File Storage

The work done by a computer would be useless if it were to disappear at the end of the work session. Yet that is exactly what would happen in most computers if the work was left in RAM, the machine's temporary memory.

RAM is designed to accommodate the task at hand, but is operational only so long as the power is switched on. The instant the power goes off, whether by accident or design, RAM is once more an empty slate. To use and re-use programs and files from session to session, the user must 'save' a newly created or freshly updated document to storage, an electronic filing system that will keep its contents in order, even with the power off.

Modern computers make use of two storage technologies - magnetic and optical. Data storage methods relying on magnetism have predominated through most of electronic computing history with optical technology only coming on the scene in the 1970s.

All magnetic storage (see disk) operates on the principle that atoms of certain elements - chiefly iron, nickel, chromium and cobalt - act as natural magnets. When a disk, drum, or tape is coated with a powdered alloy of one of these elements, usually iron dioxide, the magnetized atoms behave like microscopic bar magnets, each generating its own field

Tape was an early magnetic medium which continues to be used widely for backup and for archival storage of data. Borrowed from the music recording industry, tape a great leap forward. One drawback, however, is that taped information is stored - and recovered- in serial fashion, which means that finding a given piece of data can be a frustratingly slow process.

In 1971, IBM introduced the considerably smaller, flexible magnetic disk, the now commonplace floppy.

Since their debuts, both hard and floppy disks have shrunk steadily in size and just as steadily grown in capacity. The smallest floppy disk, at 3.5 inches, can hold as much as 1.44 megabytes of data, while a hard disk of the same size can store several times that amount of information.

Diskette Drives

The diskette drive is one of the most essential components of a computer. Computers have several drives, each with a different construction and purpose.

A hard drive is the PC's primary storage medium and is most often known as the C: drive, and a diskette drive, often called the floppy disk drive, which uses a removable hard-shelled diskette and is often known as the A: drive.

What is a Diskette? A diskette is a small, plastic disk coated with a substance (see above) that can be magnetised to store computer data.

The best thing about diskettes is that they can be used over and over again. Information can be written to them and then deleted to make room for other data. Diskettes do have a built-in safety or write-protection device to prevent data from being written over existing data.

On 5.25 diskettes, this device is a notch on the side of the diskette, the notch must be covered with a label. Normally new diskettes are packaged with a sheet of labels.

On a 3.5-inch diskette, the write-protection is the small square opening on the corner of the diskette. When the square is exposed the, the diskette is write-protected, and no new data can be written to the diskette.

It is easy to tell the type of diskette by looking at the label. Manufacturers use either 2D or DD to identify double-density (720KB), high-density diskettes (1.4 MB) are marked with HD. Most diskettes today are HD.

How the Diskette Drive Works: When a diskette is inserted into the drive, it presses against a system of levers. The first lever opens a sliding shutter that acts as a cover for the Mylar diskette inside,which records data.

A series of levers, gears and tiny springs inside the drive move two read-write heads until they touch the Mylar surface, one on each side of the diskette. These heads are tiny electromagnets, which use magnetic pulses to change the orientation of metallic particles embedded in the diskette coating.

A stepper motor then turns the shaft that has a spiral groove cut into it. And arm attached to the read-write heads rests inside the shaft's groove. As the shaft turns, the arm moves back and forth, positioning the read-write heads over the diskette. The stepper motor turns in either direction, according to signals from the circuit board

The diskette drive has a circuit board that receives data and instructions for writing the data to the diskette from the controller's board. This circuit board turns the instructions into signals that control the movement of the diskette and read-write heads.

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Hard Drives

A hard drive has five main components: Head accuator, circuit board, spindle, read/ write heads, and platters.

Head actuator - Controls the read/write heads.

Circuit board (logic board) - Receives commands from the hard drive controlelr and translates them in order to move the head actuator, which moves the read/write head across the platters.

Spindle - Spins the platters and is run by a motor.

Read/Write Heads - Move across the platters to write data to and read from the platters.

Platters - The aluminum alloy disks upon which data is stored in a hard drive.

Platters, also called disks, are the part of the hard drive where data is stored. They are rigid magnetic disks made of an aluminum alloy covered with a thin layer of iron oxide. When a computer is on, these platters constantly spin on a spindle, which is driven by a motor. Each side of every platter has a read/write head, which writes data to and reads data from each platter. They are attached to the head actuator, which precisely moves the read-write heads across the platters. An integrated circuit board, also called a logic board, allows the computer to move the read/ write heads across the platter surfaces.

