Magnetic Drum
We can say with humility and without judgment: the computing world has known strange machines in its early days. Among them, a rotating cylinder dominated three decades of technological evolution. Gustav Tauschek, an Austrian inventor, created in 1932 an object that would change the face of data storage: the magnetic drum.
A metal cylinder coated its surface with a ferromagnetic layer capturing binary information. All around the drum, read-write heads captured and inscribed data in the form of tiny magnetized dots. The constant rotation of the cylinder provided access to stored information, in a precise and regular mechanical ballet.
The first versions of the 1940s already showed the technical limitations of the era. Recording density reached only 50 bits per inch, with about twenty tracks per inch. The adjustment required watchmaker precision, so much so that technicians adjusted the gap between heads and surface to within a thousandth of an inch, using differential screws and high-precision machining.
The IBM 650, marketed from 1954, perfectly embodied this technology. Its drum, made of a cobalt-nickel alloy, measured 4 inches in diameter by 14 inches in length. It spun at the dizzying speed of 12,500 revolutions per minute on ultra-precise ball bearings. Its recording density reached 50 magnetic dots per inch, pulsing at 128 kHz. The data organization followed a particular logic: parallel storage at the bit level within each digit, but serial for the digits of a single word. Fifty words were inscribed on one circumference of the drum, each containing ten decimal digits and a sign. The system coded the minus sign by the digit 8, and the plus sign by the digit 9. A space equivalent to one digit separated each word, and five parallel tracks encoded each value.
David Macklin, a programmer at Republic Aviation in 1957, recounts the constraints imposed by this architecture. Developers had to account for the actual physical position of data on the drum to optimize access times. With its 2,000 addressable positions distributed across parallel tracks, the machine imposed a constant intellectual gymnastics. The programmer calculated the location of the next instruction or let the SOAP assembler handle it. On average, three or four executions per complete revolution completed the work, depending on the previous data positions.
The technology evolved significantly with the invention of the hydrodynamic bearing. This advance, developed by IBM for the SAGE air defense computer, made it possible to achieve densities comparable to magnetic tapes without resorting to the complex mechanical adjustments of the past.
The legacy of the magnetic drum survives in certain modern UNIX systems, where /dev/drum designates the virtual swap device. This name comes directly from the historical use of the drum as memory paging support.
The arrival of magnetic core memories signaled the gradual decline of the drum as primary memory. However, these rotating cylinders remained in use as memory extensions into the 1960s, thanks to their reliability and moderate cost compared to contemporary alternatives.
The electronic heart of the magnetic drum beat to the rhythm of about 2,000 tubes, primarily models 5955, 6211, 12AY7, 6AL5, 2D21 and 5687. Types 6211 and 5965 were similar to the 12AV7 but met IBM’s specific acceptance tests. Nearly 3,600 crystal diodes completed the logic circuits. The power supply consumed 16 to 18 KVA at 208V, 60 cycles, single-phase. Selenium rectifiers provided direct current, avoiding the need for complex electronic regulation.
Beyond simple storage, the magnetic drum shaped the design of early computers and the method of writing programs. Developers delved into the physical reality of storage to optimize their code, creating a hardware-software symbiosis characteristic of the 20th century. Its limitations in terms of access and capacity stimulated the search for alternatives. This work led to magnetic disks, faster and larger. IBM’s RAMAC, the first commercial hard disk marketed in 1956, marked the beginning of the end for the magnetic drum as a primary storage device.