Data and programs are stored in main memory, as random access memory (RAM) , before and after processing by the central processing unit (CPU) . However, RAM is volatile —its contents disappear when a computer's power is turned off. So what does one do with data that will be reused, stored on a long-term basis, or is simply too large to fit into the main memory of a computer? Mechanical storage devices, called secondary storage or external storage, are used to store data externally. Secondary storage is non-volatile—that is, data and programs are permanent.
Types of Storage Devices
There are many secondary storage devices, including magnetic drums, magnetic tapes, magnetic disks, and optical disks. These devices vary with respect to their versatility, durability, capacity, and speed.
These are very early high-speed, direct access storage devices used in the 1950s and 1960s. The magnetic drum is a metal cylinder coated with a sensitive magnetic material. The cylinder has tracks around its circumference. Each track has its own read/write head, and data are stored as magnetic spots on the tracks. Like the magnetic disk, which was introduced later, the read/write head can move quickly to any track, providing direct access to the data stored on the drum.
This is one of the oldest secondary storage devices. It was first used for storing data in the early 1950s. At that time, the tape was made of a flexible metal and was stored on reels. The metal tape was plated with a thin film of iron, which allowed data to be stored as a series of small, magnetized spots. Although the tape provided a compact form of storage, it was extremely heavy and not universally accepted. It was not until a very thin, flexible material called mylar was developed that tape processing gained wide acceptance. This plastic mylar was coated with an iron oxide that could be magnetized to store data. In the 1950s and 1960s, magnetic tape was the primary means for storing large amounts of data.
Data are stored on magnetic tape in columns. Each byte of data (eight bits ) utilizes one column across the width of the tape. Data are stored on the magnetic tape at different densities . Low density is 1,600 bytes per inch (bpi). Densities of 6,250 bpi and greater were common in the 1980s.
The old reel-to-reel magnetic tapes are being replaced by tape cartridges, which are used to back up or archive data. A tape backup is a copy of the data used to restore lost data. If one loses the data on a hard disk, he can copy the data from the tape backup. Tape cartridges used on microcomputers are similar to audiocassettes and can hold up to 35 gigabytes (GB) of data. There are several different types of tape cartridges, including quarter-inch cartridge (QIC), digital audio tape (DAT), digital linear tape (DLT), Travan, and advanced digital recording (ADR).
Magnetic tapes are an inexpensive and relatively stable way to store large volumes of data. Tapes can be used repeatedly, although time and environmental factors such as heat and humidity do affect them. The principle disadvantage of magnetic tape is that it stores data sequentially—that is, one record is stored right after another. Retrieving data from the tape is slow since the tape must be read from the beginning up to the location of the desired record. Thus, magnetic tapes are not a good choice when one needs to find information rapidly.
Currently, the most widely used secondary storage device is the magnetic disk. There are two kinds of magnetic disks: hard disks and floppy disks. Hard disks are thin, metallic platters developed in the 1950s. Each hard disk contains one or, more commonly, a series of platters that rotate on a shaft at a high rate of speed. The platters have a top and bottom surface where data can be recorded as magnetic spots. The platters have concentric circles called tracks where the data are actually stored. Access arms containing read/write heads move across the spinning platters to read or write data on the tracks. There is a read/write head for each platter surface. These read/write heads float on a cushion of air and do not actually touch the surface of the disk. When reading data from the disk, the read head senses the magnetic spots on the surface and transfers the data to main memory. When writing, the data are transferred from main memory and are stored as magnetic spots on the recording surface.
The speed of access to data on a disk is based on both the rotational speed of the disk and the speed of the access arm. The read/write heads must position themselves over the platter at the proper address and the disk pack must rotate until the proper data are located. The time it takes the read/write heads to position themselves to the correct address is referred to as access motion time. Rotational delay time is the time needed for the disk to rotate the data under the read/write head.
The read/write heads move horizontally (from left to right) to any of 200 positions, called cylinders, on the platter surface. The cylinder represents the circular tracks on the same vertical line within the disk pack. To find data on the disk, an address is used. This address consists of a cylinder number, the recording surface number, and the data record number. The computer's operating system figures out how to fit data into the fixed spaces on the disk.
