Editing Magnetic-core memory (section)

===How core memory works===
[[File:PDP-8 core memory driver module 1.jpg|thumb|One of three inter-connected modules that make up an Omnibus-based (PDP 8/e/f/m) PDP-8 core memory plane.]]
[[File:PDP-8 core memory.jpg|thumb|One of three inter-connected modules that make up an Omnibus-based PDP-8 core memory plane.  This is the middle of the three and contains the array of actual ferrite cores.]]
[[File:PDP-8 core memory driver module 2.jpg|thumb|One of three inter-connected modules that make up an Omnibus-based PDP-8 core memory plane.]]

The most common form of core memory, ''X/Y line coincident-current'', used for the main memory of a computer, consists of a large number of small [[toroid]]al [[ferrimagnetic]] [[ceramic]] [[Ferrite (magnet)|ferrite]]s (''cores'') held together in a grid structure (organized as a "stack" of layers called ''planes''), with wires woven through the holes in the cores' centers. In early systems there were four wires: ''X'', ''Y'', ''Sense'', and ''Inhibit'', but later cores combined the latter two wires into one ''Sense/Inhibit'' line.<ref name=US3329940 /> Each toroid stored one [[bit]] (0 or 1). One bit in each plane could be accessed in one cycle, so each machine [[Word (computer architecture)|word]] in an array of words was spread over a "stack" of planes. Each plane would manipulate one bit of a word in [[parallel computing|parallel]], allowing the full word to be read or written in one cycle.

Core relies on the square hysteresis loop properties of the ferrite material used to make the toroids. An electric current in a wire that passes through a core creates a magnetic field. Only a [[magnetic field]] greater than a certain intensity ("select") can cause the core to change its magnetic polarity. To select a memory location, one of the X and one of the Y lines are driven with half the current ("half-select") required to cause this change. Only the combined magnetic field generated where the X and Y lines cross (the [[logical conjunction]]) is sufficient to change the state; other cores will see only half the needed field ("half-selected"), or none at all. By driving the current through the wires in a particular direction, the resulting [[electromagnetic induction|induced]] field forces the selected core's magnetic flux to circulate in one direction or the other (clockwise or counterclockwise). One direction is a stored ''1'', while the other is a stored ''0''.

The toroidal shape of a core is preferred since the magnetic path is closed, there are no magnetic poles and thus very little external flux. This allows the cores to be packed closely together without their magnetic fields interacting. The alternating 45-degree positioning used in early core arrays was necessitated by the diagonal sense wires. With the elimination of these diagonal wires, tighter packing was possible.<ref name=US3711839 />