3-2-3 Secondary Storage

Transcription

1 3-2-3 Secondary Storage Secondary Storage Hardware: - devices that permanently hold data and information as well as programs. We will look at floppy disks, hard disks, optical disks, magnetic tape, and smart cards. 1- Floppy Disks A floppy disk, often called a diskette or simply a disk, is a removable flat piece of Mylar plastic packaged in a 3.5-inch plastic case. Data and programs are stored on the disk's coating by means of magnetized spots, following standard on/off patterns of data representation (such as ASCII). The plastic case protects the Mylar disk from being touched by human hands. Originally, when most disks were larger (5.25 inches), the disks actually were "floppy," not rigid; now only the plastic disk inside is flexible or floppy. Floppy disks are inserted into a floppy-disk drive, a device that holds, spins, reads data from, and writes data to a floppy disk. Read means that the data in secondary storage is converted to electronic signals and a copy of that data is transmitted to the computer's memory (RAM). Write means that a copy of the electronic information processed by the computer is transferred to secondary storage. Floppy disks have a write-protect notch, which allows you to prevent a diskette from being written to. In other words, it allows you to protect the data already on the disk. To write protect, use your thumbnail or the tip of a pen to move the small sliding tab on the lower right side of the disk (viewed from the back), thereby uncovering a square hole. On the diskette, data is recorded in concentric circles called tracks. Unlike on a vinyl phonograph record, these tracks are neither visible grooves nor a single spiral. Rather, they are closed concentric rings. On a formatted disk each track is divided into sectors, invisible wedgeshaped sections used for storage reference purposes. When you save data from your computer to a diskette, the data is distributed by tracks and sectors on the disk. That is, the system software uses the point at which a sector intersects a track to reference the data location. 30

2 When you insert a floppy disk into the slot (the drive gate or drive door) in the front of the disk drive, the disk is fixed in place over the spindle of the drive mechanism. The read/write head is used to transfer data between the computer and the disk.- When the disk spins inside its case, the read/write head moves back and forth over the data access area on the disk. When the disk is not in the drive, a metal clip covers this access area. An access light goes on when the disk is in use. After using the disk, you can retrieve it by pressing an eject button beside the drive. Note: Do not remove the disk when the access light is on. Let's compare the 3.5-inch floppy disk with some 3.5-inch floppy-disk cartridges, or higher-capacity removable disks-zip disks, SuperDisks, and HiFD disks. Let's consider these: (1) 3.5-inch floppy disks megabyte: The present-day standard for traditional floppy disks is 1.44 megabytes, the equivalent of 500 typewritten pages. Today's floppy carries the label 2HD, in which the 2 stands for "double-sided" (it holds data on both sides) and the HD stands for "high density" (which means it stores more data than the previous standard-dd, for "double density"). (2) Zip disks megabytes: Produced by Iomega Corp., Zip disks are special disks with a capacity of 100 or even 250 megabytes. At 100 megabytes, this is 70 times the storage capacity or, the standard floppy. Among other uses, Zip disks are used to store large spreadsheet files, database files, image files, multimedia presentation files, and Web sites. Zip disks require their own Zip disk drives, which may come installed on new computers, although external Zip drives are also available. (3) SuperDisks megabytes: Produced by Imation, SuperDisks are disks with a capacity of 120 megabytes; the SuperDisk drive can also read standard megabyte floppy disks, which Zip drives cannot do. (4) HiFD disks megabytes: Made by Sony Corp., HiFD disks have a capacity of 200 megabytes; the disk drive can also read standard 1.44-megabyte floppies. HiFD disks have 140 times the capacity of today's standard floppy disks. 31

3 2- Hard Disks Floppy disks use flexible plastic, but hard disks use metal. Hard disks are thin but rigid metal platters covered with a substance that allows data to be held in the form of magnetized spots. Hard disks are tightly sealed within an enclosed hard-disk-drive unit to prevent any foreign matter from getting inside. Data may be recorded on both sides of the disk platters. Hard disks are quite sensitive devices. The read/write head does not actually touch the disk but rather rides on a cushion of air about inch thick. The disk is sealed from impurities within a container, and the whole apparatus is manufactured under sterile conditions. Otherwise, all it would take is a human hair, a dust particle, a fingerprint smudge, or a smoke particle to cause what is called a head crash. A head crash happens when the surface of the read/write head or particles on its surface come into contact with the surface of the hard-disk platter, causing the loss of some or all of the data on the disk. A head crash can also happen when you bump a computer too hard or drop something heavy on the system cabinet. An incident of this sort could, of course, be a disaster if the data has not been backed up. There are firms that specialize in trying to retrieve (for a hefty price) data from crashed hard disks, though this cannot always be done. There are two types of hard disks-non removable and removable. (1) Non removable hard disks: A non removable hard disk, also known as a fixed disk, is housed in a microcomputer system unit and is used to store nearly all programs and most data files. Usually it consists of four 3.5-inch metallic platters sealed inside a drive case the size of a small sandwich, which contains disk platters on a drive spindle, read/write heads mounted on an access arm that moves back and forth, and power connections and circuitry. Operation is much the same as for a diskette drive: The read/write heads locate specific 32

