L>How computer systems Work: Disks And second Storage How computer systems Work: Disks And secondary StorageThe benefits of an additional StoragePicture, if girlfriend can, how many filing-cabinet drawers would certainly be forced tohold the millions of records of, say, tax records kept through the internal RevenueService or historic employee records kept by general Motors. The recordstorage rooms would have to be enormous. Computers, in contrast, permitstorage on ice or decaying in incredibly compressed form. Storage capacity isunquestionably one of the most valuable assets the the computer.Secondary storage, sometimes referred to as auxiliary storage, is storage separate from the computer system itself, whereby you deserve to store software and data on asemi long-term basis. Second storage is necessary since memory, orprimary storage, have the right to be supplied only temporarily. If you are sharing yourcomputer, you have to yield storage to someone rather after her programruns; if you are not sharing her computer, your programs and also data willdisappear native memory as soon as you turn off the computer. However, youprobably desire to store the data you have actually used or the info youhave acquired from processing; the is why secondary storage is needed.Furthermore, memory is limited in size, whereas secondary storage mediacan keep as much data as necessary. Keep in mental the characteristithedesigningfairy.com the the memory hierarchy that were described in the section on the CPU and memory:
The benefits of second storage deserve to be summarized together follows: Capacity. Institutions may store the equivalent of a roomful that data onsets the disks the take increase less room than a breadbox. A basic diskettefor a personal computer stop the equivalent of 500 printed pages, orone book. One optical disk can hold the tantamount of approximately400 books. Reliability. Data in secondary storage is usually safe, due to the fact that secondarystorage is physically reliable. Also, that is more complicated for unscrupulouspeople to tamper with data on disk 보다 data save on record in a filecabinet.Convenience. V the assist of a computer, authorized people can locateand access data quickly. Cost. Together the three previous benefits indicate far-ranging savings in warehouse costs. The is much less expensive to save data on ice cream or disk(the principal method of secondary storage) than to buy and house filingcabinets. Data that is reliable and safe is less expensive to keep thandata subject to errors. But the best savings deserve to be uncovered in the speedand convenience that filing and retrieving data. This benefits apply to every the various secondary storage gadgets but, asyou will see, some tools are far better than others. We start with a watch atthe various storage media, consisting of those supplied for personal computers,and then think about what the takes to get data organized and processed. Magnetic decaying StorageDiskettes and hard disks are magnetic media; that is, castle are based on atechnology of representing data as magnetized spots on the disk v amagnetized clues representing a 1 bit and the lack of such a clues representing a 0 bit. Reading data native the disk way converting the magnetized data to electrical impulses that deserve to be sent to the processor. Writingdata come disk is the opposite: sending electrical impulses indigenous the processorto be convert to magnetized spots on the disk. Thesurface of every disk has concentric monitor on it. The variety of tracks persurface varies v the particular form of disk.
| Storage|| Speed|| Capacity|| Relative cost ($)|| Permanent?|
| Registers|| Fastest|| Lowest|| Highest|| No|
| RAM|| really Fast|| Low/Moderate|| High|| No|
| Floppy Disk|| really Slow|| Low|| Low|| Yes|
| Hard Disk|| Moderate|| very High|| very Low|| Yes|
You are watching: File compression is only available on hard-disk drives.
DiskettesMade of flexible Mylar, a diskette deserve to record data together magnetized clues ontracks ~ above its surface. Diskettes became popular in addition to the personalcomputer. The older diskette, 5-1/4 customs in diameter, is still in use, butnewer computers use the 3-1/2 inch diskette (Figure 1). The 3-1/2 inchdiskette has the protection of a difficult plastic jacket, a size to to the right convenientlyin a shirt pocket or purse, and also the volume to organize significantly much more datathan a 5-1/4 inch diskette. Diskettes offer certain advantages which, asyou will see, space not readily available with hard disk:Portability. Diskettes easily transport data from one computer toanother. Workers, for example, lug their files from office computer system tohome computer system and ago on a diskette rather of in a briefcase. Students usage the campus computers however keep their papers on their owndiskettes.Backup. The is convenient to place an extra copy of a hard disk document on adiskette. Brand-new software. Although, because that convenience, software packages space kepton difficult disk, brand-new software the end of the box may come top top diskettes (newsoftware likewise may come on CD-ROM disks, i m sorry we will certainly discussshortly). The end of the diskettes advantageous life-time may be ~ above us. In 1998 Macintosh presented its brand-new computer, the IMAC, there is no a floppy disk drive. Alternatives such as Zip disks (discussed later), or transporting data via networks space making the low-capacity diskette come to be obsolete.For more details top top floppy disks and also drives click here. This is optional analysis .Hard Disks A tough disk is a steel platter coated with magnetic oxide that deserve to be magnetized to represent data. Difficult disks come in a variety of sizes.
