its phases. Current compounds last for between 500,000 and

1,000,000 erase cycles before the material deteriorates to a point it is unacceptable for


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data storage. To compensate for the limited life of phase-change media, the software

drivers for the drives are designed to minimize the repeated erasing of sectors and spread

sector usage across the disc.


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3. Data Organization

1. Introduction

1. Computer storage systems differ in the way the organize and allow access to the

information that they store. Engineers class the storage systems in modern computers aseither sequential or random access. From a practical standpoint the difference is

functional, how the computer system finds the information that it needs. But it also has a

historic dimensionCCsequential computer mass storage pre-dates random access. You can

also look at the difference topographically. Sequential storage is one dimensional and

random access has two (or, possibly, more) dimensions.

2. Neither technology is inherently better. Both are used in modern PCs. For a given

application, of course, one of the two is typically more suitable. But because each has its

own strengths, both will likely persist as long as PCs populate desktops, if not longer.

2. Sequential Media

1. A fundamental characteristic of tape recording is that information is stored on tape one-

dimensionallyCCin a straight line across the length of the tape. This form of storage is

called sequential because all of the bits of data are organized one after another in a strict

sequence, like those paper-based dots. In digital systems, one bit follows after the other

for the full length of the tape. Although the width of the tape may be put to use in multi-

track, and the helical recording may be used by video systems, conceptually these, too,

store information in one dimension only.

2. In the Newtonian universe (the only one that appears to make sense to the normal human

mind), the shortest distance between two points is always a straight line. Alas, in magnetic

tape systems, the shortest distance between two bits of data on a tape may also be a long

time. To read two widely separated bits on a tape, all the tape between them must be

passed over. Although all the bits in between are not to be used, they must be scanned in

the journey from the first to second bits. If you want to retrieve information not stored in

order on a tape, the tape must shuttle back and forth to find the data in the order that you

want it. All that tape movement to find data means wasted time.

3. In theory, there's nothing wrong with sequential storage schemesCCdepending on thestorage medium that's used, they can be very fast. For example, one form of solid-state

computer memory, the all-electronic shift register, moves data sequentially at nearly the

speed of light.


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4. The sequential mass storage systems of today's computers are not so blessed with speed,

however. Because of their mechanical foundations, most tape systems operate somewhat

slower than the speed of light. For example, although light can zip across the vacuum of

the universe at 186,000 miles per second (or so), cassette tape crawls along at one and

seven-eighths inches per second. Although light can get from here to the moon and back in

a few seconds, moving a cassette tape that distance would take about 10 billion times

longer, several thousand years.

5. Although no tape stretches as long as the 238,000 mile distance to the moon, sequential

data access can be irritatingly slow. Instead of delivering the near-instant response most

of today's impatient power users demand, picking a file from a tape can take as long as 10

minutes. Even the best of today's tape systems require 30 seconds or more to find a file. If

you had to load all your programs and data files from tape, you might as well take up

crocheting to tide you through the times you're forced to wait.

6. Most sequential systems store data in blocks, sometimes called exactly that, sometimes

called records. The storage system defines the structure and content of each block.

Typically, each block includes identifying information (such as a block number) and

error-control information in addition to the actual data. Blocks are stored in order on tape.

In some systems, blocks lay end to end, while others separate them with blank areas called

Inter-Record Gaps.

7. Most tape systems use multiple tracks to increase their storage (some systems spread as

many as 144 tracks across tape just one-quarter inch wide). The otherwise stationary

read-write head in the tape machine moves up and down to select the correct track.

8. Some tape standards put a directory on the tape that holds the location of information on

the tape. By consulting the directory, the drive can determine which track holds the

information you want. Instead of scanning across hundreds of megabytes to find what you

need, the tape drive can zero in on the correct track, substantially trimming the response

time of the tape system.

9. These tape media have a very straightforward design. The tape moves from left to right

past a stationary read/write head. When a current is passed through an electromagneticcoil in this head, it creates the magnetic field needed to write data onto the tape.

10. When the tape is later passed in front of this head, the moving magnetic field generated by

the magnetized particles on the tape induces a minuscule current in the head. This current

is then amplified and converted into digital data. The write current used in putting data on

the tape overpowers whatever fields already exist on the tape, both erasing them and


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imposing a new magnetic orientation to the particles representing the information to be

recorded.

