Storage devices

All common forms of backing storage record data on a moving magnetic surface.

Magnetic tape stores Information in a serial form rather like the familiar tape recorder. It has the advantage of being cheap and relatively fast with speeds of reading or writing ranging from 15.000 to 300.000 characters per second.

A reel of half-inch wide tape 2400ft long kept on a reel less than 12in in diameter can Store about 16 million characters of information. A disadvantage is that input Information must be sorted to the same sequence as that stored on the magnetic tape. Also all information stored on magnetic tape must be processed whether there are changes or not.

Magnetic drums have a surface coated with magnetic material. Information is stored in bands on the surface while the drum revolves at high speed. Each band has a read-write head and information can be read or written at speeds or up to 1.200.000 characters per second selected from any band at will. The limitation is that restricted amounts of inhumation can be stored and the cost per unit of information stored is quite high.

Magnetic disc storage is a compromise that offers the advantages of random access and rapid rates of reading and writing but requiring some time to locate the information area to be accessed. In recent years random access storage devices have been developed tremendously and can now compete in price with magnetic tape.

Interchangeable disc units which can store about 25 million characters, each accessible In less than 1 10th of a second, are now available while fixed disc units can now access information in less than 1 100th of a second. Once the information is accessed, transfer takes place at speeds of up to 300.000 characters per second.