Twistor memory is a form of computer memory formed by wrapping magnetic tape around a current-carrying wire to make a device called a twistor. Operationally, twistor memory was very similar to core memory. Twistors could also be used to make ROM memories, including a re-programmable form known as a piggyback twistor. Both forms were able to be manufactured using automated processes, which was expected to lead to much lower production costs than core-based systems. Introduced by Bell Labs in 1957, the first commercial use was in their 1ESS switch which went into operation in 1965. Twistor memory was used only briefly in the late 1960s and early 1970s, when semiconductor memory devices replaced almost all earlier memory systems. The basic ideas behind twistor memory also led to the development of bubble memory, although this had a similarly short commercial lifespan.
Core memory
Construction
In core memory, small ring-shaped 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 be affected by the induced field. The materials used for the core magnets were specially chosen to have a very "square" magnetic hysteresis pattern. This meant that fields just below a certain threshold will do nothing, but those just above this threshold will cause the core to be affected by that magnetic field; it will abruptly flip its magnetization state. The square pattern and sharp flipping states ensures that a single core can be addressed within a grid; nearby cores will see a slightly different field, and not be affected.
Data retrieval The basic operation in a core memory is writing. This is accomplished by powering a selected X and Y wire both to the current level that will, by itself, create ½ the critical magnetic field. This will cause the field at the crossing point to be greater than the core's saturation point, and the core will pick up the external field. Ones and zeros are represented by the direction of the field, which can be set simply by changing the direction of the current flow in one of the two wires. In core memory, a third wire - the sense/inhibit line - is needed to write or read a bit. Reading uses the process of writing; the X and Y lines are powered in the same fashion that they would be to write a "0" to the selected core. If that core held a "1" at that time, then the magnetic state flips to a "0" and the transition causes a short pulse of electricity to be induced into the sense/inhibit line. If no pulse is seen, then no flip occurred, thus the core already held a "0". This process is destructive; if the core did hold a "1", that pattern is destroyed during the read, and has to be re-set in a subsequent operation. The sense/inhibit 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. Between reads or writes the data was stored magnetically. This means that core memory is a non-volatile memory.
Manufacturing Manufacturing core memory was a major issue. The X and Y wires had to be threaded through the cores in a weave pattern, and the sense/inhibit line passed through every core in a plane. In spite of considerable effort, no one successfully automated the production of core memory, which remained a manual task into the 1970s. To increase memory density one had to use smaller cores, which greatly increased the difficulty of wiring them onto the lines.
… excerpt ends here. Continue reading the full article.
