For many early computers, drum memory formed the main working memory of the computer. It was so common that these computers were often referred to as drum machines. Some drum memories were also used as secondary storage.
Drums were displaced as primary computer memory by magnetic core memory which was a better balance of size, speed, cost, reliability and potential for further improvements. Similarly, drums were replaced by hard disk drives for secondary storage, which were also less expensive and denser. The manufacture of drums ceased in the 1970s.
A drum memory contained a large metal cylinder, coated on the outside surface with a ferromagnetic recording material. It could be considered the precursor to the hard disk drive (HDD), but in the form of a drum rather than a flat disk. 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 simply selected the proper head and waited for the data to appear under it as the drum turned (rotational latency). Not all drum units were designed with each track having its own head. Some, such as the English Electric DEUCE drum and the Univac FASTRAND had multiple heads moving a short distance on the drum in contrast to modern HDDs, which have one head per platter surface.
The performance of a drum with one head per track is determined almost entirely by the rotational latency, whereas in an HDD its performance includes a rotational latency delay plus the time to position the head over the desired track (seek time). In the era when drums were used as main working memory, programmers often did optimum programming--the programmer[NB 1] positioned code on the drum in such a way as to reduce the amount of time needed for the next instruction to rotate into place under the head. They did this by timing how long it would take after loading an instruction for the computer to be ready to read the next one, then placing that instruction on the drum so that it would arrive under a head just in time. This method of timing-compensation, called the "skip factor" or "interleaving" (interleaving in disk storage), was used for many years in storage memory controllers.
Tauschek's original drum memory (1932) had a capacity of about 500,000 bits (62.5 kilobytes).
One of the earliest functioning computers to employ drum memory was the Atanasoff-Berry computer (1942). It stored 3000 bits; however, it employed capacitance rather than magnetism to store the information. The outer surface of the drum was lined with electrical contacts leading to capacitors contained within.
Magnetic drums were developed for the US Navy during WW II with the work continuing at Engineering Research Associates (ERA) in 1946 and 1947. An experimental study was completed at ERA and reported to the Navy on June 19, 1947. Other early drum storage device development occurred at Birkbeck College (University of London), Harvard University, IBM and the University of Manchester. An ERA drum was the internal memory for the Atlas 1 computer delivered to the US Navy in October 1950. Thru mergers ERA became a division of UNIVAC shipping the Series 1100 drum as a part of the UNIVAC File Computer in 1956; each drum stored 180,000 characters.
The first mass-produced computer, the IBM 650, had about 8.5 kilobytes of drum memory (later doubled to about 17 kilobytes in the Model 4).
In BSD Unix and its descendants, /dev/drum was the name of the default virtual memory (swap) device, deriving from the use of drum secondary-storage devices as backup storage for pages in virtual memory.
Drum memory is referenced in The Story of Mel, in which the skilled programmer Mel optimizes programs written for a drum memory computer (the RPC 4000) by taking advantage of the time to process an instruction and the time for the drum to rotate so that the next instruction or data can be read, or optimizing in the opposite direction when the program should wait before proceeding.