Data and information must be stored until needed using a variety of storage methods. There are many types of storage media and devices.
Computer Storage Fundamentals
Data are processed and stored in a computer system through the presence or absence of electronic or magnetic signals in the computer’s circuitry or in the media it uses. This is called a “two-state” or binary representation of data, since the computer and the media can exhibit only two possible states or conditions. For example, transistors other semiconductor circuits are either in a conducting or nonconducting state. Media such as magnetic disks and tapes indicate these two states by having magnetized spots whose magnetic fields have one of two different directions, or polarities. This binary characteristic of computer circuitry and media is what makes the binary number system the basis for representing data in computers. Thus, for electronic circuits, the conducting (ON) state represents the number one, while the nonconducting (OFF) state represents the number zero. For magnetic media, the magnetic field of a magnetized sport in one direction represents a one, while magnetism in the other direction represents a zero.
The smallest element of data is called a bit, which can have a value of either zero or one. The capacity of memory chips is usually expressed in terms of bits. A byte is a basic grouping of bits that the computer operates as a single unit. Typically, it consists of eight bits and represents one character of data in most computer coding schemes. Thus, the capacity of a computer’s memory and secondary storage device is usually expressed in terms of bytes. Computer codes such as ASCII (American Standard Code for Information Interchange) use various arrangements of bits to form bytes that represent the numbers zero through nine, the letters of the alphabets, and many other characters.
Storage capacities are frequently measured in kilobytes (KB), megabytes (MB), gigabytes (GB), or terabytes (TB). Although kilo means 1,000 in the metric system, the computer industry uses K to represents 1,024 or (210) storage positions. Therefore, a capacity of 10 megabytes, for example, is really 10,485,760 storage positions, rather than 10 million positions. However, such differences are frequently disregarded in order to simplify descriptions of storage capacity. Thus, a megabyte is roughly 1 million bytes of storage, while a gigabyte is roughly 1 billion bytes and a terabyte represents about 1 trillion bytes.
Direct and Sequential Access
Primary storage media such as semiconductor memory chips are called direct access or random access memories (RAM). Magnetic disk devices are frequently called direct access storage devices (DASDs). On the other hand, media such as magnetic tapes are known as sequential access devices.
The term direct access and random access describe the same concept. They mean that an element of data or instructions (such as a byte or word) can be directly stored and retrieved by selecting and using any of the locations on the storage media. They also mean that each storage position (1) has a unique address and (2) can be individually accessed in approximately the same length of time without having to search through other storage positions. For example, each memory cell on a microelectronic semiconductor RAM chip can be individually sensed or changed in the same length of time. Also any data record stored on a magnetic or optical disk can be accessed directly in approximately the same time period.
Sequential access storage media such as magnetic tape do not have unique storage addresses that can be directly addressed. Instead, data must be stored and retrieved using a sequential or serial process. Data are recorded one after another in a predetermined sequence (such as in numeric order) on a storage medium. Locating an individual item of data requires searching much of the recorded data on the tape until the desired item is located.
Semiconductor Memory
The primary storage (main memory) of your computer consists of microelectronic semiconductor memory chips. Memory chips with capacities of 4 million bits (4 megabits) and 16 megabytes or more of memory chips can be added to your PC to increase its memory capacity. Specialized memory can help improve your computer’s performance. Examples include external cache memory of 256 or 512 kilobytes to help your microprocessor work faster, or a video graphics accelerator card with 2 megabytes or more of RAM for faster and clearer video performance. Removable credit-card-size and smaller “flash memory” RAM cards can also provide several megabytes of erasable direct access storage for PDAs or handheld PCs.
Some of the major attractions of semiconductor memory are its small size, great speed, and shock and temperature resistance. One major disadvantage of most semiconductor memory is its volatility. Uninterrupted electric power must be supplied or the contents of memory will be lost. Therefore, emergency transfer to other devices or standby electrical power (through battery packs or emergency generators) is required if data are to be saved. Another alternative is to permanently “burn in” the contents of semiconductor devices so that they cannot be erased by a loss of power.
Thus, there are two basic types of semiconductor memory: random access memory (RAM) and read only memory (ROM).
- RAM: random access memory. These memory chips are the most widely used primary storage medium. Each memory position can be both sensed (read) and changed (written), so it is also called read/write memory. This is a volatile memory.
- ROM: read only memory. Nonvolatile random access memory chips are used for permanent storage. ROM can be read but not erased or overwritten.
Frequently used control instructions in the control unit and programs in primary storage (such as parts of the operating system) can be permanently burned in to the storage cells during manufacture. This is sometimes called firmware. Variations include PROM (programmable read only memory) and EPROM (erasable programmable read only memory) that can be permanently or temporarily programmed after manufacture.
