Magnetic-core memory

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. Two or more wires pass through each core. Magnetic hysteresis 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 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 (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 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.