Magnetic-core memory
In computing, magnetic-core memory is a form of random-access memory. It predominated for roughly 20 years between 1955 and 1975, and is often just called core memory, or, informally, core.
Core memory uses toroids of a hard magnetic material. Each core stores one bit of information. Two or more wires pass through each core, forming an X-Y array of cores. When an electrical current above a certain threshold is applied to the wires, the core will become magnetized. The core to be assigned a value - or written - is selected by powering one X and one Y wire to half of the required current, such that only the single core at the intersection is written. Depending on the direction of the currents, the core will pick up a clockwise or counterclockwise magnetic field, storing a 1 or 0.
This writing process also causes electricity to be induced into nearby wires. If the new pulse being applied in the X-Y wires is the same as the last applied to that core, the existing field will do nothing, and no induction will result. If the new pulse is in the opposite direction, a pulse will be generated. This is normally picked up in a separate "sense" wire, allowing the system to know whether that core held a 1 or 0. As this readout process requires the core to be written, this process is known as destructive readout, and requires additional circuitry to reset the core to its original value if the process flipped it.
When not being read or written, the cores maintain the last value they had, even if the power is turned off. Therefore, they are a type of non-volatile memory. Depending on how it was wired, core memory could be exceptionally reliable. Read-only core rope memory, for example, was used on the mission-critical Apollo Guidance Computer essential to NASA's successful Moon landings.
Using smaller cores and wires, the memory density of core slowly increased. By the late 1960s a density of about 32 kilobits per cubic foot was typical. The cost declined over this period from about $1 per bit to about 1 cent per bit. Reaching this density requires extremely careful manufacturing, which was almost always carried out by hand in spite of repeated major efforts to automate the process. Core was almost universal until the introduction of the first semiconductor memory chips in the late 1960s, and especially dynamic random-access memory in the early 1970s. Initially around the same price as core, DRAM was smaller and simpler to use. Core was driven from the market gradually between 1973 and 1978.
Even after magnetic-core memory was replaced by semiconductor memory, main memory was often still referred to as "core", particularly by people used to the term who worked on older machines with magnetic-core memory. The process of copying the entire content of a computer's main memory to a disk file for further inspection by a system programmer is still called a "core dump". When core memory used for calculations was expensive and a scarce resource, technologies were developed to swap blocks of data "out of core" onto larger, slower storage. Algorithms whose working set size exceeds main memory came to be called out-of-core algorithms, while in-core algorithms fit in main memory.
History
Developers
The basic concept of using the square hysteresis loop of certain magnetic materials as a storage or switching device was known from the earliest days of computer development. Much of this knowledge had developed due to an understanding of transformers, which allowed amplification and switch-like performance when built using certain materials. The stable switching behavior was well known in the electrical engineering field, and its application in computer systems was immediate. For example, J. Presper Eckert and Jeffrey Chuan Chu had done some development work on the concept in 1945 at the Moore School during the ENIAC efforts.Robotics pioneer George Devol filed a patent for the first static magnetic memory on 3 April 1946. Devol's magnetic memory was further refined via 5 additional patents and ultimately used in the first industrial robot. Frederick Viehe applied for various patents on the use of transformers for building digital logic circuits in place of relay logic beginning in 1947. A fully developed core system was patented in 1947, and later purchased by IBM in 1956. This development was little-known, however, and the mainstream development of core memory is normally associated with three independent teams.
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 patent described a type of memory that would today be known as a delay-line or shift-register system. Each bit was stored using a pair of transformers, one that held the value and a second used for control. A signal generator produced a series of pulses that were sent into the control transformers at half the energy needed to flip the polarity. The pulses were timed so the field in the transformers had not faded away before the next pulse arrived. If the storage transformer's field matched the field created by the pulse, then the total energy would cause a pulse to be injected into the next transformer pair. Those that did not contain a value simply faded out. Stored values were thus moved bit by bit down the chain with every pulse. Values were read out at the end, and fed back into the start of the chain to keep the values continually cycling through the system. Such systems have the disadvantage of not being random-access, to read any particular value one has to wait for it to cycle through the chain. Wang and Woo were working at Harvard University's Computation Laboratory at the time, and the university was not interested in promoting inventions created in their labs. Wang was able to patent the system on his own.
The MIT Project Whirlwind computer required a fast memory system for real-time aircraft tracking. At first, an array of Williams tubes—a storage system based on cathode-ray tubes—was used, but proved temperamental and unreliable. Several researchers in the late 1940s conceived the idea of using magnetic cores for computer memory, but MIT computer engineer Jay Forrester received the principal patent for his invention of the coincident-current 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 was installed on Whirlwind in the summer of 1953. Papian stated: "Magnetic-Core Storage has two big advantages: greater reliability with a consequent reduction in maintenance time devoted to storage; shorter access time 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. Then we spent the following seven years in the patent courts convincing them that they had not all thought of it first."
A third developer involved in the early development of core memory was Jan A. Rajchman at RCA. A prolific inventor, Rajchman designed a unique core system using ferrite bands wrapped around thin metal tubes, building his first examples using a converted aspirin press in 1949. Rajchman later developed versions of the Williams tube and led the development of the Selectron.
Two key inventions led to the development of magnetic core memory in 1951. The first, developed by An Wang, 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, using two cores to store a bit. A Wang core shift register is in the Revolution exhibit at the Computer History Museum. The second, 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. The first use of magnetic core was in the Whirlwind computer, and Project Whirlwind's "most famous contribution was the random-access, magnetic core storage feature." Commercialization followed quickly. Magnetic core was used in peripherals of the ENIAC in 1953, the IBM 702 delivered in July 1955, and later in the 702 itself. The IBM 704 and the Ferranti Mercury used magnetic-core memory.
It was during the early 1950s that Seeburg Corporation developed one of the first commercial applications of coincident-current core memory storage in the "Tormat" memory of its new range of jukeboxes, starting with the V200 developed in 1953 and released in 1955. Numerous uses in computing, telephony and industrial process control followed.
Patent disputes
Wang's patent was not granted until 1955, and by that time magnetic-core memory was already in use. This started a long series of lawsuits, which eventually ended when IBM bought the patent outright from Wang for. Wang used the funds to greatly expand Wang Laboratories, which he had co-founded with Dr. Ge-Yao Chu, a schoolmate from China.MIT wanted to charge IBM $0.02 per bit royalty on core memory. 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.