3. Random Access Media

1. On floppy and most hard computer disks, the recorded data are organized to takeadvantage of the two dimensional aspect of the flat, wide disk surface to give even faster

access than is possible with the directory system on tape. Instead of being arranged in a

single straight line, disk-based data are spread across several concentric circles like lanes

in a circular race track or the pattern of waves rolling away from a splash. Some optical

drives follow this system, but many other optical systems modify this arrangement,

changing the concentric circles into one tightly packed spiral that continuously winds from

edge to center of the disk. But even these continuous-data systems behave much as if they

had concentric circles of information.

2. The mechanism for making this arrangement is quite elementary. The disk moves in one

dimension under the read/write head, which scans the tape in a circle as it spins and

defines a track, which runs across the surface of the disk much like one of the lanes of a

racetrack. In most disk systems the head, too, can moveCCelse the read/write head would

be stuck forever hovering over the same track and the same stored data, making it a

sequential storage system that wastes most of the usable storage surface of the disk.

3. In most of today's disks systems, the read/write moves across a radius of the disk,

perpendicular to a tangent of the tracks. The read/write head can quickly move between

the different tracks on the disk. Although the shortest distance between two points (or two

bytes) remains a straight line, to get from one byte to another, the read/write head can

take shortcuts across the lanes of the racetrack. After the head reaches the correct track, it

still must wait for the desired bit of information to cycle around under it. However, disks

spin relatively quicklyCC300 revolutions per minute for most floppy disks and up to 7200

rpm for some hard disksCCso you only need to wait a fraction of a second for the right

byte to reach your system.

4. Because the head can jump from byte to byte at widely separated locations on the disk

surface and because data can be read and retrieved in any order or at random in the two-dimensional disk system, disk storage systems are often called random access devices,

even though they fall a bit short of the mark with their need to wait while hovering over a

track.

5. The random access capability of magnetic disk systems makes the combination much

faster than sequential tape media for the mass storage of data. Disks are so superior and


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so much more convenient than tapes that tape is almost never used as a primary mass

storage system. Usually, tape plays only a secondary role as a backup system. Disks are

used to store programs and files that need to be loaded on a moment's notice.

4. Combination Technologies1. Some PC applications involve long streams of data and only occasional random access.

The most important of these is multimedia. For example, a video plays a long sequence of

data bytes and requires random access only when you want to leap between scenes.

2. Although in theory random access storage should work for such applications, practical

storage systems sometimes have problems. When a disk drive needs to gather together a

number of random blocks of data to put together a data stream, its head may have to jump

all over the disk to locate the blocks. Although the drive can read data within each block

at a rate faster than the requirements of the data stream, the delay imposed by physically

moving the drive=s head to find the next block may interrupt the smooth flow of the data

stream. As a result, a video played from an ordinary hard disk may drop frames or appear

jumpy. Sequential devices don=t suffer this problem and are eminently suited to playing

video, as your home VCR ably demonstrates. What your VCR cannot do, however, is

quickly shift from a scene at the beginning of a tape to one at the end of the tape.

3. Some PC mass storage devices consequently combine sequential and random access.

Most optical drives and A/V hard disks (discussed in Chapter 10, "Hard Disks") are

optimized for applications that require the smooth data streams of sequential media.

4. Most magneto-optical and pure optical drives organize their storage as a single

continuous track spiral, essentially the topological equivalent of sequential tape. However,

the read-write head of the drive is still capable of moving radially across the disk. It has

to, if just to follow the long spiral. Because it still can leap from one part of the

continuous track to another, the moving head also endows these systems with fast random

access speeds. Because the head can follow the track inward without jogging, without

skipping over the unreadable disk area between tracks, these continuous track systems can

smoothly read long blocks of data at high speeds. Consequently, they offer excellent

random-access speeds and high continuous data transfer rates.


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4. Data Coding

1. Introduction

1. The goal of any data storage system is to pack as much information into as small a space

as possible, at least with an acceptable level of security and reasonable hope of being ableto recover it later. Long ago archivists realized that the information content was

paramount. The form in which they stored the information was negotiable. Instead of

locking away sheaves and reams of paper, they could microfilm the images and store the

data content in a fraction of the space.

2. In digital storage, more in less space has always been the credo. Again, one of the big

secrets is having freedom to manipulate the form of the data without affecting its

information content.

