![KL CoreMemory](https://upload.wikimedia.org/wikipedia/commons/d/da/KL_CoreMemory.jpg)
Magnetic-core memory was the predominant form of
random-access
Random access (more precisely and more generally called direct access) is the ability to access an arbitrary element of a sequence in equal time or any datum from a population of addressable elements roughly as easily and efficiently as any othe ...
computer memory
In computing, memory is a device or system that is used to store information for immediate use in a computer or related computer hardware and digital electronic devices. The term ''memory'' is often synonymous with the term ''primary storage ...
for 20 years between about 1955 and 1975.
Such memory is often just called core memory, or, informally, core.
Core memory uses
toroids (rings) of a
hard magnetic material
Coercivity, also called the magnetic coercivity, coercive field or coercive force, is a measure of the ability of a ferromagnetic material to withstand an external magnetic field without becoming demagnetized. Coercivity is usually measured in ...
(usually a
semi-hard ferrite) as
transformer
A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...
cores, where each wire threaded through the core serves as a transformer winding. Two or more wires pass through each core. Magnetic
hysteresis
Hysteresis is the dependence of the state of a system on its history. For example, a magnet may have more than one possible magnetic moment in a given magnetic field, depending on how the field changed in the past. Plots of a single component of ...
allows each of the cores to "remember", or store a state.
Each core stores one
bit of information. A core can be magnetized in either the clockwise or counter-clockwise direction. The value of the bit stored in a core is zero or one according to the direction of that core's magnetization.
Electric current
An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The moving pa ...
pulses in some of the wires through a core allow the direction of the magnetization in that core to be set in either direction, thus storing a one or a zero. Another wire through each core, the sense wire, is used to detect whether the core changed state.
The process of reading the core causes the core to be reset to a zero, thus erasing it. This is called ''destructive readout''. 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
Non-volatile memory (NVM) or non-volatile storage is a type of computer memory that can retain stored information even after power is removed. In contrast, volatile memory needs constant power in order to retain data.
Non-volatile memory typic ...
'' memory.
Using smaller cores and wires, the
memory density
Density is a measure of the quantity of information bits that can be stored on a given length (''linear density'') of hard disk, track, area of Surface area, surface (''areal density''), or in a given volume (''volumetric density'') of a computer s ...
of core slowly increased, and by the late 1960s a density of about 32 kilobits per cubic foot (about 0.9 kilobits per litre) was typical. However, reaching this density required extremely careful manufacture, which was almost always carried out by hand in spite of repeated major efforts to automate the process. The cost declined over this period from about $1 per bit to about 1 cent per bit. The introduction of the first
semiconductor memory
Semiconductor memory is a digital electronic semiconductor device used for digital data storage, such as computer memory. It typically refers to devices in which data is stored within metal–oxide–semiconductor (MOS) memory cells on a sili ...
chips in the late 1960s, which initially created static random-access memory (
SRAM), began to erode the market for core memory. The first successful dynamic random-access memory (
DRAM
Dynamic random-access memory (dynamic RAM or DRAM) is a type of random-access semiconductor memory that stores each bit of data in a memory cell, usually consisting of a tiny capacitor and a transistor, both typically based on metal-oxid ...
), the
Intel 1103
The 1103 is a dynamic random-access memory (DRAM) integrated circuit (IC) developed and fabricated by Intel. Introduced in October 1970, the 1103 was the first commercially available DRAM IC; and due to its small physical size and low price relat ...
, followed in 1970. Its availability in quantity at 1 cent per bit marked the beginning of the end for core memory.
Improvements in semiconductor manufacturing led to rapid increases in storage capacity and decreases in price per kilobyte, while the costs and specs of core memory changed little. Core memory was driven from the market gradually between 1973 and 1978.
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
The Apollo Guidance Computer (AGC) was a digital computer produced for the Apollo program that was installed on board each Apollo command module (CM) and Apollo Lunar Module (LM). The AGC provided computation and electronic interfaces for guidan ...
essential to
NASA
The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research.
NASA was established in 1958, succeeding t ...
's successful Moon landings.
