LISPy things you can do in 64K bytes of core

Magnetic Core Toroids

mag core toroids

(Back to the essay)

This is a picture of some ferrite toroids in a "modern" DEC G114B UNIBUS core memory plane that I have taken through a microscope at 50x magnification. Core memory was not that densely packed in the 1960's. The G114B was manufactured in the 1970's and had a capacity of 16K × 18bits. The same capacity would have filled a rack in the 1960's, but the G114B was a single UNIBUS board about the size of a sheet of paper (letter or A4).

G114B board

The core planes themselves are located under the green plastic cover in the middle of the board. With the cover removed they look like this:

core plane

And, zooming in a little bit closer:

core plane

Each plane consists of 32 rows holding 256 bits each, giving a total of 8192 tiny little toroids per plane. This is interesting, because the G114B is specified with a capacity of 16K×18-bit words, but the planes are not organized in words internally. There are 36 planes, making a grand total of 36×8192 = 294,912 toroids on a surface area of about 200 square centimeters. This is probably how far miniaturization went before core memory was replaced by MOS memory.

All pictures by Nils M Holm, probably taken in the late 1990's with a Sony MVC-FD5 digital camera.

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