2. Flux Transitions

1. The ones and zeroes of digital information are not normally represented by the absolute

direction in which the magnetic field is oriented, but by a change from one orientation to

another so that they can take advantage of the most easily detected maximal magnetic

changeCCfrom saturation in one direction to saturation in the other. These dramatic

changes are termed flux transitions because the magnetic field or flux makes a transition

between each of its two allowed states. In the very simplest magnetic recording systems,

the occurrence of a flux transition would be the equivalent of a digital one; no transition

would be a digital zero.

2. The system must know when to expect a flux transition, or it would never know that it

had missed one. Somehow the magnetic medium and the recording system must be

synchronized with one another so that the system knows the point at which a flux

transition should occur or not. Instead of simple bit-for-bit recording, digital magnetic

storage requires an elaborate coding system to keep the data straight.

3. Certainly, assigning a single flux transition the job of storing a digital bit could be made

to work, but this obvious solution is hardly the optimal one. For instance, to prevent

errors, a direct one to one correspondence of flux to data would require that the pulse

train on the recording medium be exactly synchronized with the expectations of thecircuitry reading the data, perhaps by carefully adjusting the speed of the medium to

match the expected data rate. A mismatch would result in all of the data read or written

being in error. It might take several spins of the diskCCeach spin lasting more than a

dozen millisecondsCCto get back in sync.


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4. By including extra flux transitions on the disk to help define the meaning of each flux

change on a magnetic medium, you could eliminate the need for exact speed control or

other physical means of synchronizing the stored data. All popular magnetic recording

systems use this expedient to store data asynchronously. However, all of these

asynchronous recording schemes also impose a need for control information to help make

sense from the unsynchronized flux transition pulse train.

3. Single-Density Recording

1. In one of the earliest magnetic digital recording schemes called Frequency Modulation, or

FM recording, the place in which a flux transition containing a digital bit was going to

occur was marked by an extra transition called a clock bit. The clock bits form a periodic

train of pulses that enables the system to be synchronized. The existence of a flux change

between those corresponding to two clock bits indicated a digital 1, and no flux change

between clocks indicated a digital 0.

2. The FM system requires a reasonably loose frequency tolerance. That is, the system could

reliably detect the presence or absence of pulse bits between clock bits even if the clock

frequency was not precise. In addition, the bandwidth of the system is quite narrow, so

circuit tolerances are not critical. The disadvantage of the system is that two flux changes

were needed to record each bit of data, the least-dense practical packing of data on disk.

3. Initial digital magnetic storage devices used the FM technique, and for years it was the

prevailing standard. After improvements in data packing were achieved, FM became the

point of reference, often termed single-density recording.

4. Double-Density Recording

1. Modified Frequency Modulation recording (MFM) or double-density recording was once

the most widely used coding system for PC hard disks and is still used by many PC

floppy disk drives. Double-density recording eliminates the hard clock bits of single-

density to pack information on the magnetic medium twice as densely.

2. Instead of clock bits, digital 1's are stored as a flux transition and 0's a the lack of a

transition within a given period. To prevent flux reversals from occurring too far apart, an

extra flux reversal is always added between consecutive 0's.


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5. Group Coded Recording

1. Even though double-density recording essentially packs every flux transition with a bit of

data, it's not the most dense way of packing information on the disk. Other data coding

techniques can as much as double the information stored in a given system as compared todouble-density recording.

2. FM and MFM share a common characteristic, a one to one correspondence between bits

of data and the change recorded on the disk. Although such a correspondence is the

obvious way to encode information, it is not the only way. Moreover, the strict

correspondence does not always make the most efficient use of the storage medium.

3. The primary alternative way of encoding data is to map groups of bits to magnetic

patterns on the magnetic storage medium. Encoding information in this way is called

Group Coded Recording or GCR.

4. On the surface, group coding appears like a binary cipher. Just as in the secret codes used

by simplistic spies in which each letter of the alphabet corresponds to another, group

coding reduced to an absurdity would make a pattern like 0101 record on a disk as a

pattern of flux transitions like TTNT where T is a transition and N is no transition. Just

as simple translations buy the spy little secrecy (such transpositional codes can be broken

in minutes by anyone with a rudimentary knowledge of ciphering), they do little for the

storage system. Where they become valuable is in using special easy to record patterns of

flux transitions for each data groups, typically with more transitions than there are bits in

the data group. This technique succeeds in achieving higher real densities because the real

limit on data storage capacity is the spacing of flux transitions in the magnetic medium.

