Core, I am assuming, means "magnetic core memory". This was an
early type of persistent memory used on early computers. You could
put a core on a shelf and pull it down for use several years later,
and it would still have the data in memory as it was the day you
put it on the shelf. Core memory was used as permanent
random-access memory on early computers
A very small magnetic doughnut (a single core) would have three
wires passed through it. Two wires would be used to set or read the
position of the magnetic field of the particular core doughnut. A
third wire was used to carry the signal if the field was
reversed.
All magnetic fields can be reversed by creating a nearby
magnetic field at sufficient strength. Current in one of the
address wires was insufficient to cause a change in the magnetic
field of the doughnut core. But when both address wires carried
current at the same time, the magnetic field generated would flip
the field of the core to align with the field produced by the
current in the address wires.
When one of these tiny doughnuts switched magnetic poles a
current was then introduced into the sensor wire and read as a
change that might indicate a 1 or a zero. The machine would then
put the core back the way it was to preserve that value stored in
that memory location.
All of this wiring using these very tiny doughnuts of magnetic
material was difficult and cost a lot of money to produce. It was
all they had for permanent storage until transistors came into
common use.
