The IBM 2301 fixed-head disk, or "drum", late 1960s. The drum is a spinning
cylinder with many circular tracks and each track has its own read/write
head, so the head don't have to move. The access time, therefore, was
significantly faster than a moving-head disk, but the capacity much smaller.
Drums were often used as main memory in the 1940s and 50s. By the 1960s
they were used mainly for swapping and paging (as, for example, in our
1975-era Digital Equipment Corporation PDP-11/50).
The IBM 2301 had a capacity of 4MB, average access time of 8.6msec, data
transfer rate of 1.2MB/sec, and cost about