Although core memory is obsolete, computer memory is still sometimes called "core" even though it is made of semiconductors, particularly by people who had worked with machines having actual core memory. The files that result from saving the entire contents of memory to disk for inspection, which is nowadays commonly performed automatically when a major error occurs in a computer program, are still called "
core dumps".
History
Developers
![Project Whirlwind - core memory, circa 1951 - detail 1](https://upload.wikimedia.org/wikipedia/commons/5/5b/Project_Whirlwind_-_core_memory%2C_circa_1951_-_detail_1.JPG)
The basic concept of using the square
hysteresis
Hysteresis is the dependence of the state of a system on its history. For example, a magnet may have more than one possible magnetic moment in a given magnetic field, depending on how the field changed in the past. Plots of a single component of ...
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
transformer
A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...
s, which allowed amplification and switch-like performance when built using certain materials. The stable switching behavior was well known in the
electrical engineering
Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems which use electricity, electronics, and electromagnetism. It emerged as an identifiable occupation in the l ...
field, and its application in computer systems was immediate. For example,
J. Presper Eckert
John Adam Presper Eckert Jr. (April 9, 1919 – June 3, 1995) was an American electrical engineer and computer pioneer. With John Mauchly, he designed the first general-purpose electronic digital computer (ENIAC), presented the first course in co ...
and
Jeffrey Chuan Chu Jeffrey Chuan Chu (朱傳榘) (July 14, 1919 – June 6, 2011), born in Tianjin, Republic of China, was a pioneer computer engineer. He received his BS from the University of Minnesota and his MS from the Moore School at the University of Pennsylvan ...
had done some development work on the concept in 1945 at the
Moore School during the
ENIAC efforts.
Robotics pioneer
George Devol
George Charles Devol Jr. (February 20, 1912 – August 11, 2011) was an American inventor, best known for creating Unimate, the first industrial robot. Devol's invention earned him the title "Grandfather of Robotics". The National Inventor ...
filed a patent for the first static (non-moving) 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
An industrial robot is a robot system used for manufacturing. Industrial robots are automated, programmable and capable of movement on three or more axes.
Typical applications of robots include welding, painting, assembly, disassembly, pick a ...
. Frederick Viehe applied for various patents on the use of
transformer
A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...
s 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 is normally associated with three independent teams.
Substantial work in the field was carried out by the
Shanghai
Shanghai (; , , Standard Mandarin pronunciation: ) is one of the four direct-administered municipalities of the People's Republic of China (PRC). The city is located on the southern estuary of the Yangtze River, with the Huangpu River flow ...
-born
American
American(s) may refer to:
* American, something of, from, or related to the United States of America, commonly known as the "United States" or "America"
** Americans, citizens and nationals of the United States of America
** American ancestry, pe ...
physicist
A physicist is a scientist who specializes in the field of physics, which encompasses the interactions of matter and energy at all length and time scales in the physical universe.
Physicists generally are interested in the root or ultimate caus ...
s
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; his patent focused on using cores to create
delay-line or
shift-register
A shift register is a type of digital circuit using a cascade of flip-flops where the output of one flip-flop is connected to the input of the next. They share a single clock signal, which causes the data stored in the system to shift from one lo ...
memory systems. Wang and Woo were working at
Harvard University
Harvard University is a private Ivy League research university in Cambridge, Massachusetts. Founded in 1636 as Harvard College and named for its first benefactor, the Puritan clergyman John Harvard, it is the oldest institution of higher le ...
'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
Real-time or real time describes various operations in computing or other processes that must guarantee response times within a specified time (deadline), usually a relatively short time. A real-time process is generally one that happens in defined ...
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
Jay Wright Forrester (July 14, 1918 – November 16, 2016) was a pioneering American computer engineer and systems scientist. He is credited with being one of the inventors of magnetic core memory, the predominant form of random-access computer ...
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: (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. 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 was
Jan A. Rajchman
Jan Aleksander Rajchman (London, 10 August 1911 – 1 April 1989) was a Polish electrical engineer and computer pioneer.
Biography
Jan Aleksander was son of Ludwik Rajchman and Maria Bojańczyk. His father was a Polish bacteriologist and ...
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
Aspirin, also known as acetylsalicylic acid (ASA), is a nonsteroidal anti-inflammatory drug (NSAID) used to reduce pain, fever, and/or inflammation, and as an antithrombotic. Specific inflammatory conditions which aspirin is used to treat inc ...
press in 1949.
