Drum memory was a magneticdata storage device invented byGustav Tauschek in 1932 inAustria.[1][2] Drums were widely used in the 1950s and into the 1960s ascomputer memory.
Many early computers, called drum computers or drum machines, used drum memory as the main working memory of the computer.[3] Some drums were also used assecondary storage as for example variousIBM drum storage drives, and theUNIVAC Flying head andFASTRAND series of drums.
Drums were displaced as primary computer memory by magneticcore memory, which offered a better balance of size, speed, cost, reliability and potential for further improvements.[4] Drums were then replaced byhard disk drives forsecondary storage, which were both less expensive and offered denser storage. The manufacturing of drums ceased in the 1970s.
A drum memory or drum storage unit contained a large metal cylinder, coated on the outside surface with aferromagnetic recording material. It could be considered the precursor to thehard disk drive (HDD), but in the form of a drum (cylinder) rather than a flat disk. In most designs, one or more rows of fixedread-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 theEnglish Electric DEUCE drum and the UNIVACFASTRAND had multiple heads moving a short distance on the drum. In comparison modern HDDs have one head per platter surface, which move together.
In November 1953 Glenn E. Hagen of Logistics Research, Inc. published a paper disclosing "air floating" ofmagnetic heads in an experimental sheet metal drum.[5] 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."[6] Flying heads became standard in drums andhard disk drives.
Magnetic drum units used as primary memory were addressed by word. Drum units used as secondary storage were addressed by block. Several modes of block addressing were possible, depending on the device.
Some devices were divided into logical cylinders, and addressing by track was actually logical cylinder and track.
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 the rotational latency, whereas in an HDD with moving heads 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 didoptimum programming—the programmer—or the assembler, for example IBM’sSymbolic Optimal Assembly Program (SOAP)—positioned instructions 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.[7] 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", was used for many years in storage memory controllers.
Tauschek's original drum memory (1932) had a capacity of about 500,000bits (62.5kilobytes).[2]
One of the earliest functioning computers to employ drum memory was theAtanasoff–Berry computer (1942). It stored 3,000 bits; however, it employedcapacitance rather thanmagnetism to store the information. The outer surface of the drum was lined with electrical contacts leading tocapacitors contained within.
Magnetic drums were developed for theU.S. Navy byEngineering Research Associates (ERA) in 1946 and 1947.[8] An experimental ERA study was completed and reported to the Navy on June 19, 1947.[8] Other early drum storage device development occurred atBirkbeck College (University of London),[9]Harvard University,IBM and theUniversity of Manchester. 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 andUNIVAC 1101. Throughmergers, ERA became a division ofUNIVAC shipping the Series 1100 drum as a part of theUNIVAC File Computer in 1956; each drum stored 180,000 6-bit characters (135 kilobytes).[10]
The first mass-produced computer, theIBM 650 (1954), initially had up to 2,000 10-digit words, about 17.5kilobytes, of drum memory (later doubled to 4,000 words, about 35 kilobytes, in the Model 4).
InBSD Unix and its descendants,/dev/drum was the name of the defaultvirtual memory (swap) device,[11] deriving from the historical use of drum secondary-storage devices as backup storage forpages invirtual memory.
Magnetic drum memory units were used in theMinuteman ICBM launch control centers from the beginning in the early 1960s until theREACT upgrades in the mid-1990s.
There was a 1,070-word drum memory for data, stored as twelve 6-bit digits or characters per word
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