Magnetic Core Memory

Magnetic core memory is a type of computer memory that was developed in the early 1950's. In core memory, each bit of information is represented by the magnetic polarity of a core. A core is a small ferrite toroid that is magnetized by wires threaded through the hole in its center. These wires are called the driver lines. The polarity of a core can be changed by driving current through its driver lines. For example, north-south polarity could represent logical zero and south-north polarity could represent logical one.

Cores are arranged in matrixes called core planes. A core plane usually contains some thousand cores. There are three wires threaded through each core. They are shown in diagram D1. The wire connecting the horizontal row is called the X-driver line (cyan) and the two wires per vertical row are called the Y-driver (yellow) line and the sense/inhibit line (magenta).

The sense/inhibit line may be combined with the X-driver, too. The arrangement varies, but there are always three lines per core. To polarize a core, current must be driven through both the X-driver and Y-driver line connected to that core. Driving current through only one of the lines is not sufficient to change the polarity of a core. Therefore, each core can be selected individually by chosing the proper combination of X-driver and Y-driver lines.

The picture shows a DEC G114B UNIBUS core memory module. The core plane itself is hidden under a fiber shield that protects the cores.

Nils M Holm 2004, 2008