[ Rajchman later developed versions of the Williams tube and led development of the Selectron.
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 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 Computer History Museum (CHM) is a museum of computer history, located in Mountain View, California. The museum presents stories and artifacts of Silicon Valley and the information age, and explores the computing revolution and its impact on ...
. 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 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
The IBM 704 is a large digital mainframe computer introduced by IBM in 1954. It was the first mass-produced computer with hardware for floating-point arithmetic. The IBM 704 ''Manual of operation'' states:
The type 704 Electronic Data-Pro ...
(1954) and the Ferranti Mercury (1957) 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
An industrial process control in continuous production processes is a discipline that uses industrial control systems to achieve a production level of consistency, economy and safety which could not be achieved purely by human manual control. I ...
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
Wang Laboratories was a US computer company founded in 1951 by An Wang and G. Y. Chu. The company was successively headquartered in Cambridge, Massachusetts (1954–1963), Tewksbury, Massachusetts (1963–1976), and finally in Lowell, Massachusett ...
, 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.
Production economics
In 1953, tested but not-yet-strung cores cost each. As manufacturing volume increased, by 1970 IBM was producing 20 billion cores per year, and the price per core fell to . Core sizes shrank over the same period from around diameter in the 1950s to in 1966. The power required to flip the magnetization of one core is proportional to the volume, so this represents a drop in power consumption by a factor of 125.
The cost of complete core memory systems was dominated by the cost of stringing the wires through the cores. Forrester's coincident-current system required one of the wires to be run at 45 degrees to the cores, which proved difficult to wire by machine, so that core arrays had to be assembled under microscopes by workers with fine motor control.
In 1956, a group at IBM filed for a patent on a machine to automatically thread the first few wires through each core. This machine held the full plane of cores in a "nest" and then pushed an array of hollow needles through the cores to guide the wires. Use of this machine reduced the time taken to thread the straight X and Y select lines from 25 hours to 12 minutes on a 128 by 128 core array.
Smaller cores made the use of hollow needles impractical, but there were numerous advances in semi-automatic core threading. Support nests with guide channels were developed. Cores were permanently bonded to a backing sheet "patch" that supported them during manufacture and later use. Threading needles were butt welded to the wires, so the needle and wire diameters were the same, and efforts were made to entirely eliminate the use of needles.
The most important change, from the point of view of automation, was the combination of the sense and inhibit wires, eliminating the need for a circuitous diagonal sense wire. With small changes in layout, this also allowed much tighter packing of the cores in each patch.[Creighton D. Barnes, et al., Magnetic core storage device having a single winding for both the sensing and inhibit function, , granted 4 July 1967.][Victor L. Sell and Syed Alvi, High Density Core Memory Matrix, , granted Jan. 16, 1973.]
By the early 1960s, the cost of core fell to the point that it became nearly universal as main memory
Computer data storage is a technology consisting of computer components and recording media that are used to retain digital data. It is a core function and fundamental component of computers.
The central processing unit (CPU) of a computer ...
, replacing both inexpensive low-performance drum memory and costly high-performance systems using vacuum tube
A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric voltage, potential difference has been applied.
The type kn ...
s, and later discrete transistor
upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink).
A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch e ...
s as memory. The cost of core memory declined sharply over the lifetime of the technology: costs began at roughly per bit and dropped to roughly per bit. Core was replaced with integrated semiconductor
A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...
RAM chips in the 1970s.
An example of the scale, economics, and technology of core memory in the 1960s was the 256K 36-bit word (1.2 MiB
The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable unit ...
) core memory unit installed on the PDP-6
The PDP-6, short for Programmed Data Processor model 6, is a computer developed by Digital Equipment Corporation (DEC) during 1963 and first delivered in the summer of 1964.
It was an expansion of DEC's existing 18-bit systems to use a 36-bit da ...
at the MIT Artificial Intelligence Laboratory by 1967. This was considered "unimaginably huge" at the time, and nicknamed the "Moby Memory". It cost $380,000 ($0.04/bit) and was 69 inches wide, 50 inches tall, and 25 inches deep with its supporting circuitry (189 kilobits/cubic foot = 6.7 kilobits/litre). Its cycle time was 2.75 μs.