Hard drives operate much like a record player spinning a vinyl record. Just like a record player's arm that holds the needle, the hard drive has a movable arm that carries the read/write heads across the platter's magnetic surface. Instead of grooves etched into the record's vinyl surface, magnetic sectors formed by formatting the disk are placed in concentric patterns on the platter.

Computers record and "play back" data on hard drives much like a record player. Data is translated into a series of bits. The hard drive receives these bits and uses the read/ write heads to magnetically store or write the bits on one of the platters. This process involves converting the data to positive and negative magnetic fields on the platter's surface. The computer keeps track of where these bits are stored on the various platters so when you want to retrieve them, it can direct the read/write heads to the precise locations on the platters where the data is stored. The heads recognize the data stored as magnetic fields and send it to the computer's monitor within a few millionths of a second.

Although the storage area of a platter's surface is smooth and unbroken, formatting the hard drive (preparing it so it can store data) gives it textured features. The surface's magnetic coating is divided into a series of cylinders containing tracks; each track is divided into sectors. These hard drive divisions are like a map that lets a computer determine exactly where it stores data.

A cylinder is a set of tracks on a platter that can be accessed without read/ write head movement. Formatting a hard drive lays down an exact pattern of tracks that are the same distance from the spindle around which the platters rotate. Each group of tracks forms the shape of a cylinder.

A track is the part of the platter's surface that passes under one read/ write head while it is stationary. Tracks form concentric circles beginning with track 0 at the outer edge of the disk. How close together these tracks are placed on a platter determines the amount of data it can hold.

Each track is divided into sectors, which are identified by the side of the disk on which they are located, their track number, and the sector number within the track.

Computers use a file system to organize the files on a disk. Each operating system- Macintosh, DOS, Windows, Windows 95, OS/ 2, and Windows NT-can only work with one or two filing systems. This is one reason why data saved to a disk formatted for one system cannot be read by another system. The File Allocation Table (FAT) is the file system developed by Microsoft Corp. for DOS and Windows. The common file systems are FAT (used by DOS, Windows 3.1, and early versions of Win95), FAT32 (used by later versions of Win95 and IBM's OS/2) New Technology File System (NTFS, used only on Windows NT), and High Performance File System (HPFS, also used alternately by OS/2 and Windows NT). The FAT, FAT32, and NTFS file systems rely on clusters as an integral part of data storage. A cluster is the smallest allocation unit in these three file systems. The table is much like a road map that represents whether each cluster of a disk is free or allocated to a file.

Read/Write Heads

The read/write heads themselves vary somewhat in design, depending on the type of disk drive. Each consists of three separate heads. A single read/write head for recording and retrieving data is sandwiched between two erase heads, whose sole purpose is to remove any stray magnetic signals from the area on either side of a data track. The resulting demagnetized zones not only isolate the data track, but also help to compensate for any variations in alignment between different disk drives, ensuring that data recorded by one drive will be readable by another.

The read/write heads of alI floppy disk drives must actually touch the surface of a disk in order to record or retrieve data. To do so, the heads use a small-access window cut into the protective jacket enclosing the disk. This window, obvious on a 5.25-inch floppy, is hidden beneath a spring-loaded door on a 3.5-inch disk and is only exposed when the disk is inserted into a disk drive. A smaller, round hole, called the index hole, punched in the jacket of a 5.25-inch floppy and in the disk itself, provides a reference point for the reading and writing of data; magnetic signals recorded in a special location on a 3.5-inch disk serve the same purpose in lieu of an index hole.

Both kinds of floppies also incorporate what is known as a write-protect feature to prevent data from being written over or accidentally erased. On a 5.25-inch disk this feature takes the form of a notch cut into one side of the disk's protective jacket made of the plastic material PVC; covering the notch with a small piece of tape allows the disk to be read but not written to. On a 3.5-inch floppy, a tab in one corner of the disk can be slid open to prevent the disk from being written to.

To protect sensitive data from irrevocable loss, a special lining inside the jacket sweeps the surface of the disk as it spins, collecting dust and bits of magnetic material as they are shed by the disk. To a disk drive, a particle of dust the size of a bacterium looms as large as a boulder and can all too easily disrupt the reading and writing of data. A human hair, measuring just 40 microns thick, has the same potential for disaster as a log across a superhighway and can wreak similarly devastating havoc.

Inevitably, some of the microscopic litter escapes the lining and gradually accumulates in the recording gap that forms part of a read/write head. In time enough debris can build up to clog the gap, leading to read/write errors and eventually rendering the head useless. A special head-cleaning disk may remedy the problem and reduce the likelihood of disk or head failure.

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