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Chapter 6: Storage
OBJECTIVES:
After completing this chapter, you will be able to:
1. Describe the characteristics of magnetic disks
2. Differentiate between floppy disks and Zip disks
3. Describe the characteristics of a hard disk
4. Describe the characteristics of optical discs
5. Differentiate among various CD and DVD formats
6. Identify the uses of tape
7. Discuss PC Cards and the various types of miniature mobile storage media
8. Identify uses of microfilm and microfiche
CONTENTS:
STORAGE
MAGNETIC DISKS
Floppy Disks
Zip Disks
Hard Disks
OPTICAL DISCS
CD-ROMs
CD-Rs and CD-RWs
DVD-ROMs
Recordable and Rewritable DVDs
TAPE
PC CARDS
MINIATURE MOBILE STORAGE MEDIA
Flash Memory Cards
USB Flash Drives
Smart Cards
MICROFILM AND MICROFICHE
ENTERPRISE STORAGE
PUTTING IT ALL TOGETHER
CHAPTER SUMMARY
COMPANIES ON THE CUTTING EDGE
Maxtor
SanDisk Corporation
TECHNOLOGY TRAILBLAZERS
Al Shugart
Mark Dean
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Storage holds data, instructions, and information for future use. For example, the home user might store letters, budgets, bank statements, a household inventory, records of stock purchases, tax data, addresses of friends and relatives, daily schedules, e-mail messages, home work assignments, recipes, digital photographs, music, and videos. A business user accesses many stored items, including customer orders and invoices, vendor payments, payroll records, tax data, inventory records, presentations, digital photographs, contracts, marketing literature, contacts, appointments, schedules, e-mail messages, and Web pages. Other users store diagrams, drawings, blueprints, designs, marketing literature, corporate newsletters, product catalogs,
FIGURE 6-1 A variety of storage media.
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videos, audio recordings, multimedia presentations, and digital photographs. All computers also use storage to hold system and application software.
Storage requirements among users vary greatly. Home users typically have much smaller storage requirements than business users. For example, a home user may need 80 billion bytes of storage, while large businesses may require 50 trillion bytes of storage.
A storage medium (media is the plural), also called secondary storage, is the physical material on which a computer keeps data, instructions, and information. Examples of storage media are floppy disks, Zip disks, hard disks, CDs and DVDs, tape, PC Cards, flash memory cards, USB flash drives, and microfiche (Figure 6-1).
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Capacity is the number of bytes (characters) a storage medium can hold. Figure 6-2 identifies the terms manufacturers use to define the capacity of storage media. For example, a typical floppy disk can store up to 1.44 MB of data (approximately 1.4 million bytes) and a typical hard disk has 80 GB (approximately 80 billion bytes) of storage capacity.
A storage device is the computer hardware that records and/or retrieves items to and from storage media. Writing is the process of transferring data, instructions, and information from memory to a storage medium. Reading is the process of transferring these items from a storage medium into memory. When storage devices write data on storage media, they are creating output. Similarly, when storage devices read from storage media, they function as a source of input. Nevertheless, they are categorized as storage devices, not as input or output devices.
The speed of storage devices is defined by access time. Access time measures the amount of time it takes a storage device to locate an item on a storage medium. The access time of storage devices is slow, compared with the access time of memory. Memory (chips) accesses items in billionths of a second (nanoseconds). Storage a storage medium is measured by the amount devices, by contrast, access items in thousandths of a second (milliseconds).
Magnetic disks use magnetic particles to store items such as data, instructions, and information on a disk’s surface. Depending on how the magnetic particles are aligned, they represent either a 0 bit or a 1 bit.
Magnetic disks store data and instructions in tracks and sectors (Figure 6-3). A track is a narrow recording band that forms a full circle on the surface of the disk. The disk’s storage locations consist of pie-shaped sections, which break the tracks sectors into small arcs called sectors. On a magnetic disk, a sector typically stores up to 512 bytes of data.
