8. Storage Structure

-> MAIN MEMORY
main memory (main store, main storage, RAM, primary memory). The storage that is closely associated with the processor of a computer system and from which the program instruction and data can be directly retrieved and to which the resulting data is written prior to transfer to backing store or output device. In modern machines this is semiconductor memory but in earlier machines core stores and delay lines were used.

The majority of storage activity generated by a processor in the execution of a program is directed at the main memory. In a modern processor, however, there is usually a further small high-speed memory interposed between the processor and main memory that holds recently accessed main-memory data for rapid re-access. This small high-speed memory is known as a cache. The main memory is normally used in conjunction with a backing store with a much larger capacity.

-> MAGNETIC DISK
magnetic disk. Plastic disk coated with magnetic material and used for storing computer programs and data (information) as a series of magnetic spots. Most computers contain a hard disk unit for general storage. Hard magnetic disks can store larger amounts of data and come in cartridges that slot into a special drive unit. Computers also often have a disk drive for inserting portable, lower-capacity compact discs or floppy disks. Data is stored magnetically on both sides of a floppy disk, and is read by magnetic heads in the computer as the disk rotates.

It provide the bulk of secondary storage for modern computer systems. Conceptually, disks are relatively simple. Each disk platter has a flat circular shape, like a CD. Common platter diameters range from 1.8 to 5.25 inches. the two surfaces of a platter are covered with a magnetic material. We store information be recording it magnetically on the platters.

Moving-head disk mechanism


-> MAGNETIC TAPES
magnetic tape. It was used as an early secondary-storage medium. Although it is relatively permanent and can hold large quantities of data, its access time is slow in comparison to that of main memory. In addition, random access to magnetic tape is about a thousand times slower than random access to magnetic disk, so tapes are not very useful for secondary storage. Tapes are used mainly for backup, for storage of infrequently used information, and as a medium for transfering information from one system to another.

A tape is kept in a spool, and is wound or rewound past a read-write head. Moving to the correct spot on a tape can take minutes, but once positioned, tape drives can write data at speeds comparable to disk drives. Tape capacities vary greatly, depending on the particular kind of tape drive. Some tapes hold 2 to 3 times more data than does a large disk drive. Tapes and their drivers are usually categorized by width, including 4, 8, and 19 millimeters, 1/4 and 1/2 inch.