The microcomputer utilizes a hard disk, but it also uses floppy disks, also called floppy diskettes. Floppy disks are the second kind of magnetic disk and are removable. The two common standard sizes of floppy disks are 8.9 centimeters (3.5 inches) and 13.3 centimeters (5.25 inches). It is difficult today to find a microcomputer that uses the 5.25-inch floppy disk. Floppy disks are made up of one platter constructed from polyester film rather than metal. This polyester film is covered with a magnetic coating. Data are stored on the floppy disk in much the same manner as on a hard disk—in the form of magnetic spots on tracks. However, floppy disks use sectors to store data rather than cylinders. In this method of storing data, the disk is divided into pie-shaped pieces called sectors. Each sector is assigned a unique address to locate data. Sectors are created on a floppy disk by formatting it. Floppy disks have a much lower data storage capacity (usually 1.44MB for a 3.5-inch floppy disk) and a much slower data access rate than hard disks.
As software packages such as Microsoft Office became popular, the need for larger data capacity for items such as text and graphics in a portable form became necessary. The zip drive and zip disk were introduced in the 1990s as a relatively inexpensive large-storage-capacity floppy disk. The zip disk is 8.9 centimeters (3.5 inches), removable, and provides about 100 megabytes (MB) of data storage. In late 1998, a 250 MB version of the zip disk was introduced. While this zip disk has double the storage capacity of its predecessor, it is still only 8.9 centimeters (3.5 inches) in size!
The main advantages of using a magnetic disk as a secondary storage device are its speed and direct access capability. Data can be easily and rapidly read, written, or accessed. Floppy disks provide the added advantage of portability. Disadvantages of using a magnetic disk as a secondary storage device include cost, environmental factors, user misuse and abuse, head crashes, and update problems.
The magnetic disk is more expensive than magnetic tape (DAT). The February 1998 CompUSA catalog reported that DAT cost $49.95 and stored 10,000 MB. This is a $.005 cost per megabyte. An 18 GB hard drive cost $230.00 or $.08 per megabyte. A 3.5 inch diskette cost $.50 and stored 1.4MB. This is a cost of $.35 per megabyte. A Zip Plus Drive cost $100.00 and stored 100MG to 250MG. This is a cost of $1.00 per megabyte. It was reported by Dataquest, a research firm, that the average cost of data storage across various media was 15 cents per megabyte in the year 2000. This cost decreased to 3 cents per megabyte in 2002, as noted by Effy Oz in Management Information Systems.
Magnetic disks, both hard and floppy, are also susceptible to environmental factors such as dust, dirt, and smoke. Any of these factors will cause a hard disk to fail. Because of this, hard disks are sealed. Floppy disks are also vulnerable to environmental factors. Also, because of their portability, the floppy disk is vulnerable to misuse or abuse by users.
Head crashes can occur with any magnetic disk technology. This is when the read/write head touches the surface of the disk platter, destroying it and all of the stored data. Head crashes are normally caused by misalignment of the platter and the read/write head. A head crash renders a magnetic disk unusable. Another disadvantage of any magnetic disk is that when updating data, the old data are written over, destroying them instantly and permanently. Unless proper precautions are taken, data may be written over by mistake.
These are the newest secondary storage devices. Originally optical disks were called optical disk-read-only memory (OD-ROM) and are now called compact disk-read only memory (CD-ROM). Data are not recorded on optical disks magnetically, but with a laser device that burns microscopic holes on the surface of the disk. Binary information (0s and 1s) is encoded by the length of these bumps and the space between them. Optical disks can store much more data than floppy disks. Data can be stored in the form of text as well as pictures, sound, and full-motion video. The disks are not as sensitive to dust, dirt, and smoke as magnetic disks are, and they are portable.
CD-ROM is read-only storage. No new data can be written to it. Once recorded, the CD can be read an indefinite number of times. This is commonly referred to as write once/read many (WORM). CD-ROM has been used for storage of large financial, legal, educational, or demographic databases, and by the music industry. Encyclopedias and multimedia applications are also stored using CD-ROM technology.
Individuals and organizations can now record their own CD-ROMs using compact disk-recordable (CD-R) technology. CD-ReWritable (CDRW) is a newer technology that has been developed to allow users to create rewritable optical disks. Currently CD-RW has slower access speeds and is more expensive than magnetic storage devices.
Another optical storage medium is the digital video disk or digital versatile disk (DVD). This optical disk is the same size as a CD-ROM, but has much higher storage capacity. DVDs can store large amounts of data, video, graphics, digitized text, and audio, and are portable. It is likely that DVD will eventually replace the CD-ROM.
see also Codes; Computer System Interfaces; Memory.
Charles R. Woratschek and Terri L. Lenox
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