Drum memory

Lore & Background

Drum memory contained a large metal cylinder coated on the outside surface with a ferromagnetic recording material. In most designs, one or more rows of fixed read-write heads ran along the long axis of the drum, one for each track. The drum's controller selected the proper head and waited for the data to appear under it as the drum turned (rotational latency). Some drum units, such as the English Electric DEUCE drum and the UNIVAC FASTRAND, had multiple heads moving a short distance on the drum. In November 1953 Glenn E. Hagen of Logistics Research, Inc. published a paper disclosing 'air floating' of magnetic heads in an experimental sheet metal drum, and a US patent filed in January 1954 by Heard Baumeister of IBM disclosed a 'spring loaded and air supported shoe for poising a magnetic head above a rapidly rotating magnetic drum.' Flying heads became standard in drums and hard disk drives.

Magnetic drum units used as primary memory were addressed by word; those used as secondary storage were addressed by block. Several modes of block addressing were possible, including by track, by track and sector, by track and record number, or by key content. Some devices were divided into logical cylinders. The performance of a drum with one head per track is comparable to that of a disk with one head per track and is determined almost entirely by rotational latency. In the era when drums were used as main working memory, programmers often did optimum programming—positioning instructions on the drum to reduce the time needed for the next instruction to rotate into place under the head. This method of timing-compensation, called the 'skip factor' or 'interleaving', was used for many years in storage memory controllers.

Reader's Guide

Drum memory was displaced as primary computer memory by magnetic core memory, which offered a better balance of size, speed, cost, reliability and potential for further improvements. Drums were then replaced by hard disk drives for secondary storage, which were both less expensive and offered denser storage. The manufacturing of drums ceased in the 1970s. One of the earliest functioning computers to employ drum memory was the Atanasoff–Berry computer (1942), though it used capacitance rather than magnetism. Magnetic drums were developed for the U.S. Navy by Engineering Research Associates (ERA) in 1946 and 1947. An ERA drum was the internal memory for the ATLAS-I computer delivered to the U.S. Navy in October 1950 and later sold commercially as the ERA 1101 and UNIVAC 1101. Through mergers, ERA became a division of UNIVAC shipping the Series 1100 drum as a part of the UNIVAC File Computer in 1956. The first mass-produced computer, the IBM 650 (1954), initially had up to 2,000 10-digit words of drum memory. In BSD Unix and its descendants, 'drum' was the name of the default virtual memory (swap) device, deriving from the historical use of drum secondary-storage devices as backup storage for pages in virtual memory. Magnetic drum memory units were used in the Minuteman ICBM launch control centers from the beginning in the early 1960s until the REACT upgrades in the mid-1990s.

Did You Know?

• Gustav Tauschek invented drum memory in 1932 in Austria with a capacity of about 500,000 bits (62.5 kilobytes).
• In BSD Unix, 'drum' was the name of the default virtual memory (swap) device, reflecting historical use of drum secondary-storage for paging.