tough diskfor mainframes and minicomputers might be as huge as 14 inches in diameter. Several disks deserve to be assembled into a decaying pack. Over there aredifferent varieties of disk packs, through the variety of platters differing bymodel. Every disk in the pack has actually top and also bottom surface on which torecord data. Numerous disk devices, however, do not record data ~ above the peak ofthe height platter or ~ above the bottom of the bottom platter.A disk drive is a machine that permits data to be read from a disk orwritten ~ above a disk. A disk pack is mounted on a disk drive the is a separateunit linked to the computer. Huge computers have dozens or everhundreds of disc drives. In a disk fill all disks revolve at the same timealthough just one disk is being check out or created on at any type of one time. Themechanism for analysis or composing data top top a disk is an access arm; that movesa read/write head into position over a details track. Theread/write head ~ above the end of the access arm hovers just over the trackbut does not actually touch the surface. Once a read/write head does accidentally touch the decaying surface, this is referred to as a head crash and also all data isdestroyed. Data can additionally be ruined if a read/write head meet evenminuscule international matter on the disc surface. A disk packhas a series of access arms that slip in between the disks in the pack. 2 read/write heads space on every arm, one dealing with up for the surfaceabove it and also one facing down for the surface listed below it. However, just oneread/write head have the right to operate at any type of one time.In some disk cd driver the access arms deserve to be retracted; then the disk packcan be removed from the drive. Most disk packs, however, combine thedisks, accessibility arms, and also read/write heads in a sealed module referred to as a Winchester disk. Winchester disc assemblies are placed together in clean rooms soeven microscope dust particles execute not gain on the disk surface.Hard disks for personal computers space 5-1/4 customs or 3-1/2 customs disks insealed modules and also even gigabytes are not unusual. Hard diskcapacity for personal computers has actually soared in recent years; capacities ofhundreds of megabytes are common and also gigabytes are not unusual.Although an individual probably cannot imagine generating enough output-letters, budgets, reports, and also so forth-to fill a tough disk, softwarepackages take up a lot of of space and have the right to make a dent quite quickly. Furthermore, graphithedesigningfairy.com images and audio and video files require large filecapacities. Perhaps an ext important 보다 capacity, however, is the convenience of speed. Personal computer users discover accessing records on a tough diskis significantly faster and thus an ext convenient than accessing papers on adiskette. For more details on difficult disks and also drives discovered in contemporary Pthedesigningfairy.com, click here. This is optional reading .Removable Storage: Zip Disks
|Figure 2: difficult Disk and also Drive|
Personal computer users, who never seem come have enough hard diskstorage space, may turn come a removable difficult disk cartridge. Once full, aremovable difficult disk cartridge have the right to be changed with a new one. In effect, aremovable cartridge is together portable as a diskette, yet the disc cartridgeholds much an ext data. Removable units also are essential to businessesconcerned v security, since the units have the right to be used during businesshours yet hidden away during off hours. A disadvantage the a removablehard disk is that it takes longer to accessibility data than a integrated hard drive.The most famous removable disk media is the Zip drive from Iomega (Figure 3). End 100"s of millions have actually been sold, making it the de facto standard. The disc cartridges look prefer a floppy disk, but are contempt bigger in every dimensions. Larger Zip disks hold 100MB, newer ones host 250MB and cost $8-$10 a item (Floppies host 1.4MB and also cost approximately $2). The drive sells for about $80- $125. Many new Pthedesigningfairy.com come v Zip drives developed in enhancement to floppy drives. Zip disks space a an excellent way to store big files and software programs.For an ext details onremovable warehouse such together Zip drives, click below . This is optional analysis .Hard Disks in GroupsA concept of using several little disks that job-related together as a unit is calleda redundant selection of cheap disks, or just RAID. Thegroup of connected disks operates together if that were just one large disk, yet itspeeds increase reading and also writing by having multiple access paths. The datafile for, say, aircraft factory tools, might be spread throughout several disks; thus,if the computer system is provided to look at up devices for numerous workers, the computerneed not check out the data in turn yet instead check out them in ~ the very same time inparallel. Furthermore, data defense is improved because if a disk fails, thedisk system can reconstruct data on an extra disk; thus, computer system operations can continue uninterrupted. This is significant data insurance.How Data Is organized on a DiskThere is more than one way of physically arranging data on a disk. Themethods we will take into consideration here are the sector an approach and the cylindermethod.The ar MethodIn the sector method each