Description
The term "core" comes from conventional transformer
A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...
s whose windings surround a magnetic core
A magnetic core is a piece of magnetic material with a high magnetic permeability used to confine and guide magnetic fields in electrical, electromechanical and magnetic devices such as electromagnets, transformers, electric motors, generators, in ...
. In core memory, the wires pass once through any given core—they are single-turn devices. The properties of materials used for memory cores are dramatically different from those used in power transformers. The magnetic material for a core memory requires a high degree of magnetic remanence
Remanence or remanent magnetization or residual magnetism is the magnetization left behind in a ferromagnetic material (such as iron) after an external magnetic field is removed. Colloquially, when a magnet is "magnetized", it has remanence. The ...
, the ability to stay highly magnetized, and a low coercivity so that less energy is required to change the magnetization direction. The core can take two states, encoding one bit. The core memory contents are retained even when the memory system is powered down ( non-volatile memory). However, when the core is read, it is reset to a "zero" value. Circuits in the computer memory system then restore the information in an immediate re-write cycle.
How core memory works
The most common form of core memory, ''X/Y line coincident-current'', used for the main memory of a computer, consists of a large number of small toroidal ferrimagnetic ceramic
A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain ...
ferrites (''cores'') held together in a grid structure (organized as a "stack" of layers called ''planes''), with wires woven through the holes in the cores' centers. In early systems there were four wires: ''X'', ''Y'', ''Sense'', and ''Inhibit'', but later cores combined the latter two wires into one ''Sense/Inhibit'' line. Each toroid stored one bit (0 or 1). One bit in each plane could be accessed in one cycle, so each machine word
A word is a basic element of language that carries an semantics, objective or pragmatics, practical semantics, meaning, can be used on its own, and is uninterruptible. Despite the fact that language speakers often have an intuitive grasp of w ...
in an array of words was spread over a "stack" of planes. Each plane would manipulate one bit of a word in parallel
Parallel is a geometric term of location which may refer to:
Computing
* Parallel algorithm
* Parallel computing
* Parallel metaheuristic
* Parallel (software), a UNIX utility for running programs in parallel
* Parallel Sysplex, a cluster of IBM ...
, allowing the full word to be read or written in one cycle.
Core relies on the "square loop" properties of the ferrite material used to make the toroids. An electric current in a wire that passes through a core creates a magnetic field. Only a magnetic field
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
greater than a certain intensity ("select") can cause the core to change its magnetic polarity. To select a memory location, one of the X and one of the Y lines are driven with half the current ("half-select") required to cause this change. Only the combined magnetic field generated where the X and Y lines cross (a logical AND function) is sufficient to change the state; other cores will see only half the needed field ("half-selected"), or none at all. By driving the current through the wires in a particular direction, the resulting induced
Induce may refer to:
* Induced consumption
* Induced innovation
* Induced character
* Induced coma
* Induced menopause
* Induced metric
* Induced path
* Induced topology
* Induce (musician), American musician
See also
* Inducement (disambiguation ...
field forces the selected core's magnetic flux to circulate in one direction or the other (clockwise or counterclockwise). One direction is a stored ''1'', while the other is a stored ''0''.
The toroidal shape of a core is preferred since the magnetic path is closed, there are no magnetic poles and thus very little external flux. This allows the cores to be packed closely together without allowing their magnetic fields to interact. The alternating 45-degree positioning used in early core arrays was necessitated by the diagonal sense wires. With the elimination of these diagonal wires, tighter packing was possible.
Reading and writing
To read a bit of core memory, the circuitry tries to flip the bit to the polarity assigned to the 0 state, by driving the selected X and Y lines that intersect at that core.
* If the bit was already 0, the physical state of the core is unaffected.
* If the bit was previously 1, then the core changes magnetic polarity. This change, after a delay, induces a voltage pulse into the Sense line.
The detection of such a pulse means that the bit had most recently contained a 1. Absence of the pulse means that the bit had contained a 0. The delay in sensing the voltage pulse is called the access time of the core memory.
