Magnetic Tapes

Magnetic tape was used as an early secondary-storage media. Although it is relatively permanent, and can hold large numbers of data, magnetic tape is quite slow in comparison to the access time of main memory. Even more important, magnetic tape is limited to sequential access. Thus, it is unsuitable for providing the random access needed for most secondary-storage requirements. Tapes are used mainly for backup, for storage of infrequently used information, and as a medium for transferring 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 rather than milliseconds; once positioned, however, tape drives can write data at densities and speeds approaching those of disk drives. Capacities vary depending on the length and width of the tape, and on the density at which the head can read and write. A tape drive is usually named by its width. Thus, there are 8-millimeter, ¼-inch, and ½-inch (also known as 9-track) tape drives. The 8-millimeter tape drives have the highest density, due to the technology they use; they currently store 5 gigabytes of data (a gigabyte is one billion bytes) on a 350-foot tape.

Last Updated Jul.28/99