Magnetic Disk Storage
Magnetic disks are the most common form of secondary storage for your computer system. That’s because they provide fast access and high storage capacities at a reasonable cost. Magnetic disk drives contain metal disks that are coated on both sides with an iron oxide recording material. Several disks are mounted together on a vertical shaft, which typically rotates the disks at speeds of 3,600 to 7,600 revolutions per minute (rpm). Electromagnetic read/write heads are positioned by access arms between the slightly separated disks to read and write data on concentric, circular tracks. Data are recorded on tracks in the form of tiny magnetized spots to form the binary digits of common computer codes. Thousands of bytes can be recorded on each tracks, and there are several hundred data tracks on each disk surface, thus providing you with billions of storage positions for your software and data.
Types of Magnetic Disks
There are several types of magnetic disk arrangements, including removable disk cartridges as well as fixed disk units. Removable disk devices are popular because they are transportable and can be used to store backup copies of your data offline for convenience and security.
- Floppy disks or magnetic diskettes, consist of polyester film disks covered with an iron oxide compound. A single disk is mounted and rotates freely inside a protective flexible or hard plastic jacket, which has access openings to accommodate the read/write head of a disk drive unit. The 31/2 inch floppy disk, with capacities of 1.44 megabytes, is the most widely used version, with a newer LS-120 technology offering 120 megabytes of storage.
- Hard disk drives combine magnetic disks, access arms, and read/write heads into a sealed module. This allows higher speeds, greater data-recording densities, and closer tolerances within a sealed, more stable environment. Fixed or removable disk cartridge versions are available. Capacities of hard drives range from several hundred megabytes to gigabytes of storage.
- RAID. Disk arrays of interconnected microcomputer hard disk drives have replaced large-capacity mainframe disk drives to provide many gigabytes of online storage. Known as RAID (redundant arrays of independent disks), they combine from 6 to more than 100 small hard disk drives and their control microprocessors into a single unit. RAID units provide large capacities with high access speeds since data are accessed in parallel over multiple paths from many disks. RAID units also provide a fault tolerant capability, since their redundant design offers multiple copies of data on several disks. If one disk fails, data can be recovered from backup copies automatically stored on other disks.
Magnetic Tape Storage
Magnetic tape is still being used as a secondary storage medium in business applications. They read/write heads of magnetic tape drives record data in the form of magnetized spots on the iron oxide coating of the plastic tape. Magnetic tape devices include tape reels and cartridges in mainframes and midrange systems, and small cassettes or cartridges for PCs. Magnetic tape cartridges have replaced tape reels in many applications, and can hold over 200 megabytes.
One growing business application of magnetic tape involves the use of 36-track magnetic tape cartridges in robotic automated drive assemblies that can hold hundreds of cartridges. These devices serve as slower, but lower cost, storage to supplement magnetic disks to meet massive data warehouse and other business storage requirements. Other major applications for magnetic tape includes long-term archival storage and backup storage for PCs and other systems.
Optical Disk Storage
Optical disks are a fast-growing storage medium. The version for use with micro computers is called CD-ROM (compact disk- read only memory). CD-ROM technology use 12-centimeter (4.7 inch) compact disks (CDs) similar to those used in stereo music systems. Each disk can store more than 600 megabytes. That’s the equivalent of over 400 1.44 megabyte floppy disks or more than 300,000 double-spaced pages of text. A laser records data by burning permanent microscopic pits in a spiral track on a master disk from which compact disks can be mass produced. Then CD-ROM disk drives use a laser device to read the binary codes formed by those pits.
CD-R (compact disk — record able) is another optical disk technology. It enables computers with CD-R disk drive units to record their own data once on a CD, then be able to read the data indefinitely. The major limitation of CD-ROM and CD-R disks is that recorded data cannot be erased. However, CD-RW
(CD-rewritable) optical disk systems have now become available which record and erase data by using a laser to heat a microscopic point on the disk’s surface. In CD-RW versions using magneto optical technology, a magnetic coil changes the spot’s reflective properties from one direction to another, thus recording a binary one or zero. A laser device can then read the binary codes on the disk by sensing the direction of reflected light.
Optical disk capacities and capabilities have increased dramatically with the emergence of an optical disk technology called DVD (digital video disk or digital versatile disk), which can hold from 4.0 to 8.5 gigabytes of multimedia data on each side of a compact disk. The large capacities and high quality images and sound of DVD technology are expected to eventually replace CD-ROM and CD-RW technologies for data storage, and promise to accelerate the sue of DVD drives for multimedia products that can be used in both computers and home entertainment systems.
Blu-Ray and HD-DVD are two new optical storage technologies that fight as the successor of DVD. Both HD-DVD and Blu-ray are high-definition optical disc formats, incompatible with each other. They were meant to replace the existing DVD format, which was and still is used by a large user community. HD-DVDs and Blu-ray discs are similar in terms of the higher storage capacities and a superior picture quality offered by them. Their storage capacities are higher than those of CDs and DVDs. However, a single layer HD-DVD can store 15 GB data whereas a single-layer Blu-ray disc stores 25 GB data.