Magnetic-core Memory - History

History

Frederick Viehe applied for various patents on the use of transformers for building digital logic circuits in place of relay logic beginning in 1947, issued through 1960, and assigned to IBM, working in his home laboratory. The first application was the Electronic Relay Circuit, revised and called the Memory Transformer. He was issued a patent for core memory manufacture in 1966. 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 electromechanical 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.

The MIT Whirlwind computer required a fast memory system for real-time aircraft tracking use. At first, Williams tubes—a storage system based on cathode ray tubes—were used, but these devices were always temperamental and unreliable. Several researchers in the late 1940s, including Jay Forrester, conceived the idea of using magnetic cores for computer memory, but Forrester received the principal patent for his invention of the co-incident core memory that enabled the 3D storage of information. William Papian of Project Whirlwind cited one of these efforts, Harvard's "Static Magnetic Delay Line", in an internal memo. The first Core memory of 32 x 32 x 16 bits was installed on Whirlwind in the summer of 1953. Papian, described: "Magnetic-Core Storage has two big advantages: (1) greater reliability with a consequent reduction in maintenance time devoted to storage; (2) shorter access time (core access time is 9 microseconds: tube access time is approximately 25 microseconds) thus increasing the speed of computer operation."

In April 2011, Forrester recalled, "the Wang use of cores did not have any influence on my development of random-access memory. The Wang memory was expensive and complicated. As I recall, which may not be entirely correct, it used two cores per binary bit and was essentially a delay line that moved a bit forward. To the extent that I may have focused on it, the approach was not suitable for our purposes." He describes the invention and associated events, in 1975. Forrester has since observed "It took us about seven years to convince the industry that random-access magnetic-core memory was the solution to a missing link in computer technology," Forrester later said. "Then we spent the following seven years in the patent courts convincing them that they had not all thought of it first."

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 enabling the construction of a serial, one dimensional shift register of o(50) bits, using two cores to store a bit. A Wang core shift register is in the Revolution exhibit at the Computer History Museum. The second, Jay Forrester's, was the coincident-current system, which enabled a small number of wires to control a large number of cores enabling 3D memory arrays of several million bits e.g. 8K x 8K x 64 bits.

Forrester's coincident-current system required one of the wires to be run at 45 degrees to the cores, which proved impossible to wire by machine, so that core arrays had to be assembled under microscopes by workers with fine motor control. Initially, garment workers were used. Successful automated assembly was only achieved in the 1970s around the time core memory became obsolete; because of this, automated assembly never entered the actual industrial production of core memory. It was during the early 1950s that Seeburg developed the use of this coincident current ferrite core memory storage in the "Tormat" memory of its new range of jukeboxes, starting with the V200 released in 1955. Development work was completed in 1953.

Some manufacturers employed Scandinavian seamstresses who had been laid off due to mechanization of the textile industry, for example to replace the memory of the Swedish computer BESK with core memory in 1956. By the late 1950s industrial plants had been set up in the Far East to build core. Inside, hundreds of workers strung cores for low pay. This lowered the cost of core to the point where it became largely universal as main memory by the early 1960s, replacing both inexpensive low-performance drum memory and costly high-performance systems using vacuum tubes, and later transistors, as memory. The cost of core memory declined sharply over the lifetime of the technology: costs began at roughly US$1.00 per bit and dropped to roughly US$0.01 per bit. Core was replaced by integrated semiconductor RAM chips in the 1970s.

Wang's patent was not granted until 1955, and by that time core was already in use. This started a long series of lawsuits, which eventually ended when IBM bought the patent outright from Wang for US$500,000. Wang used the funds to greatly expand Wang Laboratories, which he had co-founded with Dr. Ge-Yao Chu, a school mate from China. In 1964, after years of legal wrangling, IBM paid MIT $13 million for rights to Forrester's patent—the largest patent settlement to that date.

Core memory was part of a family of related technologies, now largely forgotten, which exploited the magnetic properties of materials to perform switching and amplification. By the 1950s vacuum tube electronics was well developed and very sophisticated, but tubes had a limited lifetime, used much more power and were much larger than semiconductor or magnetic technology, and their operating characteristics changed over their life. Magnetic devices had many of the virtues of the discrete and integrated solid-state devices that would replace them, and were extensively used in military applications. A notable example was the portable (truck-based) MOBIDIC computer developed by Sylvania for the United States Army Signal Corps in the late 1950s. The contents of electronic memory were lost when power was disconnected, but core memory was non-volatile and kept its contents.