Three types of magnetic disks are floppy disks, Zip disks, and hard disks. Some of these disks are portable; others are not. With respect to a storage medium, the term portable means you can remove the medium from one computer and carry it to another computer. The following sections discuss types of magnetic disks.
FIGURE 6-2 The capacity of a storage medium is measured by the amount of bytes it can hold.
FIGURE 6.3 Tracks form circles on the surface of a magnetic disk. The disk’s storage locations are divided into pie-shaped sections, which break the tracks into small arcs called sectors.
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A floppy disk, also called a diskette, is a portable, inexpensive storage medium that consists of a thin, circular, flexible plastic Mylar film with a magnetic coating enclosed in a square- shaped plastic shell. A typical floppy disk is 3.5 inches wide and can store up to 500 double- spaced pages of text, several digital photographs, or a small audio file. Floppy disks are not as widely used as they were 15 years ago because of their low storage capacity. They are used, however, for specific applications. Some users work with floppy disks to transport small files to and from nonnetworked personal computers, such as from school or work to home.
A floppy disk drive is a device that reads from and writes on a floppy disk. A user inserts a floppy disk in and removes it from a floppy disk drive. Desktop personal computers and note book computers may have a floppy disk drive installed inside the system unit (Figure 6-4a).
Some users purchase an external floppy disk drive, in which the drive is a separate device with a cable that plugs in a port on the system unit (Figure 6-4b). These external drives are attached to the computer only when the user needs to access items on a floppy disk.
You can read from and write on a floppy disk any number of times. To protect a floppy disk from accidentally being erased, the plastic outer shell on the disk contains a write-protect notch in its corner. A write-protect notch is a small opening that has a tab you slide to cover or expose the notch. If the write-protect notch is open, the drive cannot write on the floppy disk. If the write-protect notch is covered, or closed, the drive can write on the floppy disk. A floppy disk drive can read from a floppy disk whether the write-protect notch is open or closed.
As mentioned earlier, a floppy disk stores data in tracks and sectors. A typical floppy disk stores data on both sides of the disk, has 80 tracks on each side of the recording surface, and has 18 sectors per track. The actual number of available bytes on a floppy disk is 1,474,560.
A Zip disk is a type of portable magnetic media that can store from 100 MB to 750 MB of data. The larger capacity Zip disks hold about 500 times more than a standard floppy disk. These large capacities make it easy to transport many files or large items such as graphics, audio, or video files. Another popular use of Zip disks is to back up important data and information. A backup is a duplicate of a file, program, or disk that you can use in case the original is lost, damaged, or destroyed. A Zip drive is a high- capacity disk drive developed by Iomega Corporation that reads from and writes on a Zip disk (Figure 6-5).
For more information, visit scsite.com/dcf2e/ ch6/weblink and then click Floppy Disks.
FIGURE 6-4 Two types of floppy disk drives.
For more information, visit scsite.com/dcf2e/ ch6/weblink and then click Zip Disks.
FIGURE 6-5 An external Zip drive.
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A hard disk is a storage device that contains one or more inflexible, circular platters that store data, instructions, and information. People use hard disks to store all types of documents, spreadsheets, presentations, databases, e-mail messages, Web pages, digital photographs, music, videos, and software. Businesses use hard disks to store correspondence, reports, financial records, e-mail messages, customer orders and invoices, payroll records, inventory records, presentations, contracts, marketing literature, schedules, and Web sites.
The system unit on most desktop and note book computers contains at least one hard disk. The entire device is enclosed in an airtight, sealed case to protect it from contamination. A hard disk that is mounted inside the system unit sometimes is called a fixed disk because it is not portable (Figure 6-6).
Current personal computer hard disks have storage capacities from 40 to 300 GB and more (read Looking Ahead 6-1 for a look at the next generation of hard disk storage capacities). The storage capacity of the average hard disk is more than 40,000 times that of a standard floppy disk. Like floppy disks and Zip disks, hard disks store data magnetically. Hard disks also are read/write storage media. That is, you can read from and write on a hard disk any number of times.
