The earliest form of computer memory is composed of magnetic drums. It was a few years later before it was superseded by core memory and semiconductor memory. Back then, almost all computers made use of magnetic drums. In fact, earlier computer models were also referred to as drum machines because of it.
Magnetic drums were invented by Gustav Tauschek, the genius who was also behind the inventions of the punch card machine and the optical character recognition machine. It was a cylindrical metal coated with ferromagnetic recording material. Along its body, there are numerous read-write heads that are designed to be stationary. These heads would emit an electric pulse that can recognize and change a magnetic orientation. Each track has its own head so the user need not trouble himself with assigning a head for the track to be used. Instead, the user only needs to choose the right head and the data will be shown once the drum rotates to it. The read head will then recognize the instruction in binary form, either in 1’s or 0’s.
Since the instructions will be written and read as the drum rotates, the speed with which the memory works on fully depends on the rotational latency of the drum. This leads to a delay in the time spent to work on the computer. Many users have to learn how to position the instructions in the drum to minimize the amount of time spent between writing the instructions to them being read. Despite this shortcoming, many computers utilized drums as their main memory. Some of these include the ABC computer, IBM 650 and the PDP-11/45 machine.
Seeing how the market accepted the drums, many other developments followed, including the English electric DEUCE drum and the Univac FASTRAND. The differences lie in the number of heads. Some of these do not have one head per track which leads to more delay to account for the time to position the head on the track.
In the end, the delay becomes a huge operational hindrance that drums were soon replaced by the magnetic core memory.
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