Drum memory of a Polish ZAM-41 computer

Drum memory is a data storage device and was an early form of computer memory that was widely used in the 1950s and into the 1960s, invented by Gustav Tauschek in 1932 in Austria. For many machines, a drum formed the main working memory of the machine, with data and programs being loaded on to or off the drum using media such as paper tape or punch cards. Drums were so commonly used for the main working memory that the machines were often referred to as drum machines. Drums were later replaced as the main working memory by core memory, which was faster and had no moving parts, and which lasted until semiconductor memory entered the scene.

A drum is a large metal cylinder that is coated on the outside surface with a ferromagnetic recording material. It is, simply put, a hard disk platter in the form of a drum rather than a flat disk. A row of read-write heads runs along the long axis of the drum, one for each track.

A key difference between a drum and a modern disk is that on a drum, the heads do not have to move, or seek, in order to find the track they are looking for. This means that the time to read (or write) any particular piece of data is shorter than it would be on a disk; the controller simply waits for the data to appear under the proper head as the drum turns. The performance of the drum or of a fixed head disk is defined almost entirely by the rotational speed, whereas in a moving head disk, both rotational speed and head movement rates are important.

Performance was still an issue, however. So, when a drum was used as the main working memory of the machine, programmers often took to hand-writing the code onto the drum in a particular fashion in order to reduce the amount of time needed to find the next instruction. They did this by carefully timing how long it would take for a particular instruction to run and the computer to ready itself to read the next instruction, then placing that instruction on the drum so that it was just arriving under the heads at that point in time. This method of timing compensation is called the Skip Factor or interleave, and is still used today in modern hard disk controllers.

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

Wikimedia Commons has media related to:

Drum memory

• The Story of Mel – the classic story about one programmer's drum machine hand-coding antics
• 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