LOOKING AHEAD 6 1: Heat Increases Disk Capacity
Things are heating up in the data storage industry. Engineers at IBM Research are testing the use of heat to record data inexpensively on magnetic media, such as hard disks.
Within the next ten years, the researchers predict that this new technique will allow storage of more than one terabit per square inch; today’s magnetic media can store approximately 60 gigabits per square inch. With this capacity, a hard disk drive that can store seven terabits will be commonplace.
IBM calls this new storage system Millipede. It uses heated tips mounted on the ends of cantilevers, in a fashion similar to the way the stylus on an old phonograph sat on the grooves of vinyl records. For more information, visit scsite.com/dcf2e!ch6/looking and then click Heated Storage.
FAQ 6-1: Have personal computer hard disk capacities grown much since their inception?
Yes, hard disk capacities have grown phenomenally over the past several years, as shown in the chart below. This trend is expected to continue at a rate of 60 percent annually. For more information, visit scsite.com/dcf2e/ch6/faq and then click Hard Disk Capacities.
FIGURE 6-6 The hard disk in a desktop computer is enclosed inside an airtight, sealed case inside the system unit.
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CHARACTERISTICS OF A HARD DISK
Characteristics of a hard disk include capacity, platters, read/write heads, cylinders, sectors and tracks, revolutions per minute, transfer rate, and access time. Figure 6-7 shows sample characteristics of a 120 GB hard disk. The following paragraphs discuss each of these characteristics.
The capacity of a hard disk is determined from the number of platters it contains, together with composition of the magnetic coating on the platters. A platter is made of aluminum, glass, or ceramic and is coated with an alloy material that allows items to be recorded magnetically on its surface. The coating usually is three millionths of an inch thick.
On desktop computers, platters most often have a size of approximately 3.5 inches in diameter. A typical hard disk has multiple platters stacked on top of one another. Each platter has two read/write heads, one for each side. The hard disk has arms that move the read/write heads to the proper location on the platter (Figure 6-8).
FIGURE 6-7 Characteristics of a sample 120 GB hard disk. The actual disk’s capacity sometimes is different from the advertised capacity because of bad sectors on the disk.
FIGURE 6.8 HOW A HARD DISK WORKS
Step 1: The circuit board controls the movement of the head actuator and a small motor.
Step 2: A small motor spins the platters while the computer is running.
Step 3: When software requests a disk access, the read/write heads determine the current or new location of the data.
Step 4: The head actuator positions the read/write head arms over the correct location on the platters to read or write data.
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The location of the read/write heads often is referred to by its cylinder. A cylinder is the vertical section of a track that passes through all platters (Figure 6-9). A single movement of the read/ write head arms accesses all the platters in a cylinder. If a hard disk has two platters (four sides), each with 1,000 tracks, then it will have 1,000 cylinders with each cylinder consisting of 4 tracks (2 tracks for each platter).
While the computer is running, the platters in the hard disk rotate at a high rate of speed. This spinning, which usually is 5,400 to 15,000 revolutions per minute (rpm), allows nearly instant access to all tracks and sectors on the platters. The platters typically continue to spin until power is removed from the computer. (On many computers, the hard disk stops spinning after a specified time to save power.) The spinning motion creates a cushion of air between the platter and its read/write head. This cushion ensures that the read/write head floats above the platter instead of making direct contact with the platter surface. The distance between the read/write head and the platter is about two millionths of one inch.
As shown in Figure 6-10, this close clearance leaves no room for any type of contamination. Dirt, hair, dust, smoke, and other particles could cause the hard disk to have a head crash. A head crash occurs when a read/write head touches the surface of a platter, usually resulting in a loss of data or sometimes loss of the entire drive. Access time for today’s hard disks ranges from approximately 3 to 12 ms (milliseconds). The average hard disk access time is at least seven times faster than the average floppy disk drive.
