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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 called simply base memory. Main memory uses toroids (rings) of a hard magnetic material (usually a semi-hard ferrite) as the core of the transformer. Each core stores one bit of information. A core can be magnetized clockwise or counterclockwise. The bit value stored in a core is zero or one depending on the magnetization direction of that core. Pulses of electric current in some wires through a core allow you to set the direction of magnetization in that core in either direction, thus storing either a zero or a zero. Another wire through each core, the sense wire, is used to detect if the core has changed state. The core reading process causes the core to be reset to zero, thereby erasing it. This is called destructive reading. When not read or written, the cores retain the last value they had, even if the power is off. Therefore they are, a type of non-volatile memory.

 

Wikipedia Magnetic Core Memory