4 instructions or data files according to track or sector. Microcomputer hard drives with capacities measured in tens of gigabytes-up to 320 gigabytes, according to current ads-are becoming essential because today's programs are so huge. Microsoft Office alone is 100 megabytes. As for speed, hard disks allow faster access to data than floppy disks do, because a hard disk spins many times faster. Computer ads frequently specify speeds in revolutions per minute. A floppy disk drive rotates at only 360 rpm; a 7200-rpm hard drive is going about 300 miles per hour. In addition, ads may specify the type of hard-disk controller, a special-purpose circuit board that positions the disk and read/write heads and manages the flow of data and instructions to and from the disk. Popular harddisk controllers are Ultra ATA (or EIDE) and SCSI. Commonly found on new PCs, Ultra ATA (advanced technology attachment) allows fast data transfer and high storage capacity; it is also known as EIDE (enhanced integrated drive electronics). Ultra ATA can support only one or two hard disks. By contrast, SCSI (small computer system interface), pronounced "scuzzy," supports several disk drives as well as other peripheral devices by linking them in a daisy chain of up to seven devices. SCSI controllers are faster and have more storage capacity than EIDE controllers; they are typically found in servers and workstations. (2) Removable hard disks: Removable hard disks, or hard disk cartridges, consist of one or two platters enclosed along with read/write heads in a hard plastic case, which is inserted into a microcomputer's cartridge drive. Typical capacity is now terabytes. 33

5 3- Optical Disks: CDs & DVDs An optical disk is a removable disk, usually 4.75 inches in diameter and less than one-twentieth of an inch thick, on which data is written and read through the use of laser beams. An audio CD holds up to 74 minutes (2 billion bits' worth) of highfidelity stereo sound. Some optical disks are used strictly for digital data storage, but many are used to distribute multimedia programs that combine text, visuals, and sound. With an optical disk, there is no mechanical arm, as with floppy disks and hard disks. Instead, a high-power laser beam is used to write data by burning tiny pits or indentations into the surface of a hard plastic disk. To read the data, a lowpower laser light scans the disk surface: Pitted areas are not reflected and are interpreted as 0 bits; smooth areas are reflected and are interpreted as 1 bits. Because the pits are so tiny, a great deal more data can be represented than is possible in the same amount of space on a diskette and many hard disks. An optical disk can hold over 4.7 gigabytes of data, the equivalent of 1 million typewritten pages. Nearly every PC marketed today contains a CD-ROM or DVD-ROM drive, which can also read audio CDs. These, along with their recordable and rewritable variations, are the two principal types of optical disk technology used with computers. (1) CD-ROM - for reading only: For microcomputer users, the best known type of optical disk is the CD-ROM. CD-ROM (compact disk-read-only memory) is an optical-disk format that is used to hold prerecorded text, graphics, and sound. Like music CDs, a CD-ROM is a read-only disk. Read only means the disk's content is recorded at the time of manufacture and cannot be written on or erased by the user. As the user, you have access only to the data imprinted by the disk's manufacturer. A CD-ROM disk can hold up to 650 megabytes of data, equal to over 300,000 pages of text. A CD-ROM drive's speed is important because with slower drives images and sounds may appear choppy. In computer ads, drive speeds are indicated by the symbol "X," as in "44X," 34

6 which is a high speed. "X" denotes the original data transfer rate of 150 kilobytes per second. (The data transfer rate is the time the drive takes to transmit data to another device. A 44X drive runs at 44 times 150, or 6600 kilobytes (6.6 megabytes) per second. If an ad carries the word "Max," as in "44X Max," this indicates the device's maximum speed. Drives range in speed from 16X to 48X; the faster ones are more expensive. (2) CD-RW - for rewriting many times: A CD-RW (compact disk- rewritable) disk, also known as an erasable optical disk, allows users to record and erase data so that the disk can be used over and over again. Special CD-RW drives and software are required. CD-RW disks are useful for archiving and backing up large amounts of data or work in multimedia production or desktop publishing. However, because they are relatively slow they are no substitute for a hard disk. (3) DVD-ROM - the versatile video disk: A DVD-ROM (digital versatile disk or digital video disk, with read-only memory) is a CD disk with extremely high capacity, able to store gigabytes. How is this done? Like a CD or CD-ROM, the surface of a DVD contains microscopic pits, which represent the Os and is of digital code that can be read by a laser. The pits on the DVD, however, are much smaller and grouped more closely together than those on a CD, allowing far more information to be represented. Also, the laser beam used focuses on pits roughly half the size of those on current audio CDs. In addition, the DVD format allows for two layers of data-defining pits, not just one. Finally, engineers have succeeded in squeezing more data into fewer pits, principally through data compression. Many new computer systems now come with a DVD drive as standard equipment; these drives can also take standard CD-ROM disks. DVDs have enormous potential to replace CDs for archival storage, mass distribution of software, and entertainment. They not only store far more data but are different in quality from CDs. As one writer points out, DVDs encompass much more: multiple dialogue 35

