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Core memory was a common form of random access memory (RAM) from the mid-1950s to the mid-'70s, and It was developed at MIT in 1951. The memory made use of magnetic rings called cores that had wires passing through them for selecting and detecting the contents of the cores. With the introduction of memory based on semiconductor technology, core memory became obsolete, though some still call the main memory of a computer the core memory.
Core memory is also known as magnetic-core memory.
The function of core memory was based on hysteresis of the magnetic material used to make the rings. Each core in the core memory was used to store one bit of information. The cores can be magnetized via clockwise and counterclockwise direction. The value stored in the core depended upon the direction of magnetization. Access to core memory involved read and write cycles. The read cycle would cause the memory contents to be lost, whereas the write cycle would restore the contents of the memory location. A read cycle must be followed by a write cycle. Another salient feature of core memory is non-volatility, meaning its contents are not lost once power is removed. Special logic was included in the memory controller to ensure the memory contents were not altered unless the power supplies were at their normal values.
Non-volatility was one of the biggest advantages with core memory in the early years of memory development.
Core memory was fairly slow and initially expensive to fabricate. Being magnetic in nature, it was vulnerable to the effects of interference. Adjustments with respect to sense levels, drive currents and memory timing were required in the case of core memory. Time-consuming applications were required to diagnose hardware problems in core memory.