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Magnetic Memory

January 19, 2017

Microprocessors, and their predecessor central processing units (CPUs), have gotten the lion's share of attention in computer hardware. Faster clock rates and novel architectures have always generated a lot of media coverage, but all that finesse would be useless with computer memory.

Various technologies were used to implement data memory in early computers. Data were stored as patches of electric charge in cathode ray tubes (the Williams tube), and acoustic waves in a vat of mercury (a delay line memory, see figure). As can be imagined, the volume density of data storage for such devices was very small, and there was a long access time to a particular data bit.

Fig. 16 of US Patent No. 2,629,827,  'Memory system,' by John Presper Eckert, Jr., and John W Mauchly, February 24, 1953

A mercury delay line computer memory system, fig. 16 of US Patent No. 2,629,827, "Memory system," by John Presper Eckert, Jr., and John W. Mauchly, February 24, 1953. Elements 440 are quartz wafers that act as transmitters and receivers of the acoustic wave. Mercury was used as the wave medium, since its high density (13.5 g/cc at room temperature) results in a relatively low speed of sound (1,451.4 meters per second). This patent disclosed, also, delay lines implemented by electric charging, or exciting phosphors, at the circumference of a rotating disk. (via Google Patents.[1]

Magnetic storage of information was known long before the first electronic computers. Valdemar Poulsen, a prolific Danish electrical engineer, invented magnetic wire recording in 1898. It was just a short conceptual leap from wire recording to the drum memory in which data were written onto the magnetic surface of a drum. An array of many read/write heads could be placed at the circumference of the drum to increase the volume density of information. The density of 0.5 megabits per drum, or less, is small by today's standards. The BSD operating system still incorporates a /dev/drum memory device.

Since the magnetic drum is a sequential access memory device, the speed of data access is slow. On average, you need to wait a half rotation of the drum to get a particular data bit. That's why engineers were excited around 1950 when they noted the magnetic square-loop response and rapid switching speed of some ferrite materials, such as (Fe,Mg,Mn)3O4.[2]

It's easy to magnetize some materials, notably iron, by passing an electric current through a surrounding solenoid coil, generating a magnetizing field. The problem is that most such materials won't retain their magnetic moment when the current is removed, and the process is typically very slow. The ferrite materials have a B-H loop, the graph of their magnetization as a function of applied magnetic field, that's nearly a square. That means that application of reversing currents will switch the ferrite cleanly and quickly from one magnetic state to another - a perfect feature for a memory element.

Figure caption

The dust jacket of Emerson W. Pugh's "Memories that Shaped an Industry: Decisions Leading to IBM System/360," a book that recalls the history of ferrite core memory.[3]

The book features some of the pioneers of this technology.

Photographs, by row, left to right, from top, Erich Bloch, John W. Gibson, Bill Rhodes, Mike Haynes, and Jay Forrester

(Scan of my copy.)

This period of computer development in the 1950s was long before the Planar processing that allowed considerable circuit complexity at minimal cost. The bits of a core memory were individual ferrite toroids that were addressed by current-carrying wires running through them, and these were laboriously assembled by hand into memory arrays (see photo). For those who think that off-shoring of labor is a recent development, most of this hand assembly was done by low paid workers in East Asia.[3]

Ferrite core memory detail

A ferrite core memory array.

The individual cores are 0.090" diameter, 0.030" thick, with a 0.060" diameter hole.[3]

(Portion of a Wikimedia Commons photo by Jud McCranie.)

Initially, an entire day's hand assembly was required to create a 64x64 bit array, but this was reduced to about 12 minutes with automated tooling.[3] The cost per bit likewise decreased from $1.00 to $0.01. The first core memories had an access time of about 6 μs, and this was improved to 0.6 μs (600 ns) within a decade. This later memory access speed is roughly equivalent to the earliest personal computers, such as the Apple II computer. Switching at such high rates was difficult with the electronics of the time, since nearly half an ampere's current was required in the two bit address lines (see figure).

Coincident-current selection of a ferrite core memory element

Coincident-current selection of a ferrite core memory element.

Selection is understood by application of the right-hand rule to the current in a quarter turn of wire.

(Drawn by the author using Inkscape.)

The pace of technology is unrelenting. Soon, semiconductor memory technologies offered better performance at a better price, and ferrite core memories were essentially obsolete within fifteen years of their introduction. Another magnetic memory technology, bubble memory, which made use of semiconductor planar-processing techniques, made a brief appearance after ferrite core; but those, too, were obsolete by the late-1980s.

Figure caption

A magnetic bubble memory module.

(Photo by the author, via Wikimedia Commons.)

Erich Bloch, one of the principals of core memory development, and the top image on the dust jacket of Pugh's book, shown above, outlived core memory by many decades. Bloch died November 25, 2016, at age 91.[4] Bloch, born of Jewish parents in Sulzburg, Germany, on January 9, 1925, came to the United States after being orphaned during the Holocaust.[4]

Bloch received his B.S. degree in electrical engineering from the University of Buffalo, and he went to work for prominent New York manufacturer, IBM. At IBM, Bloch was on the development team for the IBM 7030, an early transistorized mainframe computer that executed a half million instructions per second. The IBM 7030 was followed by the System/360, the development of which IBM invested $5 billion, a tremendous amount of money at the time.[4] The System/360 was a commercial succes, and I ran my computer programs on one during my graduate school days. Bloch retired from IBM in 1984, and he was director of the National Science Foundation from 1984 to 1990 under Ronald Reagan.[4]


  1. John Presper Eckert, Jr., and John W Mauchly, "Memory system," US Patent No. 2,629,827, February 24, 1953.
  2. Ernst Albers-Schonberg, "Process of making molded ceramic ferromagnetic products," US Patent No. 2,700,023, January 18, 1955.
  3. Emerson W. Pugh, "Memories that Shaped an Industry: Decisions Leading to IBM System/360," MIT Press (April, 1984, Out of Print).
  4. Steven Musil, "Erich Bloch, IBM mainframe pioneer, dies at 91," CNET, December 1, 2016.

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