FIGURE 6-9 A cylinder is the vertical section of track through all platters on a hard disk.
FIGURE 6-10 The clearance between a disk read/write head and the platter is about two millionths of an inch. A smoke particle, dust particle, human hair, or other contaminant could render the drive unusable.
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MINIATURE HARD DISKS Many mobile devices and consumer electronics include miniature hard disks to provide users with greater storage capacities than flash memory. These tiny hard disks (Figure 6-11) are found in devices such as music players, digital cameras, smart phones, and PDAs. Miniature hard disks have storage capacities that range from 2 GB to 100 GB.
PORTABLE HARD DISKS
Portable hard disks either are external or removable and have storage capacities up to 250 GB or higher.
An external hard disk, shown in the upper-left picture in Figure 6-12, is a separate free-standing hard disk that connects with a cable to a USB port or FireWire port on the system unit. As with the internal hard disk, the entire hard disk is enclosed in an airtight, sealed case.
A removable hard disk is a hard disk that you insert and remove from either a dock or a drive. Removable hard disks that insert in a dock are self-contained units. Removable hard disks that insert in a drive, shown in the bottom-right picture in Figure 6-12, operate in a manner similar to a floppy disk drive, reading from and writing on the removable hard disk.
Portable hard disks offer the following advantages over internal hard disks (fixed disks):
• Transport a large number of files
• Back up important files or an entire internal hard disk
• Easily store large audio and video files
• Secure your data; for example, at the end of a work session, remove the hard disk and lock it up, leaving no data in the computer
• Add storage space to a notebook computer or Tablet PC
• Add storage space to a desktop computer without having to open the system unit
• Share a drive with multiple computers
As the prices of portable hard disks drop, increasingly more users will purchase one to supplement a home or office internal hard disk.
HARD DISK CONTROLLERS
A disk controller consists of a special-purpose chip and electronic circuits that control the transfer of data, instructions, and information from a disk to and from the system bus and other components in the computer. That is, it controls the interface between the hard disk and the system bus. A disk controller for a hard disk, called the hard disk controller, may be part of a hard disk or the motherboard, or it may be a separate adapter card inside in the system unit.
In their personal computer advertisements, vendors usually state the type of hard disk interface supported by the hard disk controller. Thus, you should understand the types of available hard disk interfaces. In addition to USB and FireWire (external hard disk interfaces), three other types of hard disk interfaces for internal use in personal computers are SATA, EIDE, and SCSI.
FIGURE 6-1 1 This hard disk has a form factor of 0.85 inch and storage capacities up to 4 GB.
FIGURE 6.12 Examples of portable hard disks.
For more information, visit scsite.com/dcf2e/ ch6/weblink and then click Hard Disks.
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• SATA (Serial Advanced Technology Attachment), the newest type of hard disk interface, uses serial signals to transfer data, instructions, and information. The primary advantage of SATA interfaces is their cables are thinner, longer, more flexible, and less susceptible to interference than cables used by hard disks that use parallel signals. SATA interfaces also support connections to CD and DVD drives.
• EIDE (Enhanced Integrated Drive Electronics) is a hard disk interface that uses parallel signals to transfer data, instructions, and information. EIDE interfaces can support up to four hard disks at 137 GB per disk. EIDE interfaces also provide connections for CD and DVD drives and tape drives.
• SCSI interfaces, which also use parallel signals, can support up to eight or fifteen peripheral devices. Supported devices include hard disks, CD and DVD drives, tape drives, printers, scanners, network cards, and much more. Some computers have a built-in SCSI interface, while others use an adapter card to add a SCSI interface.
ONLINE STORAGE
Some users choose online storage instead of storing data locally on a hard disk. Online storage is a service on the Web that provides hard disk storage to computer users, usually for a minimal monthly fee (Figure 6-13). Fee arrangements vary. For example, one online storage service charges $10 per month for 5 GB of storage.
