Drum memory

Drum memory of a Polish ZAM-41 computer

Drum memory from the BESK computer

Drum memory was a magnetic data storage device invented by Gustav Tauschek in 1932 in Austria and was widely used in the 1950s and into the 1960s as computer memory. Some drum memories were also used as secondary storage.

For many computers, drum memory formed the main working memory of the computer. Drums were so commonly used for the primary computer memory that these computers were often referred to as drum machines.

Drums were displaced as the primary computer memory by core memory which was faster (no moving parts), less expensive and more dense. For the same reasons, drums were displaced as secondary storage by hard disk drives. Drums stopped being manufactured in the 1970s, eclipsed by the advent of inexpensive semiconductor memory.

Design

A drum was a large metal cylinder that was coated on the outside surface with a ferromagnetic recording material. It could be considered the precursor to the hard disk platter, but in the form of a drum rather than a flat disk. In many cases a row of fixed read-write heads ran along the long axis of the drum, one for each track.

A difference between many drums and a modern hard disk drive is that some drums had one track per head, so that the heads did not have to move to the track to access data; the controller simply waited for the data to appear under the relevant head as the drum turned. In a modern hard disk drive, delay can include the seek time to move the head to the desired track, plus a rotational latency, the time taken by the disk to rotate wanted data into position, whereas the performance of a drum with its head per track is determined almost entirely by the rotational latency. Particularly while drums were used as main working memory, programmers often took to positioning code onto the drum in such a way as to reduce the amount of time needed to find the next instruction. 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 is called the skip factor or interleave, and was used for many years in hard disk controllers.

The head per track scheme was not inherent to drum technology; a few drum stores such as the English Electric DEUCE drum and the Univac FASTRAND had one or more moving heads.

Use and legacy

Tauschek's original drum memory 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. However it employed regenerative capacitor memory rather than magnetism to store information; the outer surface of the drum was lined with electrical contacts leading to circuitry contained within.

The first mass-produced computer, the IBM 650, had about 8.5 kilobytes of drum memory, which in a later model (number 4) was doubled to about 17 kilobytes.

As late as 1980, PDP-11/45 machines that used drums for swapping (and magnetic core memory) were still in use at many of the original UNIX sites.

In modern-day BSD Unix and its descendants, /dev/drum is the name of the default virtual memory (swap) device, deriving from the use of drum secondary-storage devices as backing store for pages in virtual memory.

See also

• CAB500
• Karlqvist gap
• Manchester Mark 1
• Random-access memory
• Wisconsin Integrally Synchronized Computer

External links

• The Story of Mel: the classic story about one programmer's drum machine hand-coding antics: Mel Kaye.
• Librascope LGP-30: The drum memory computer referenced in the above story, also referenced on Librascope LGP-30.
• Librascope RPC-4000: Another drum memory computer referenced in the above story
• Oral history interview with Dean Babcock