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Twistor memory
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Twistor memory

Twistor is a form of computer memory, similar to core memory, formed by wrapping magnetic tape around a current-carrying wire. Although the developers, Bell Labs, had high hopes for Twistor, it was used for only a brief time in the marketplace between about 1968 and the mid-1970s. In this period all previous forms of memory were quickly replaced by RAM chips, which were considerably faster and less expensive. Twistor is largely the brainchild of a single person, Andrew Bobeck, who later developed bubble memory after generalizing some of the concepts used in Twistor.

In core memory small circular magnets, the cores, are threaded by two crossed wires, X and Y, to make a matrix known as a plane. When one X and one Y wire are powered, a magnetic field is generated at a 45 degree angle to the wires. The core magnets sit on the wires at a 45 degree angle, so the single core wrapped around the crossing point of the powered X and Y wires will pick up the induced field. Core was non-volatile, fast compared to earlier systems, and fairly dense. The main concern with core was the time needed to wire up all the tiny little magnets, which grew in complexity as the density of the core planes was increased.

Twistor was similar in concept, but instead of small circular magnets, twistor used magnetic tape to store the patterns. The tape was wrapped around one set of the wires, say X, in such a way that it formed a 45 degree helix. The Y wires were replaced by solenoids wrapping a number of twisor wires. Selection of a particular bit was the same as in core, with one X and Y line being powered, generating a field at 45 degrees. The magentic tape was specifically selected to only allow magnetization along the length of the tape, so only a single point of the twistor would have the right direction of field to become magnetized.

In core a third wire, the sense/inhibit line, is needed to write or read a bit. This line is shared by all of the cores in a particular plane, meaning that only one bit can be read (or written) at once. Core planes were typically stacked in order to store one bit of a word per plane, and a word could be read or written in a single operation by working all of the planes at once.

One major advantage of twisor is that the entire row of twistors enclosed by one of the Y solenoids can be read or written at the same time, because there is no need for a sense/inhibit line. Unlike core, the X and Y lines do not both have to be driven in order to read a bit, instead only the Y is used and the X can be used as the sense line. Each of the X lines in a stack of twistor can then be read at the same time.

Oddly enough, twistor typically also included a single plane of core. In this case the core was used to select which solenoid to power, as opposed to being used as memory, the single magnetized core feeding power into a particular solenoid. This is actually the original purpose of core, to select a single element out of an X and Y grid of powered lines, the concept of using the generated magnetic field as a storage system developed later.

The major advantage to twistor was mechanical however; twistor elements could be built using completely automated machines. Typically the magnetic tape was wound around a copper wire, with a stack of such wires being embedded in a mylar sheet. The sheet could then be folded accordion-style to make a smaller block, and then inserted through a row of solenoids.

It was this reason that led Bell to spend so much time developing the system, because they believed twistor could lead to considerably lower cost memory systems, leaving them supplying the majority of computer makers. Instead the solid-state memory systems "came out of nowhere" and twistor never had time to become established before all core disappeared.

Twistor was used in a number of applications however. Much of the development funding was supplied by the US Air Force, as twistor was to be used as the main memory in the Nike X project. Bell also used twistor as the main memory in their groundbreaking ESS series of electronic telephone switches, and did so up to the 4ESS introduced in 1976 and sold into the 1980s.

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