Following any such read, the bit contains a 0. This illustrates why a core memory access is called a ''destructive read'': Any operation that reads the contents of a core erases those contents, and they must immediately be recreated.
To write a bit of core memory, the circuitry assumes there has been a read operation and the bit is in the 0 state.
* To write a 1 bit, the selected X and Y lines are driven, with current in the opposite direction as for the read operation. As with the read, the core at the intersection of the X and Y lines changes magnetic polarity.
* To write a 0 bit, two methods can be applied. The first one is the same as reading process with current in the original direction. The second has reversed logic. Write 0 bit, in other words, is to inhibit the writing of a 1 bit. The same amount of current is also sent through the Inhibit line. This reduces the net current flowing through the respective core to half the select current, inhibiting change of polarity.
The access time plus the time to rewrite is the memory cycle time.
The Sense wire is used only during the read, and the Inhibit wire is used only during the write. For this reason, later core systems combined the two into a single wire, and used circuitry in the memory controller to switch the function of the wire. However, when Sense wire crosses too many cores, the half select current can also induce a considerable voltage across the whole line due to the superposition of the voltage at each single core. This potential risk of "misread" limits the minimum number of the Sense wire. Increasing Sense wires requires more decode circuits.
Core memory controllers were designed so that every read was followed immediately by a write (because the read forced all bits to 0, and because the write assumed this had happened). Computers began to take advantage of this fact. For example, a value in memory could be read and incremented (as for example by the AOS
instruction on the PDP-6
The PDP-6, short for Programmed Data Processor model 6, is a computer developed by Digital Equipment Corporation (DEC) during 1963 and first delivered in the summer of 1964.
It was an expansion of DEC's existing 18-bit systems to use a 36-bit da ...
) almost as quickly as it could be read; the hardware simply incremented the value between the read phase and the write phase of a single memory cycle (perhaps signalling the memory controller to pause briefly in the middle of the cycle). This might be twice as fast as the process of obtaining the value with a read-write cycle, incrementing the value in some processor register, and then writing the new value with another read-write cycle.
Other forms of core memory
''Word line'' core memory was often used to provide register memory. Other names for this type are ''linear select'' and ''2-D''. This form of core memory typically wove three wires through each core on the plane, ''word read'', ''word write'', and ''bit sense/write''. To read or clear words, the full current is applied to one or more ''word read'' lines; this clears the selected cores and any that flip induce voltage pulses in their ''bit sense/write'' lines. For read, normally only one ''word read'' line would be selected; but for clear, multiple ''word read'' lines could be selected while the ''bit sense/write'' lines ignored. To write words, the half current is applied to one or more ''word write'' lines, and half current is applied to each ''bit sense/write'' line for a bit to be set. In some designs, the ''word read'' and ''word write'' lines were combined into a single wire, resulting in a memory array with just two wires per bit. For write, multiple ''word write'' lines could be selected. This offered a performance advantage over ''X/Y line coincident-current'' in that multiple words could be cleared or written with the same value in a single cycle. A typical machine's register set usually used only one small plane of this form of core memory. Some very large memories were built with this technology, for example the Extended Core Storage
Extension, extend or extended may refer to:
Mathematics
Logic or set theory
* Axiom of extensionality
* Extensible cardinal
* Extension (model theory)
* Extension (predicate logic), the set of tuples of values that satisfy the predicate
* E ...
(ECS) auxiliary memory in the CDC 6600, which was up to 2 million 60-bit words.
Another form of core memory called core rope memory provided read-only storage
Read-only memory (ROM) is a type of non-volatile memory used in computers and other electronic devices. Data stored in ROM cannot be electronically modified after the manufacture of the memory device. Read-only memory is useful for storing sof ...
. In this case, the cores, which had more linear magnetic materials, were simply used as transformer
A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...
s; no information was actually stored magnetically within the individual cores. Each bit of the word had one core. Reading the contents of a given memory address generated a pulse of current in a wire corresponding to that address. Each address wire was threaded either through a core to signify a binary or around the outside of that core, to signify a binary As expected, the cores were much larger physically than those of read-write core memory. This type of memory was exceptionally reliable. An example was the Apollo Guidance Computer
The Apollo Guidance Computer (AGC) was a digital computer produced for the Apollo program that was installed on board each Apollo command module (CM) and Apollo Lunar Module (LM). The AGC provided computation and electronic interfaces for guidan ...
used for the NASA
The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research.
