RAM is a very fast and efficient way of storing data. However, it has two disadvantages which make it impossible to use it as a computer's only storage space. The first, as mentioned above, is that it loses all of its data when the computer is turned off. This might be okay if you were to leave your computer on all day every day. But what about unexpected shutdowns such as a power outage, or someone tripping over the power cord? All of the data on the computer would be lost forever. The other disadvantage is that RAM is fairly expensive. The price fluctuates, but is normally at an average of about $30 per MB. So what if you have a lot of programs or a really big data file or both? You would be spending a fortune! For these two reasons, it is necessary for auxiliary storage devices. Most auxiliary storage devices are much slower than RAM, but are far cheaper by the MB, and retain their data even after the computer is turned off. Many types of auxiliary storage devices are described below.
The read/write head contains a coil wound around an iron core. An individual bit is converted into two pulses. The pulses go through the coil, and this magnetizes the surface of the disk. Each of the pulses is stored as a band of magnetism. If the drive is to store a bit representing a 1, the bands of magnetism are reversed. If the drive is to store a bit representing a 0, the bands of magnetism are in the same direction. When a second bit is stored, the first band is always the opposite polarity of the previous band to indicate it is beginning a new bit. To read the bits, the disk spins past the head, which creates an electric signal in the coil that goes to the computer.
This diagram shows how data is stored with magnetic fields. The arrows represent the direction of magnetism.
Before any magnetic drive can become usable, it must be formatted. This is done by writing a pattern of 1s and 0s which divide the disk radially into sectors and concentric circles. The read/write head reads these magnetic signposts and determines where it is on the disk. Two or more sectors on a single track constitute a cluster or block. A cluster is the smallest unit of information in DOS. The number of bytes in a cluster varies according to the version of DOS used to format the disk, and it also depends on the disk's size. A cluster may hold 256 bytes, and this means that even if one were to store only 5 bytes, the whole cluster would be used up. On the disk's sector 0, the drive creates a special file called FAT (File Allocation Table). The FAT contains the structure of DOS's directory, and has information on which clusters store which files.
Disks and disk drives store information when the computer is off. A disk drive can write information to a disk and read information from a disk. As with all auxiliary storage, nothing on the disk changes when the power to the computer is turned off. They are by far the most common type of auxiliary storage. It is almost unheard of to have a computer without both types: floppy drive and hard drive. A computer without a floppy drive can't ever boot in order to copy the operating system to the hard drive. A computer without a hard drive can't boot without a boot floppy every time, and can't have any software installed. . .ever. Several types of disk drives have been developed to accommodate different storage needs.
Floppy disk drives are the most common type of disk drives, and they are used to store information on floppy diskettes. They can store between 360 kilobytes and 1440 kilobytes per disk, depending on the type of disk drive and diskette. An advantage of the floppy disk drive is that you can take information from one computer and put it on another. But the amount of information stored on one disk is much less than other permanent storage devices. These disk drives also read and write slowly, and they are more easily damaged.
Hard Disks hold much more information than floppy disks, from 10 million bytes to nine billion bytes. As with RAM, a million bytes is called a megabyte, and a billion bytes is called a gigabyte. The disk cannot be removed from the drive, and for this reason, it is referred to as a fixed disk or permanent storage. Usually, a hard disk is located inside the system unit. It contains one or more disks and read/write heads which read the information on a disk and write information to the disk. A hard disk can store more information than a floppy disk, and it can also read and write information much faster than a floppy disk drive.
