GoKawiilOne interesting side effect of the twistor concept was noticed in production: under certain conditions, passing a current through one of the electrical wires running inside the tape would cause the magnetic fields on the tape to move in the direction of the current. If used properly, it allowed the stored bits to be pushed down the tape and pop off the end, forming a type of delay-line memory , but one where the propagation of the fields was under computer control, as opposed to automatically advancing at a set rate defined by the materials used. However, such a system had few advantages over twistor memory, especially as it did not allow random access.
Twistor memory is essentially a version of core memory that replaces the "cores" with pieces of magnetic tape . The main advantage of twistor memory is its ability to be assembled by automated machines, as opposed to core memory, which was almost entirely manually assembled. AT&T had great hopes for twistor memory, believing that it would greatly reduce the cost of computer memory and put them in an industry leading position. Instead, DRAM memories came onto the market in the early 1970s and rapidly replaced all previous random-access memory systems. Twistor memory ended up being used only in a few applications, many of them AT&T's own computers.
Bubble memory is largely the brainchild of a single person, Andrew Bobeck . Bobeck had worked on many kinds of magnetics-related projects through the 1960s, and two of his projects put him in a particularly good position for the development of bubble memory. The first was the development of the first magnetic-core memory system driven by a transistor -based controller, and the second was the development of twistor memory .
Development [ edit ]
Bubble domain visualization by using CMOS-MagView
Bubble memory driver coils (windings, or field coils) and guides (T bar guides in this case). The guides, or propagation elements, are on top of a magnetic film, which is on top of a substrate chip. This is mounted to a PCB (not shown) and then surrounded by two windings, shown in yellow and blue.
In 1967, Bobeck joined a team at Bell Labs and started work on improving twistor memory. The memory density of twistor memory was a function of the size of the wires; the length of any one wire determined how many bits it held, and many such wires were laid side-by-side to produce a larger memory system.
Conventional magnetic materials, like the magnetic tape used in twistor memory, allowed the magnetic signal to be placed at any location and to move in any direction. Paul Charles Michaelis working with permalloy magnetic thin films discovered that it was possible to move magnetic signals in orthogonal directions within the film. This seminal work led to a patent application.[2] The memory device and method of propagation were described in a paper presented at the 13th Annual Conference on Magnetism and Magnetic Materials, Boston, Massachusetts, 15 September 1967. The device used anisotropic thin magnetic films that required different magnetic pulse combinations for orthogonal propagation directions. The propagation velocity was also dependent on the hard and easy magnetic axes. This difference suggested that an isotropic magnetic medium would be desirable.
This led to the possibility of making a memory system similar to the moving-domain twistor concept, but using a single block of magnetic material instead of many twistor wires. Starting work extending this concept using orthoferrite, Bobeck noticed an additional interesting effect. With the magnetic tape materials used in twistor memory, the data had to be stored on relatively large patches known as domains. Attempts to magnetize smaller areas would fail. With orthoferrite, if the patch was written and then a magnetic field was applied to the entire material, the patch would shrink down into a tiny circle, which he called a bubble. These bubbles were much smaller than the domains of normal media like tape, which suggested that very high area densities were possible.
Five significant discoveries took place at Bell Labs:
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