STORAGE DEVICES

Storage media are classified as primary storage or secondary storage on the basis of combinations*of cost, capacity, and ac­cess time. The cost of storage devices is expressed as the cost per bit of data stored. The most common units of cost are cents, millicents (0.001 cents) and microcents (0.000001 cents). The time required for the computer to locate and transfer data to and from a storage medium is called the access time for that medi­um. Capacities range from a few hundred bytes of primary stor­age for very small computers to many billions of bytes of archi­val storage for very large computer systems.

Memories may be classified as electronic or electromechani­cal. Electronic memories have no moving mechanical parts, and


91 Unit 7. Storage

data can be transferred into and out of them at very high speeds. Electromechanical memories depend upon moving mechanical parts for their operation, such as mechanisms for rotating mag­netic tapes and disks. Their data access time is longer than is that of electronic memories; however they cost less per bit stored and have larger capacities for data storage. For these reasons most computer systems use electronic memory for primary storage and electromechanical memory for secondary storage.

Primary storage has the least capacity and is the most expen­sive; however, it has the fastest access time. The principal pri­mary storage circuit elements are solid-state devices: magnetic cores and semiconductors. For many years magnetic cores were the principal elements used in digital computers for primary storage. The two principal types of semiconductors used for memory are bipolar and metal-oxide semiconductors (MOS). The former is faster, the latter is more commonly used at present. Because data can be accessed randomly, semiconduc­tor memories are referred to as random-access memory, or RAM.

There is a wide range of secondary storage devices. Typical hardware devices are rotating electromechanical devices. Mag­netic tapes, disks, and drums are the secondary storage hardware most often used in computer systems for sequential processing. Magnetic tape, which was invented by the Germans during World War II for sound recording, is the oldest secondary stor­age medium in common use. Data are recorded in the form of small magnetized "dots" that can be arranged to represent coded patterns of bits.

Tape devices range from large-capacity, high-data-rate units used with large data processing systems to cassettes and cartridges used with small systems. Magnetic disk storage, introduced in the early 1960s, has replaced magnetic tape as the main meth­od of secondary storage. As contrasted with magnetic tapes, magnetic discs can perform both sequential and random pro­cessing. They are classified as moving-head, fixed-head, or com­bination moving-head and fixed-head devices. Magnetic discs are the predominant secondary storage media. They include flexible, or floppy discs, called diskettes. The "floppies" were introduced by IBM in 1972 and are still a popular storage me­dium to meet the demands of the microcomputer market.



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