The hard drive has a sealed metal casing which prevents any dust from getting in and causing the hard drive to crash. A printed circuit board, called a logic board, is located under the hard drive. The logic board receives information from the drive's controller, and it creates these instructions into voltage fluctuations which move the head across the drive's surface. The board also makes sure that the drive is rotating at a constant speed. EIDE hard drives have the disk controller integrated with the logic board. The spindle, which is connected to an electric motor, rotates at several thousand revolutions per minute. The spindle is connected to many magnetically coated platters. A head actuator moves the read/write heads across the platters to the exact locations on the circular tracks. Then, the heads write the data to the platters or read the data already on them. When the software or operating system you are using tells the drive to read or write data, the hard disk controller moves the read/write heads to a file on the drive called FAT (File Allocation Table). This file keeps track of all the clusters each file is located in. The operating system reads the FAT to determine which clusters are free or which cluster a file exists. The operating system stores part of a file in the first cluster it finds free, and a single file can be strewn among several clusters. Whenever something is written to the drive, the operating system orders the read/write heads to update the FAT.
Internal Hard Drive
A compact disk player is a digital device. A compact disk contains a spiral track of binary codes in the form of small pits. The disk rotates at 500 revolutions per minute at the the center, where the track begins. At the edge, it rotates at 200 revolutions per minute. The linear speed of the disk is constant as it passes over the optical read-out system that decodes the tracks. Mirrors and lenses fire a beam of laser light at the spiral track on the bottom of the CD. When the disk rotates, the laser beam traverses across the disk from the center to the edge. There are pits and lands on the surface of the CD, and a laser is used to decode them. When the laser hits a land area, it is reflected back, but when it hits a pit, the light is scattered. The light sensitive photodiode detects the reflected light when the laser hits a land, but registers nothing when the laser hits a pit. The on/off signals are converted into electric signals, and can be converted into binary code.
A writable CD-ROM can write to CD's only once. The changes in the CD are permanent, making the recordable compact disk a write-once read many (WORM) medium.
A laser aims a low energy beam at the compact disk. The disk mainly consists of a clear polycarbonate plastic. On top of the polycarbonate plastic is a layer of dyed color material, a thin layer of gold to reflect the laser, a protective layer of lacquer, and an a layer of scratch-resistant polymer material.
The laser write head follows a spiral groove cut into the plastic layer. The groove is called an atip, which means absolute timing in pregroove. The path is wavy, and the frequency of the waves varies from the start to finish. When the laser beam shines on the path, it reflects off the wave pattern. By reading the frequency of waves, the CD can calculate where the write head is location relative to the disc.
The dye layer absorbs light at a specific frequency. When the dye absorbs the energy from the laser beam, it can be one of three things: the dye can become bleached, it can cause the polycarbonate layer to become distorted, or it can create a bubble. The result is a distortion called a stripe along the spiral track. If the laser is off, no mark is created. The lengths of the stripes and the spaces between them vary, and the CD drive uses the varying lengths to write information in a special code which compresses the data and checks for errors.
The CD-recordable drive or a read-only CD drive focuses a low power laser beam at the CD to read the data. When the laser hits a stripe, it is scattered. When it hits a place where a mark has not been made, the laser the gold layer reflects the laser beam back. Every time the beam is reflected back, the head generates a pulse of electricity. The pattern of pulses is decompressed into data, error checked, and is passed along to the PC in the form of 1s and 0s.
The software which is used to record data to the CD puts the data in a specific format which automatically corrects for errors and creates a table of contents. A common CD format is ISO 9096.
Magneto-Optical disk drives use lasers to pack data very tightly so that you can save hundreds of megabytes of information on a portable disk.
The electromagnetic read/write head of the drive generates a relatively large magnetic field on the magneto-optical disk. But the crystalline metal alloy covering the disk is stable enough not to be affected by the magnetism alone. A thin laser beam is then focused on the surface of the disk, and the energy heats up a tiny spot on the disk, raising it to a critical temperature called the Curie point. The heat loosens the metallic crystals in the alloy, just enough so they can be moved by the electromagnet. The electromagnet aligns the crystals in one of two directions, each one representing a one or a zero.
When reading data from the magneto-optical disk, a weak laser beam is focused along the tracks of data. The crystals polarize the light from the laser. The alignment of crystals representing 0 bits polarize the light in one direction, and the alignment of crystals representing 1 bit polarize the light in another direction. The polarized light is reflected to a photo diode, which senses in which direction the light is polarized, and then translates that information into 1s and 0s.