track is split into sectors that organize a specificnumber of characters. Data top top the monitor is accessed by referring to the surface number, track number, and sector number wherein thedata is stored. The sector technique is supplied for diskettes and also disk packs.Zone RecordingThe reality that a decaying is circular presents a problem: The ranges aroundthe monitor on the external of the decaying are higher than that of the monitor orthe inside. A provided amount that data that takes increase 1 customs of a monitor on theinside that a disk can be spread over several inches ~ above a track near the external of a disk. This method that the tracks on the external are no storingdata together efficiently.Zone record involves separating a disk right into zones to take benefit ofthe storage easily accessible on every tracks, through assigning an ext sectors to tracks inouter zones than come those in inner zones. Since each sector onthe disk holds the very same amount the data, much more sectors mean much more datastorage 보다 if every tracks had the same variety of sectors.The Cylinder MethodA way to theorem data top top a disk load is the cylinder method. The organization in this instance is vertical. The function is toreduce the time it bring away to move the accessibility arms that a disk pack into position. When the access arms room in position, they room in the very same verticalposition on every disk surfaces.To evaluate this, mean you had an north disk pack on i m sorry youwished to record data. You might be tempted to document the data horizontally-to start with the first surface, fill track 000, then to fill track 001, track002, and also so on, and then move to the 2nd surface and also again to fill tracks000, 001, 002, and also so forth. Each brand-new track and new surface, however,would require activity of the accessibility arms, a relatively slow mechanicalprocess.Recording the data vertically, on the various other hand, substantially reducesaccess arm movement. The data is taped on the tracks that deserve to beaccessed by one positioning of the access arms-that is, ~ above one cylinder. Tovisualize cylinder organization, pretend a cylindrically shaped item, suchas a believe can, were figuratively dropped straight down through all the disksin the decaying pack. All the tracks therefore encountered, in the same place oneach disk surface, consist of a cylinder. The cylinder method, then, meansall tracks of a certain cylinder on a disk pack are lined up one in ~ theother, and also all the vertical monitor of one cylinder are obtainable by theread/write heads through one positioning of the access arms mechanism.Tracks within a cylinder are numbered follow to this upright perspective: A 20-surface disk pack includes cylinder monitor numbered 0 through19, peak to bottom. Optical disk StorageThe explosive development in warehouse needs has driven the computer industry toprovide cheaper, much more compact, and more versatile storage tools withgreater capacity. This demanding shopping list is a description of the optical disk, like a CD. The an innovation works prefer this: A laser hits a great ofmetallic material spread over the surface of a disk. As soon as data is beingentered, warm from the laser produces tiny spots top top the disk surface. Toread the data, the laser scans the disk, and a lens picks up different lightreflections native the miscellaneous spots.Optical storage an innovation is categorized follow to that read/writecapability. Read-only media are taped on by the manufacturer and also canbe review from but not written to by the user. Together a disk cannot, obviously,be supplied for your files, however manufacturers deserve to use it to supply software.Applications software program packages sometimes encompass a dozen diskettes ormore; all these can fit on one optical disk v plenty that room come spare.The most prominent optical technology is the CD-ROM, because that compact disk read-only memory. The decaying inits drive is displayed in figure 3.
|Figure 3: Iomega Zip Disk|
CD-ROM has a major advantage over other optical diskdesigns: The disk style is the same to that of audio compact disks, so thesame dust-free manufacturing plants the are currently stamping the end digitalversions the Mozart or mar Chapin Carpenter deserve to easily convert to producing anything from software to one encyclopedia. Furthermore, CD-ROMstorage is large -up come 660 megabytes every disk, the equivalent ofover 400 3-1/2 inch diskettes.When purchase a computer the speed of the CD-ROM journey is advertised using an "X" factor, like 12X, or 24X. This shows the rate at which the CD have the right to transfer data come the CPU - the higher the Xfactor, the faster the CD. Contemporary computers currently offer a create CD journey or, CD-RW as an option. CD-RW is a write-once, read-many media. Through a CD-RW drive, you can produce your own CDs.This offers an inexpensive, convenient, safe way to store huge volumes the data such together favorite songs, photographs, etc.For an ext details top top CD technology, click here. This is optional analysis .
|Figure 3: Compact disc (CD) and also Drive)|
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DVDsDigital versatile Disk (DVD) drives are currently widely available in computers and home entertain centers. DVD-ROM drives can read data, such as stored advertising videos for playing. DVD-RW permit DVDs come be produced on a computer.