NASA was established in 1958, succeeding t ...
Moon landings.
Physical characteristics
The performance of early core memories can be characterized in today's terms as being very roughly comparable to a clock rate of 1 MHz
The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that one he ...
(equivalent to early 1980s home computers, like the Apple II
The Apple II (stylized as ) is an 8-bit home computer and one of the world's first highly successful mass-produced microcomputer products. It was designed primarily by Steve Wozniak; Jerry Manock developed the design of Apple II's foam-m ...
and Commodore 64
The Commodore 64, also known as the C64, is an 8-bit home computer introduced in January 1982 by Commodore International (first shown at the Consumer Electronics Show, January 7–10, 1982, in Las Vegas). It has been listed in the Guinness ...
). Early core memory systems had cycle times of about 6 µs, which had fallen to 1.2 µs by the early 1970s, and by the mid-70s it was down to 600 ns (0.6 µs). Some designs had substantially higher performance: the CDC 6600 had a memory cycle time of 1.0 µs in 1964, using cores that required a half-select current of 200 mA. Everything possible was done in order to decrease access times and increase data rates (bandwidth), including the simultaneous use of multiple grids of core, each storing one bit of a data word. For instance, a machine might use 32 grids of core with a single bit of the 32-bit
In computer architecture, 32-bit computing refers to computer systems with a processor, memory, and other major system components that operate on data in 32-bit units. Compared to smaller bit widths, 32-bit computers can perform large calculation ...
word in each one, and the controller could access the entire 32-bit word in a single read/write cycle.
Core memory is non-volatile storage—it can retain its contents indefinitely without power. It is also relatively unaffected by EMP and radiation. These were important advantages for some applications like first-generation industrial programmable controllers
A programmable logic controller (PLC) or programmable controller is an industrial computer that has been Rugged computer, ruggedized and adapted for the control of manufacturing processes, such as assembly lines, machines, robotic devices, or ...
, military installations and vehicles like fighter aircraft
Fighter aircraft are fixed-wing military aircraft designed primarily for air-to-air combat. In military conflict, the role of fighter aircraft is to establish air superiority of the battlespace. Domination of the airspace above a battlefield ...
, as well as spacecraft
A spacecraft is a vehicle or machine designed to fly in outer space. A type of artificial satellite, spacecraft are used for a variety of purposes, including communications, Earth observation, meteorology, navigation, space colonization, p ...
, and led to core being used for a number of years after availability of semiconductor
A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...
MOS memory (see also MOSFET
The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...
). For example, the Space Shuttle
The Space Shuttle is a retired, partially reusable low Earth orbital spacecraft system operated from 1981 to 2011 by the U.S. National Aeronautics and Space Administration (NASA) as part of the Space Shuttle program. Its official program na ...
IBM AP-101B flight computers used core memory, which preserved the contents of memory even through the ''Challenger
Challenger, Challengers, or The Challengers may refer to:
Entertainment
Comics and manga
* Challenger (character), comic book character
* ''Challengers'' (manga), manga by Hinako Takanaga
Film and TV
* ''The Challengers'' (TV series), a 1979 ...
''s disintegration and subsequent plunge into the sea in 1986. Another characteristic of early core was that the coercive force was very temperature-sensitive; the proper half-select current at one temperature is not the proper half-select current at another temperature. So a memory controller would include a temperature sensor (typically a thermistor
A thermistor is a type of resistor whose resistance is strongly dependent on temperature, more so than in standard resistors. The word thermistor is a portmanteau of ''thermal'' and ''resistor''.
Thermistors are divided based on their conduction ...
) to adjust the current levels correctly for temperature changes. An example of this is the core memory used by Digital Equipment Corporation
Digital Equipment Corporation (DEC ), using the trademark Digital, was a major American company in the computer industry from the 1960s to the 1990s. The company was co-founded by Ken Olsen and Harlan Anderson in 1957. Olsen was president unt ...
for their PDP-1 computer; this strategy continued through all of the follow-on core memory systems built by DEC for their PDP line of air-cooled computers. Another method of handling the temperature sensitivity was to enclose the magnetic core "stack" in a temperature controlled oven. Examples of this are the heated-air core memory of the IBM 1620 (which could take up to 30 minutes to reach operating temperature
An operating temperature is the allowable temperature range of the local ambient environment at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the de ...