The characteristics of the magnetic medium, the speed at which the disk spins, and the

design of the disk read/write head together determine the minimum and maximum spacing

of the flux changes in the medium. If the flux changes are too close together, the

read/write head might not be able to distinguish between them; if they are too far apart,

they cannot be reliably detected.

5. By tinkering with the artificial restraints on data storage, more information can be packed

within the limits of flux transition spacing in the medium.6. Run Length Limited, or RLL, is one special case of Group Coded Recording designed to

use a complex form of data manipulation to fit more information in the storage medium

without exceeding the range limits of its capability to handle flux transitions. In the most

common form of RLL, termed 2,7, each byte of data is translated into a pattern of 16 flux

transitions.


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7. Although this manipulation requires double the number of flux transition bits to store a

given amount of information, it has the virtue that only a tiny fraction of the total number

of 16-bit codes is needed to unambiguously store all the possible eight-bit data codes.

There are 256 eight-bit codes and 65,536 sixteen-bit codes. Consequently, the engineer

designing the system has a great range of 16-bit codes to choose from for each byte of

data. If he's particularly astute, he can find patterns of flux translations that are

particularly easy to record on the disk. In the 2,7 RLL system, the 16-bit patterns are

chosen so that between two and seven digital zeroes are between each set of digital ones in

the resulting 16-bit data stream of flux transitions. The 16-bit code patterns that do not

enforce the 2,7 rule are made illegal and never appear in the data stream that goes to the

magnetic storage device.

8. Although the coding scheme requires twice as many bits to encode its data, the pulses in

the data stream better fit within the flux transition limits of the recording medium. In fact,

the 2,7 RLL code ensures that flux transitions will be three times farther apart than in

double-density recording, because only the digital ones cause flux changes, and they are

always spaced at least three binary places apart. Although there are twice as many code

bits in the data stream because of the 8 to 16 bit translation, their corresponding flux

transitions will be three times closer together on the magnetic medium while still

maintaining the same spacing as would be produced by MFM. The overall gain in storage

density achieved by 2,7 RLL over MFM is 50 percent.

9. The disadvantage of the greater recording density is that much more complex control

electronics and wider bandwidth electronics in the storage device are required to handle

the higher data throughput.

6. Advanced RLL

1. A more advanced RLL coding system improves not only on the storage density that can

be achieved on a disk but is also more tolerate of old-fashioned disks. This newer system

differs from 2,7 RLL in that it uses a different code that changes the bit pattern so that the

number of sequential zeros is between three and nine. This system, known for obvious

reasons as 3,9 RLL or Advanced RLL still uses an 8 to 16 bit code translation, but itensures that digital ones will never be closer than every four bits. As a result, it allows

data to be packed into flux transitions four times denser. The net gain, allowing for the

loss in data translation, amounts to 100 percent. Information can be stored about twice as

densely with 3,9 RLL as ordinary double-density recording techniques.


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4. Most compression systems work by reducing recurrent patterns in the data stream

into short tokens. For example, the two-byte pattern at could be coded as a single

byte such as @, cutting the storage requirement in half. Most compression systems

don't permanently assign tokens to bit patterns but instead make the assignments on

the fly. They work on individual blocks of data one at a time, starting afresh with

each block. Consequently, the patterns stored by the tokens of one block may be

entirely different from those used in the next block. The key to decoding the patterns

from the tokens is included as part of the data stream.

5. Disk compression systems put data compression technology to work by increasing

the apparent capacity of your disk drives. Generally, they work by creating a virtual

drive with expanded capacity with which you can work as if it were a normal (but

larger) disk drive. The compression system automatically takes care of compressing

and decompressing your data as you work with it. The information is stored in

compressed form on your physical disk drive, which is hidden from you.

6. The compression ratio compares the resultant storage requirements to those required

by the uncompressed data. For example, a compression ratio of 90 percent would

reduce storage requirements by 90 percent. The compressed data could be stored in

10 percent of the space required by its original form. Most data compression

systems achieve about a 50 percent compression ratio on the mix of data found that

most people use.

