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Appendix F:

Bubble Memories:

Certain materials (ie. gadolinium gallium garnet) are magnetizable easily in only one direction. A film of these materials can be created so that it's magnetizable in an up-down direction. The magnetic fields tend to stick together, so you get a pattern that is kind of like air bubbles in water squished between glass, half with the north pole facing up, half with the south, floating inside the film. When a vertical magnetic field is imposed on this, the areas in opposite alignment to this field shrink to circles, or 'bubbles'.

A bubble can be formed by reversing the field in a small spot, and can be destroyed by increasing the field.

The bubbles are anchored to tiny magnetic posts arranged in lines. Usually a 'V V V' shape or a 'T T T' shape. Another magnetic field is applied across the chip, which is picked up by the posts and holds the bubble. The field is rotated 90 degrees, and the bubble is attracted to another part of the post. After four rotations, a bubble gets moved to the next post:

    o                             o              o
     \/   \/       \/   \/      \/   \/      \/   \/

    o_|_   _|_      _|_   _|_     _|_o  _|_      _|_ o _|_     _|_  o_|_
         |           o  |             |              |             |
I hope that diagram makes sense.

These bubbles move in long thin loops arranged in rows. At the end of the row, the bits to be read are copied to another loop that shift to read and write units that create or destroy bubbles. Access time for a particular bit depends on where it is, so it's not consistent.

One of the limitations with bubble memories, why they were superceded, was the slow access. A large bubble memory would require large loops, so accessing a bit could require cycling through a huge number of other bits first. The speed of propagation is limited by how fast magnetic fields could be switched back and forth, a limit of about 1 MHz. On the plus side, they are non-volatile, but eeproms, flash memories, and ferroelectric technologies are also non-volatile and and are faster.

Ferroelectric and Ferromagnetic (core) Memories: . . .

Ferroelectric materials are analogous to ferromagnetic materials, though neither actually need to contain any iron. Ferromagnetic materials, used in core memories, will retain a magnetic field that's been applied to it.

Core memories consist of ferromagnetic rings strung together on tiny wires. The wires will induce magnetic fields in the rings, which can later be read back. Usually reading this memory will erase it, so once a bit is read, it is written back. This type of memory is expensive because it has to be constructed physically, but is very fast and non-volatile. Unfortunately it's also large and heavy, compared to other technologies.

Ferroelectric materials retain an electric field rather than a magnetic field. like core memories, they are fast and non-volatile, but bits have to be rewritten when read. Unlike core memories, ferroelectric memories can be fabricated on silicon chips.

Legend reports that a Swedish jet prototype (the Viggen I believe) once crashed, but the magnetic tape flight recorders weren't fast enough to record the cause of the crash. The flight computers used core memory, though, so they were hooked up and read out, and the still contained the data microseconds before the crash occurred, allowing the cause to be determined. A similar trick was used when investigating the crash of the Space Shuttle Challenger.

On a similar note, the IBM 7740 communication controller was shipped with diagnostics code in its core memory, so it could be checked out on arrival without a host machine being operational. Faulty military equipment using core memory often had to be escorted by military security personnel because the data within it could not be erased until it was repaired.

Interestingly enough, newer flight recorders have replaced magnetic tape with flash memories, which is a newer and more reliable form of EEPROM (Electronically Erasable Programmable ROM). This actually has nothing to do with either ferromagnetic or ferroelectric memories, though. Oh well, this is an appendix. Who reads appendices anyway?

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