, about and the heated-oil-bath core memory of the IBM 7090, early IBM 7094
The IBM 7090 is a second-generation transistorized version of the earlier IBM 709 vacuum tube mainframe computer that was designed for "large-scale scientific and technological applications". The 7090 is the fourth member of the IBM 700/7000 ser ...
s, and IBM 7030.
Core was heated instead of cooled because the primary requirement was a ''consistent'' temperature, and it was easier (and cheaper) to maintain a constant temperature well above room temperature than one at or below it.
In 1980, the price of a 16 kW (kiloword
In computing, a word is the natural unit of data used by a particular processor design. A word is a fixed-sized datum handled as a unit by the instruction set or the hardware of the processor. The number of bits or digits in a word (the ''word s ...
, equivalent to 32 kB) core memory board that fitted into a DEC Q-bus computer was around . At that time, core array and supporting electronics fit on a single printed circuit board about 25 × 20 cm in size, the core array was mounted a few mm above the PCB and was protected with a metal or plastic plate.
Diagnosing hardware problems in core memory required time-consuming diagnostic programs to be run. While a quick test checked if every bit could contain a one and a zero, these diagnostics tested the core memory with worst-case patterns and had to run for several hours. As most computers had just a single core memory board, these diagnostics also moved themselves around in memory, making it possible to test every bit. An advanced test was called a " Schmoo test" in which the half-select currents were modified along with the time at which the sense line was tested ("strobed"). The data plot of this test seemed to resemble a cartoon character called " Schmoo," and the name stuck. In many occasions, errors could be resolved by gently tapping the printed circuit board with the core array on a table. This slightly changed the positions of the cores along the wires running through them, and could fix the problem. The procedure was seldom needed, as core memory proved to be very reliable compared to other computer components of the day.
File:8 bytes vs. 8Gbytes.jpg, This microSDHC card holds 8 billion bytes (8 GB). It rests on a section of magnetic-core memory that uses 64 cores to hold eight bytes. The microSDHC card holds over one billion times more bytes in much less physical space.
File:Magnetic-core memory, 18x24 bits.jpg, Magnetic-core memory, 18×24 bits, with a US quarter
The quarter, short for quarter dollar, is a United States coin worth 25 cents, one-quarter of a dollar. The coin sports the profile of George Washington on its obverse, and after 1998 its reverse design has changed frequently. It has been produ ...
for scale
File:Magnetic-core memory close-up.JPG, Magnetic-core memory close-up
File:Magnetic-core memory, at angle.jpg, At an angle
See also
* Bubble memory
* Core dump
* Core rope memory
* Delay-line memory
* Electronic calculators
An electronic calculator is typically a portable electronic device used to perform calculations, ranging from basic arithmetic to complex mathematics.
The first solid-state electronic calculator was created in the early 1960s. Pocket-sized d ...
* Ferroelectric RAM
* Magnetoresistive random-access memory
* Read-mostly memory Read-mostly memory (RMM) is a type of memory that can be read fast, but written to only slowly.
Historically, the term was used to refer to different types of memory over time:
In 1970, it was used by Intel and Energy Conversion Devices to refer ...
(RMM)
* Thin-film memory
* Twistor memory
References
External links
Interactive Java Tutorial - Magnetic Core Memory
National High Magnetic Field Laboratory
at Columbia University
*
accessed 15 April 2006
''Byte magazine
''Byte'' (stylized as ''BYTE'') was a microcomputer magazine, influential in the late 1970s and throughout the 1980s because of its wide-ranging editorial coverage. "''Byte'' magazine, the leading publication serving the homebrew market ..."
'' ...
'', July 1976
Casio ''AL-1000'' calculator
– Shows close-ups of the magnetic core memory in this desktop electronic calculator from the mid-1960s.
Still used core memory
in multiple devices in a German computer museum
*
{{Authority control
History of electronic engineering
Non-volatile memory
Types of RAM