Users subscribe to an online storage service for a variety of reasons:
• To access files on the Internet hard disk from any computer or device that has Internet access
• To allow others to access files on their Internet hard disk so others can listen to an audio file, watch a video clip, or view a picture — instead of e-mailing the file to them
• To view time-critical data and images immediately while away from the main office or location; for example, doctors can view x-ray images from another hospital, home, or office
• To store offsite backups of data
Once users subscribe to the online storage service, they can save on the Internet hard disk in the same manner they save on their local hard disk.
FIGURE 6.1 3 An example of one Web site advertising its online storage service.
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Test your knowledge of pages 220 through 228 in Quiz Yourself 6-1.
Instructions: Find the true statement below. Then, rewrite the remaining false statements so they are true.
1. Hard disks contain one or more inflexible, circular platters that magnetically store data, instructions, and information.
2. SATA is a hard disk interface that uses parallel signals to transfer data, instructions, and information.
3. Storage media is the computer hardware that records and/or retrieves items to and from a storage device.
4. Three types of manual disks are floppy disks, Zip disks, and hard disks.
Quiz Yourself Online: To further check your knowledge of magnetic disks, floppy disks, Zip disks, and hard disks, visit scsite.com/dcf2e/ch6/quiz and then click Objectives 1 — 3.
FIGURE 6-14 on optical disc drives, you push a button to slide out a tray, insert the disc with the label side up, and then push the same button to close the tray.
An optical disc is a type of optical storage media that consists of a flat, round, portable, disc made of metal, plastic, and lacquer. These discs usually are 4.75 inches in diameter and less than one-twentieth of an inch thick.
Optical discs primarily store software, data, digital photographs, movies, and music. Some optical disc for mats are read only, meaning users cannot write (save) on the media. Others are read/write, which allows users to save on the disc just as they save on a hard disk.
Nearly every personal computer today includes some type of optical disc drive installed in a drive bay. On these drives, you push a button to slide out a tray, insert the disc, and then push the same button to close the tray (Figure 6-14). Other convenient features on most of these drives include a volume control button and a headphone port (or jack) so you can use headphones to listen to audio without disturbing others nearby.
With some discs, you can read and/or write on one side only. Manufacturers usually place a silk-screened label on the top layer of these single-sided discs. You insert a single-sided disc in the drive with the label side up. Other discs are double-sided. Simply remove the disc from the drive, flip it over, and reinsert it in the drive to use the other side of the disc. Double-sided discs often have no label; instead each side of the disc is identified with small writing around the center of the disc.
Optical discs store items by using microscopic pits (indentations) and lands (flat areas) that are in the middle layer of the disc. A high-powered laser light creates the pits. A lower-powered laser light reads items from the disc by reflecting light through the bottom of the disc, which usually is either solid gold or silver in color. The reflected light is converted into a series of bits the computer can process.
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Manufacturers guarantee that a properly cared for high-quality optical disc will last 5 years but could last up to 100 years. Figure 6-15 offers some guidelines for the proper care of optical discs.
Many different formats of optical discs exist today. Two general categories are CDs and DVDs, with DVDs having a much greater storage capacity than CDs. Specific formats include CD-ROM, CD-R, CD-RW, DVD-ROM, DVD-R, DVD+R, DVD-RW, DVD+RW, and DVD+RAM.
Figure 6-16 identifies each of these optical disc for mats and specifies whether a user can read from the disc, write to the disc, and/or erase the disc. The following sections describe characteristics unique to each of these disc formats.
FIGURE 6-15 Some guidelines for the proper care of optical discs.
FIGURE 6-16 Manufacturers sell CD-ROM and DVD-ROM media prerecorded (written) with audio, video, and software. Users cannot change the contents of these discs. Users, however, can purchase the other formats of CDs and DVDs as blank media and record (write) their own data, instruction...
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