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The problem of memory
Forrester had solved most of the problems in the design of Whirlwind but one remained - memory. At the time most memory was serial. The first fast large memories were based on mercury delay lines that kept a serial stream of bits circulating as sound pulses. The Williams tube was a faster version of the same principle. A CRT display tube formed a pattern of bits as spots of light which was recirculated using a photocell and feedback amplifier. It was faster but the tubes burned out far too frequently.
The Whirlwind used a modified form of the Williams tube. An additional flood gun maintained the pattern of dots while a writing gun was used to alter the pattern. Thirty two such tubes were needed to provide the 4KBytes of storage that the Whirlwind needed. Given a tube life of one month and cost $1000 the running cost of the machine was very high - $1 per bit per month.
Forrester realised that storage was the critical problem. The whole project depended on finding a more reliable and more economical method of storage.
He started to think about ways of making a 2D or 3D form of storage rather than the one-dimensional recirculating method of storage represented by the delay line and Williams tube. After spending much time thinking about the problem Forrester encountered an article on the use of magnetic materials as amplifiers. He ordered some of the material and built an array that passed current through rings of the material to magnetise it in one of two directions.
This worked but it was too slow. Then the breakthrough! Forrester came up with a scheme that involved threading rings of the magnetic material on an x-y grid of wires. Each ring or core was threaded onto a unique pair of x-y wires. A third read/write wire was threaded through all of the cores. To read or write a bit half of the current needed to change the magnetisation of a core was placed on one of the x wires and on one of the y wires. Only the core at the intersection of the two wires was subject to a current sufficient to change its polarity. This enabled a direct access to each bit in the array.
The original patent for core memory
Today this sounds like an obvious method but then it was quite new. Forrester had doubts that it would work. Perhaps the repeated exposure to half the current needed to change the polarity would eventually cause a slow degradation in the state of the core. It didn't and coincident current core memory worked! A special test bed computer was built just to verify the principle. Then in 1953 Whirlwind was equipped with a new core memory that doubled its speed, improved its reliability and made it cheaper to keep running.
Close up of part of a Whirlwind core
Core memory was just what the developing computer industry need - you would think that the world would beat a path to Forrester's door. It didn't at first but as Forrester himself said
"it took three or four years for the industry accepted the notion and then it took the next seven years to convince them that they hadn't all thought of it first!"