Magnetic core memory is a type of non-volatile computer memory. With the use of small ceramic or ferrite magnetic rings it stores digital information by changing the polarity of the magnetic field of the rings. Used in electronic computers mainly from 1955 to 1975 instead of drum memories, cheap but slow, and thermoionic valves, fast but expensive, was replaced by semiconductor memories.
Core memory is used in the modules shown in the figure. The first module, consisting of multiple core memories stacked and originally mounted inside a metal container, belongs to a DEC PDP-8 computer, has an address equal to 12 bits and has a capacity of 1024 words. The second and third modules, on the other hand, consist of a single memory mounted on a printed circuit board, and belong respectively to a CII Mitra 15 computer, and to an Olivetti computer. The CII module has an address equal to 16 bits and has a capacity of 4096 words.
Insights
The magnetic rings are arranged on a two-dimensional grid and crossed by four wires, X, Y, Sense and Inhibition, which serve to carry the memory management signals. X and Y indicate the coordinates of the ring to be read or written, Sense reads the stored value, Inhibition prevents writing in that cell.
The rings, if subjected to a magnetic field, maintain it until a new magnetic field of adequate intensity arrives, capable of reversing the one stored previously by the ring.
To read the stored value from a ring, the corresponding X and Y lines are powered in order to register the value 0. If the ring has registered the value 1, an electrical pulse will be emitted on the Sense line. If the ring has registered the value 0, no pulse will be emitted. Note that reading changes the state of the memory, clearing it, and therefore requires a subsequent write operation to keep the stored value.
To write a value in a ring, as for writing, the corresponding X and Y lines are powered so that the value 1 is recorded. If the value to be recorded is 0, the Inhibition line is powered so as to neutralize the field magnetic created by X and Y. If, on the other hand, the value to be stored corresponds to 1, only X and Y are powered.
Fun facts
To maintain magnetic properties and thus operate reliably, magnetic core memory required a temperature-controlled environment. Depending on the model, it therefore had to be air-cooled or preheated to 41 ° C by hot air or oil baths. Consequently, it was necessary to wait up to 30 minutes before it could be used. Although magnetic core memory was a relatively reliable technology at the time, identifying faults was an extremely lengthy process. In fact, in order to be able to identify exactly which ring generated problems, it was necessary to carry out numerous reading and writing operations, comparing the read values with those written each time. Also, often a hotel The malfunction could be solved by simply tapping the memory, thereby realigning the rings.
Bibliography
Wikipedia: https://en.wikipedia.org/wiki/Magnetic-core_memory