The DVD is a level disk, the dimension of a CD - 4.7 customs diameter and also .05 inch thick. Data room stored in a little indentation in a spiral track, as with in the CD. DVD disks are review by a laser beam of shorter wave-length than used by the CD ROM drives. This permits for smaller sized indentations and also increased storage capacity. The data great is only half as thick as in the CD-ROM. This opens the opportunity to write data in twolayers. The external gold great is semi transparent, to permit reading that the underlying silver- layer. The laser beam is set to two different intensities, strongest for reading the underlying silver- layer. A 4.7 GB side of a DVD can hold 135 minutes peak quality video clip with 6 monitor stereo. This calls for a transmission rate of 4692 bits every second. The 17 GB disk hold 200 hours top quality music recording. DVD movies room made in two "codes." region one is USA and also Canada, when Europe and also Asia is an ar two. Once you beat movies, her hardware (MPEG decoder. MGEG is the data coding because that movies similar to JPEG for pictures.) must complement the DVD region. The movies room made in 2 formats, each v their own coding. The DVD cd driver come in 2X, 4X, etc. Versions, choose the CD-ROM"s. The DVD drives will not change the magnetic tough disks. The difficult disks room being boosted as promptly as DVD, and also they absolutely offer the fastest look for time and transmission price (currently 5-10 MB/second). No optic media can keep up through this. But the DVD will undoubtedly obtain a place as the follower to the CD ROM and is playing critical role in the blending of computers and also entertainment centers. For more detail ~ above DVD technology, click here . This is optional analysis . Magnetic Tape warehouse We conserved magnetic ice cream storage because that last since it has taken a subordinaterole in warehouse technology. Magnetic ice looks like the tape offered in musiccassettes plastic tape with a magnetic coating. As in other magneticmedia, data is stored together extremely small magnetic spots. Tapes come in anumber that forms, consisting of l/2-inch-wide ice wound ~ above a reel, l/4-inch-wide tape in data cartridges and cassettes, and tapes that look like plain music cassettes however are draft to keep data instead of music. The quantity of data top top a tape is to express in regards to density, whichis the number of characters per customs (cpi) or bytes per inch (bpi) that canbe save on the tape.The highest-capacity ice cream is the digital audio tape, or DAT, which provides adifferent an approach of recording data. Utilizing a method called helical scanrecording, dat wraps roughly a rotating read/write head the spins vertically as it moves. This places the data in diagonal line bands that run throughout thetape quite than down its length. This an approach produces high thickness andfaster access to data.Two reels are used, a supply reel and a take-up reel. The supply reel,which has the tape with data on it or on i m sorry data will be recorded, is thereel the is changed. The take-up reel constantly stays v the magnetic tapeunit. Numerous cartridges and cassettes have the supply and also take-up reels builtinto the same case.Tape now has actually a limited role because disk has proved the exceptional storage medium. Disk data is rather reliable, especially within a sealed module.Furthermore, as we will certainly see, decaying data have the right to be accessed directly, as opposedto data ~ above tape, which deserve to be accessed only by passing by every the dataahead of the on the tape. Consequently, the primary function of tape this particular day is asan inexpensive backup medium.Backup SystemsAlthough a hard disk is very reliable device, a tough disk journey issubject come electromechanical failure that reason loss that data. Furthermore,data files, specifically those accessed by several users, space subject to errorsintroduced by users. There is likewise the possibility of errors presented bysoftware. With any an approach of data storage, a backup system a method ofstoring data in more than one location to defend it from damage and also errors is vital. As we have already noted, magnetic ice is used mostly forbackup purposes. For an individual computer users, straightforward and inexpensiveway to earlier up a hard disk file is to just copy it to a diskette at any time itis updated. Yet this is not handy for a device with many files or manyusers.Personal computer users have the option of to buy their very own tapebackup system, come be used on a consistent basis for copying all data fromhard disk to a high-capacity tape. Data hence saved deserve to berestored to the tough disk later if needed. A vital advantage the a ice cream backupsystem is the it have the right to copy the whole hard disk in minutes, conserving you thetrouble of swapping diskettes in and out of the machine.A ascendancy of thumb among computer specialists is to calculation disk needsgenerously and also then double that amount. But estimating future demands israrely easy. Many users, therefore, make later adjustments like including a removable hard disk cartridge to accommodate widening storageneeds. To quote plenty of a computer user, "I simply couldn"t envision how I might use all that diskspace. Currently I deserve to imagine even the extra disk pour it until it is full up."
|Figure 4: DVD Disk and Drive|