Drum

Drums were the predecessor magnetic storage technology to disks; they were (as the name suggests) in physical form a drum, and generally had a head per track (like fixed-head disks).

Like fixed-head disks, this avoided the delays involved in having the head(s) seek to the correct track, and was also mechanically simpler (an important point at this early stage of development), but did require more components; drums also needed more volume for their installation.

The very first drums were actually used for main memory on some very early computers; and also on slightly later, but low-end, ones (e.g. the IBM 650 and Bendix G-15). Such use often required the use of optimum programming, to control the execution delays (due to rotational latency) which resulted if instructions were not placed in the optimum positions on the drum. Some drums were actually used to store registers, storing several registers in each track, and using multiple heads on that track to give faster access to the contents of the registers.

Once core memory appeared, drums were only seen in the role of high-speed secondary storage; often being used for swapping or paging purposes.

Drums fell out of use because they took more physical room than a disk with the equivalent surface area. A number of disk platters, giving in total a significantly greater amount of storage, could be stacked in the volume occupied by a single drum.