Core Trivia

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Some ferrite core toroids are so small that they cannot be viewed with the naked eye. The first picture on this page was taken with a first-generation digital camera through a microscope. It shows some cores of a DEC G114B core plane. The cores on the photograph have a capacity of about 50 bits. The region shown on the picture has a size of about 2.5 x 2.5mm². On the G114B plane, which is a "modern" core memory, the toroids are packed quite dense. The X-driver and Y-driver line can be seen clearly. The sense/inhibit line is in the dark.

The typical access speed of core memory ranges from 1200ns to 600ns. Accessing core memory using certain patterns can generate a lot of eletrical noise. Therefore, the parameters for pulse timing and sense amplification must be adjusted carefully.

Accessing memory locations in close neighborhood repeatedly can heat the core plane, resulting in increased resistance of the driver lines. The driver logic detects core heating using a thermal resistor and adjusts the driver current accordingly.

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Since a write cycle must always follow a read cycle when accessing core, some CPUs perform so-called read/modify/write operations. In such an operation, the CPU keeps the memory bus locked after reading the memory. It then performs the requested operation on the machine word extracted and writes the altered value back before unlocking the bus. This way, one write and one read cycle can be saved.

Some core memory subsystems use core stacks where multiple planes are arranged on top of each other. An additional Z-driver line is used to select a plane of the stack. Advantages of core stacks are higher density and shorter signal paths.

Core memory generates a lot of heat. For example, the G114B core plane with a capacity of 16K x 18 bits (which is equal to 32K bytes with ECC) has a power dissipation of more than 200 Watts.

Magnetic core memory is still used in spacecrafts, since cores are much less sensitive to cosmic radiation than modern transistor memory is.

Nils M Holm 2004, 2008