7 tracks and screen formats, and best of all, smashing sound and video." The theater-quality video and sound, of course, is what makes DVD a challenger to videotape as a vehicle for movie rentals. As with CDs, DVDs have their recordable and rewritable variants. DVD-R (DVD-recordable) disks allow one-time recording by the consumer. Two types of reusable disks are DVD-RW (DVD-rewritable) and DVD-RAM (DVD-random access memory), both of which can be recorded on and erased more than once. 4- Magnetic Tape Similar to the tape used on an audio tape recorder (but of higher density), magnetic tape is thin plastic tape coated with a substance that can be magnetized. Data is represented by magnetized spots (representing 1s) or non-magnetized spots (representing 0s). Today, "mag tape" is used mainly for backup and archiving-that is, for maintaining historical records-where there is no need for quick access. On large computers, tapes are used on magnetic-tape units or reels, and in cartridges. On microcomputers, tape is used in the form of tape cartridges, modules resembling audio cassettes that contain tape in rectangular, plastic housings. There are three common types of drive tapes-qic, DAT, and (most expensive) DLT. A comparison among main, secondary, and cache memories: The auxiliary memory has a large storage capacity, is relatively inexpensive, but has low access speed compared to main memory. The cache memory is very small, relatively expensive, and has very high access speed. Thus as the memory access speed increases, so does its relative cost. The overall goal of using a memory hierarchy is to obtain the highest-possible average access speed while minimizing the total cost of the entire memory system. Auxiliary and cache memories are used for different purpose. The cache holds those parts of the program and data that are most heavily used, while the auxiliary memory holds those parts that are not presently used by CPU. Moreover, the CPU has direct access to both cache and 36

8 main memory but not to auxiliary memory. The transfer from auxiliary to main memory is usually done by means of direct memory access of large blocks of data. The typical access time ratio between cache and main memory is about 1 to 7. For example, a typical cache memory may have an access time of 100 ns, while main memory access time may be 700 ns. Auxiliary memory average access time is usually 1000 times that of main memory. 3-3 Ports & Cables: A. port is a connecting socket or jack on the outside of the system unit into which are plugged different kinds of cables. A port allows you to plug in a cable to connect a peripheral device, such as a monitor, printer, or modem, so that it can communicate with the computer system. Ports are of several types: (1) Serial ports - for transmit ting slow data over long distances: A line connected to a serial port will send bits one after the other, like cars on a one-lane highway. Because bits must follow each other one at a time, a serial port is usually used to connect devices that do not require fast transmission of data, such as keyboard, mouse, monitors, and modems. It is also useful for sending data over a long distance. The standard for PC serial ports is the 9-pin or 25-pin RS-232C connector. (2) Parallel ports for transmitting data over small distances: A line connected to a parallel port allows 8 bits (1 byte) to be transmitted simultaneously, like cars on an eight-lane highway. Parallel lines move information faster than serial lines do, but - they can transmit information efficiently only up to 15 feet. Thus, parallel ports are used principally for connecting printers or external disk or magnetic -tape backup storage devices. 37

9 (3) SCSI ports - for transmitting fast data to up to seven devices in a daisy chain: Pronounced. "scuzzy," a SCSI (small computer system interface) port allows data to be transmitted in a "daisy chain" for up to 7 devices at speeds (32 bits at a time) higher than those possible with serial and parallel ports. Among the devices that may be connected are external hard-disk drives, CD-ROM drives, scanners, and magnetic-tape backup units. The term daisy chain means that several devices are connected in series to each other, so that data for the seventh device, for example, has to go through the other six devices first. Sometimes the equipment on the chain is inside the computer, an internal daisy chain; sometimes it is outside the computer, an external daisy chain. (4) USB ports - for transmitting data to up to 127 devices in a daisy chain: A USB (universal serial bus) port can theoretically connect up to 127 peripheral devices daisy-chained to one general-purpose port. USB ports are useful for peripherals such as digital cameras, digital speakers, scanners, high-speed modems, and joysticks. The so-called USB hot plug or hot swappable allows such devices to be connected or disconnected even while the PC is running. In addition, USB permits Plug and Play, which allows peripheral devices and expansion cards to be automatically configured while they are being installed. This avoids the hassle of setting switching and creating special files that plagued earlier users. In 2000, computer makers were planning to introduce USB 2.0, expected to run 40 times faster than the earlier version. Can you really connect up to 127 devices on a single chain? An Intel engineer did set a world record at an industry trade show before a live audience by connecting 111 peripheral devices to a single USB port on a PC. (5) Medicated ports - for keyboard, mouse, phone, and so on: So far, we have been considering general-purpose ports, but the back of a computer also 38

