IBM 650 magnetic drum memory
A non-volatile memory widely used in early computers, magnetic drum memory consisted of a rotating cylinder encircled by strips of ferromagnetic medium and a row of stationary read/write heads, one for each strip. In many ways, it was a 3-D analog of a spinning disk hard drive. For its time, it was a low-cost approach that provided a significant jump in capacity and speed.
The magnetic drum memory drove the success of the IBM 650 supercomputer, released in 1953. The 650’s 16-inch-long, 4-inch-diameter drum rotated at 12,500 rpm and could store a whopping 8.5 KB of data. Instead of waiting for a flying read/write head moving across a spinning disc to access data, the system had to wait for the data to come around in its rotation. This introduced latency, but clever programmers used this technique to their advantage by intentionally writing the code to separate a set of instructions involved with a specific memory address by the amount of idle time a certain operation might need. By the time the drum had finished the rotation and returned to the data bit, the operation was concluded and the CPU was ready to write the result. (Image courtesy of IBM)