It becomes critical to backup hard drives because disk sizes have reached up to 4 gigabytes and more. It is virtually impossible to copy all of your files on such large drives on to floppy disks. Prices under $500 dollars make tape backup drives affordable and a good idea. The ability for them to copy a gigabyte of information makes them simple for using on the biggest hard drives. The two most popular types of tape backups are quarter-inch cartridge and digital audio tape.
When a backup command is issued, the software for the quarter inch cartridge reads your hard disk's file allocation table (FAT) to locate the files you've ordered it to backup. The software writes the directory information to a 32K buffer in RAM, and then copies the files to the same buffer. Each file is preceded with information that identifies the file and its location on the hard disk.
Usually a tape drive's controller contains chips that handle error correction, and the backup software moves the full buffer from RAM to the controller's own buffer where the chips attach error correction codes. Once the data is transferred to the controller, the RAM buffer is free to receive the next block of information from the disk.
The tape drive's controller sends signals to the tape drive mechanism to start the tape rolling. QIC drives need the tape to be kept tight. This is done by using an elastic belt which is wrapped around the reels of the tape and which stretches slightly as it grips the tape. The belt ensures that the pulling force of the take-up real equals the resistance of the supply reel. Thus, the tape presses against the drive head at a constant pressure, minimizing read/write errors.
Many tape drive heads have a three part read-while-write head. The write head is located in between two read heads. The controller sends a section of data to the drive's write head, which transfers the data to the magnetic coating on the tape. Depending on which way the tape is moving, one of the read heads reads the data that's just been written by the write head. The read head makes sure that data on the tape matches what the write head sent to the tape. If the data is correct, the controller empties its buffer and the drive moves onto the next section of disk data. If the data isn't correct, the data is rewritten on the next section of tape.
A QIC tape usually contains 20 to 32 parallel tracks, and the end of the tape is marked with punched holes. When the drive head approaches the end of a spool, the holes signal the drive to move up or down to the next track and continue recording. Each track is partitioned into blocks of 512 or 1,024 bytes, and segments contain around 32 blocks. Eight blocks in a segment contain error correction codes, and also at the end of each block, the drive computes a cyclic redundancy check (CRC) for further error correction. Backup software usually reserve space for a directory of backed-up files at the beginning of track 0 or in a separate directory track.
The zip drive is a very popular storage device. It reads and writes to 100 MB cartridges. A major advantage of the zip drive is that it is very small and easy to plug in/unplug. So you can easily bring the whole drive where it is needed--not just the cartridge.
The SyQuest Drive is fairly uncommon for normal use, but has a strong demand in the graphics market. They use proprietary-format drives.
These amazing storage devices still won't be commercially available until later this year or early next year, but when the are released, they are expected to replace the CD-ROM. They use similar technology to the CD-ROM, but with a few changes which make it much faster and have many times more storage capacity. The first added upgrade is the laser. Current CD-ROMs use a low frequency red laser. The DVD-ROM will be using the recently invented blue laser. Being of a higher energy level, the blue laser has a lower wavelength and can thus read and write data closer together. Secondly, the DVD-ROM will have the laser pass through a specially shaped lens which will divide it into 2 separate focus points. This means that the DVDs will be dual layered. It's like having 2 CDs jammed together into 1. And to top all that off, the DVDs are dual sided. So that again doubles the capacity of the disc. This means that DVDs will be able to hold 4 times as much as a CD plus the added space because of the blue laser!
The Pinnacle Apex drive is a very promising storage device. It can supposedly hold 4.6 GB. However, it was supposed to come out almost a year ago and still hasn't.
These drives are a dual purpose storage device. They can read CDs as well as write 650 MB cartridges.
This is the next generation of the floppy drive. It reads and writes 120 MB floppy sized disks, as well as reading and writing normal 1.44 MB floppy disks.