page_title: Magnetic-core memory
hierarchical_section_title: Magnetic-core memory
caption_attribution_description: English: Core Memory Module
context_page_description: Magnetic-core memory was the predominant form of random-access computer memory for 20 years between about 1955 and 1975.
  Such memory is often just called core memory, or, informally, core.
  Core memory uses toroids of a hard magnetic material as transformer cores, where each wire threaded through the core serves as a transformer winding. Three or four wires pass through each core.
  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 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 memory.
context_section_description: Magnetic-core memory was the predominant form of random-access computer memory 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 (usually a semi-hard ferrite) as transformer cores, where each wire threaded through the core serves as a transformer winding. Three or four wires pass through each core.
  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 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 memory.
  Using smaller cores and wires, the memory density of core slowly increased, and by the late 1960s a density of about 32 kilobits per cubic foot was typical. However, reaching this density required extremely careful manufacture, 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 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), the Intel 1103, 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 essential to NASA'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 real core memory.  The files that result from saving the entire contents of memory to disk for debugging purposes when a major error occurs are still anachronistically called "core dumps".
caption: A 32 x 32 core memory plane storing 1024 bits (or 128 bytes) of data
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