7. Because the compression ratio varies with the kind of data you store, the ultimate

capacity of a disk that uses compression is impossible to predict. The available

capacity reported by DOS on a compressed drive is only an estimate based on the

assumed compression ratio of the system. You can change this assumption to

increase the reported remaining capacity of your disk drive, but the actual remaining

capacity (which depends on the data you store, not the assumption) will not change.

2. Lossless Versus Lossy Compression

1. Most compression systems assume that you want to get back every byte and every

bit that you store. You don't want numbers disappearing from your spreadsheets orcommands from your programs. You assume that decompressing the compressed

data will yield everything you started withCCwithout losing a bit. The processes

that deliver that result are called lossless compression systems.

2. Sometimes, however, your data may contain more detail than you need. For

example, you might scan a photo with a true-color 24-bit scanner and display it on


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an ordinary VGA system with a color range of only 256 hues. All the precise color

information in your scan is wasted on your display, and the substantial disk space

you use for storing it could be put to better use.

3. Analog images converted to digital form and analog audio recordings that are

digitized often contain subtle nuances beyond the perception of most people. Some

data reduction schemes called lossy compression systems ignore these fine nuances.

Although the reconstituted data does not exactly replicate the original, for viewing

or listening to the restored data, lossy compression is often good enough. Because

lossy compression systems work faster than lossless schemes and because their

resulting compression ratios are higher, they are often used in time- and space-

sensitive applicationsCCdigital image and sound storage.

3. Compression Implementations

1. Introduction

(1) Compression is a data transformation much like all the other manipulations

made by a microprocessor. Consequently, an ordinary software program can

convert your PC's microprocessor into an excellent data compressor.

(2) Such software-only compression systems like that built into MS DOS

Versions 6.0 and later (and upgrade PC DOS Versions 6.1 and later) can be

used to increase disk or tape capacity. Some software compression systems

work as software drivers. They intercept the data stream headed for your hard

disk, reroute it through a compression algorithm run by your PC's

microprocessor, and pass the result to your disk instead of the original data.

When the compressed data is later read, the compressed data stream is

captured, its bytes processed by a complementary decompression algorithm,

and the results passed to your application software.


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(3) With older commercial, software-only disk compression systems, these

software drivers loaded through your system's CONFIG.SYS and

AUTOEXEC.BAT files. This arrangement made the operation of the

compression system and your PC confusing. The designers of the compression

systems tried to make their disk compression invisible. Because of the way

that DOS was designed, the only way the disk compression device driver

could create a compressed drive was to give it a new name (drive letter). The

disk compression software automatically switched the letter of the compressed

drive with the letter assigned to your physical boot drive. The larger capacity

compressed drive thus appeared to be drive C: (your boot drive), but your

boot drive was actually hidden under some other name. Your CONFIG.SYS

and AUTOEXEC.BAT files had to remain uncompressed for DOS to read

and load them so that it could load the drivers to read the compressed parts of

your disk drives. Although you expected to find these files on your boot drive

(nominally drive C:), the files were really on the hidden physical drive.

(4) The real solution was to change the structure of DOS, which Microsoft did

with Version 6.0. Before MS DOS 6.0 reads your PC's CONFIG.SYS file, it

checks for another configuration file, DRVSPACE.BIN, which holds the disk

compression driver. If present, this driver gets loaded first, so the compression

system is operational even before DOS reads your CONFIG.SYS file. As a

result, your CONFIG.SYS is stored on the virtual compressed drive in

compressed format, and you access it the same way you would any file. (Your

physical drive is still renamed something else, usually Drive H:, but you never

need to access it.)

(5) The advantage of software-only compression is that you pay for nothing other

than the programCCnothing if you rely on a recent version of DOSCCyet you

almost miraculously get two times more storage space. As with any software,

however, software-only disk compression imposes additional system overhead.

In older PCs with 386 and earlier microprocessors, this overhead can besufficient to slow disk response. You can take a load off your older

microprocessor hardware-based compression using a compression coprocessor

board. The coprocessor substitutes its power for that of your microprocessor

in compression operations, eliminating the performance handicap of software

compression.


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2. DriveSpace

(1) The most popular disk compression system today is DriveSpace, if only by

virtue of being a standard part of Microsoft=s operating system offerings.

DriveSpace is in its third incarnation. Microsoft created the first version as anenhancement to DOS 6.22 to resolve patent litigation with Stac Electronics,

the develop

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Hardware Students Guide - Chapter 8 Mass Storage