10 has other, dedicated- ports, ports for special purposes. Among these are the round ports for connecting the keyboard and the mouse. There are also jacks for speakers and microphones and modem-to-telephone jacks. Finally, there is one connector that is not a port at all-the power plug socket, into which you insert the power cord that brings electricity from a wall plug. (6) Infrared ports-for cable less connections over a few feet: When you use a handheld remote unit to change channels on a TV set, you're using invisible radio waves of the type known as infrared waves. An infrared port allows a computer to make a cable less connection with infrared-capable devices, such as some printers. This type of connection requires an unobstructed line of sight between transmitting and receiving ports, and they can only be a few feet apart. 3-4 Expandability Buses & Cards Today many new microcomputer systems can be expanded. As mentioned earlier, expansion is a way of increasing a computer's capabilities by adding hardware to perform tasks that are not part of the basic system. Upgrading means changing to newer, usually more powerful or sophisticated versions. (Computer ads often make no distinction between "expansion" and "upgrading." Their main interest is simply to sell you more hardware or software.) Whether or not a computer can be expanded depends on its "architecture"-closed or open. Closed architecture means a computer has no expansion slots; open architecture means it does have expansion slots. Expansion slots are sockets on the motherboard into which you can plug expansion cards. Expansion cards-also known as expansion boards, adapter cards, interface cards, plug-in boards, controller cards, add-ins, or add-ons-are circuit boards that provide more memory or that control peripheral devices. Common expansion cards connect to the monitor graphics card), speakers and microphones (sound card), and network (network card), as we'll discuss. Most computers have four to eight expansion slots, some of which may already contain expansion cards included in your initial PC 39

11 purchase. Expansion cards are made to connect with different types of buses on the motherboard. The bus that connects the CPU within itself and to main memory is the system bus. The bus that connects the CPU with expansion slots on the motherboard and thus with peripheral devices is the expansion bus. We already alluded to the universal serial bus (USB), whose purpose, in fact, is to eliminate the need for expansion slots and expansion cards, since you can just connect USB devices in a daisy chain outside the system unit. Three expansion buses to be aware of are ISA, PCI, and AGP. (1) ISA bus -For ordinary low-speed uses: The ISA (industry standard architecture) bus is the most widely used expansion bus. It is also the oldest and, at 8 or 16 bits, the slowest at transmitting data, though it is still used for mouse s, modem cards, and low-speed network cards. (2) PCI bus - for higher speed uses: The PCI (peripheral component interconnect) bus is a higher-speed bus, and at 32 or 64 bits wide it is over four times faster than ISA buses. PCI is widely used to connect graphics cards, sound cards, modems, and high-speed network cards. (3) AGP bus- for even higher speed and 3D graphics. The AGP (Accelerated Graphics Port) bus transmits data at even higher speeds and was designed to support video and three-dimensional (3D) graphics. An AGP bus is twice as fast as a PCI bus. AGP ISA Among the types of expansion cards are graphics, sound, modem, and network interface cards. 40

12 (1) Graphics cards-for monitors: Graphics cards are included in all PCs. Also called a video card or video adapter, a graphics card converts signals from the computer into video signals that can be displayed as images on a monitor. A threedimensional AGP card is an example of a graphics card. The power of an AGP graphics card is often expressed in megabytes, as in 8, 16, or 32 MB. (2) Sound cards-for speakers and audio output: A sound card is used to transmit digital sounds through speakers, microphones, and headsets. Sound cards come installed on most new PCs. Cards such as PCI Wavetable sound cards are used to add music and sound effects to computer video games. Wavetable synthesis is a method of creating music based on a wave table, which is a collection of digitized sound samples taken from recordings of actual instruments. The sound samples are then stored on a sound card and are edited and mixed together to produce music. Wavetable synthesis produces higher quality audio output than other sound techniques. (3) Modem cards - for remote communication via phone lines: Very occasionally you may see a modem that is outside the computer. Most new PCs, however, come with internal modems-modems installed inside as circuit cards. The modem not only sends and receives digital data over telephone lines to and from other computers but can also transmit voice and fax signals. (4) Network interface cards - for remote communication via cable: A network interface card allows the transmission of data over a cable net, work, which connects various computers and other devices such as printers. 41