Magnetic core memory

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Magnetic core memory is an early form of random access computer memory. It uses tiny magnetic rings (see photographs below, with indication of scale), the cores, through which wires are threaded to store information via the polarity of their magnetic field. Such memory is often just called core memory, or, informally, core.

Although core memory has long been superseded by semiconductor memory, memory is still occasionally called "core"; in particular a file recording all the contents of memory after a system error is usually called a core dump.



Frederick Viehe first patented magnetic core memory in 1947, having developed the device in his home laboratory.[1] Independently, substantial work in the field was carried out by the Shanghai-born American physicists An Wang and Way-Dong Woo, who created the pulse transfer controlling device in 1949. The name referred to the way that the magnetic field of the cores could be used to control the switching of current in electro-mechanical systems. Wang and Woo were working at Harvard University's Computation Laboratory at the time but, unlike MIT, Harvard was not interested in promoting inventions created in their labs. Instead Wang was able to patent the system on his own while Woo was ill.

Jay Forrester's group, working on the Whirlwind project at MIT, became aware of this work. This machine required a fast memory system for real-time flight simulator use. At first, Williams tubes&mdash a storage system based on cathode ray tubes—were used, but these devices were always temperamental and unreliable.

Two key inventions led to the development of magnetic core memory in 1951. The first, An Wang's, was the write-after-read cycle, which solved the problem of how to use a storage medium in which the act of reading erased the data read. The second, Jay Forrester's, was the coincident-current system, which enabled a small number of wires to control a large number of cores (see Description section below for details).

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