Magnetic Bubble Memory Definition/Meaning:
A type of digital memory in which data is represented by
magnetic bubbles that are made to move through a stationary planar medium; the
bubbles are tiny circular areas (stable magnetic domains) in which the medium is
magnetized in the reverse direction to the rest of the medium. Bubble memory
thus differs from magnetic tape and disk stores, in which the medium moves and
me data bits are stationary with respect to it. It belongs to the class of
magnetic domain memory but is the only form in current use.
In the commonest form the medium is a thin layer of complex garnet, deposited
epitaxially on a substrate of a simpler garnet A pattern of metal shapes is
deposited on the surface of the epitaxial layer. The preferred direction of
magnetization of the epitaxial layer is perpendicular to its plane, in either
direction. When a suitable biasing magnetic field is applied perpendicular to
the layer, magnetic bubbles can form, magnetized in the opposite sense to this
field. If an additional magnetic field is applied in the plane of the layer the
bubbles will move through the layer, the pattern of metal shapes influencing the
way in which they move. Usually this field is made to rotate regularly,
typically at 100,000 revolutions per second, and the metal shapes are arranged
in regular rows so that each bubble moves one step through the pattern for each
cycle of rotation. Means are provided for inserting bubbles at the entry to the
pattern and detecting their arrival at the exit; the device therefore acts as a
shift register.
A typical bubble device stores one megabit of data. There is
usually only one input point and one output point per device so that data flow
is serial. However, within the device it is usual to divide the data path into a
series of parallel loops, with means of distributing the data stream between
them, so that the access time to a
particular item of data is only a fraction of the time taken to fill the whole
device with data. Typically the data rate is 100 kilobits per second and the
access time 30 milliseconds. A number of such devices can be used together to
provide a greater total capacity, and if required these can be arranged so that
several are accessed in parallel to give a higher data rate.
The performance and
cost per bit of bubble memory is intermediate between that of semiconductor
memory and magnetic disk. It is nonvolatile (if the bias field is provided by
permanent magnets, which is usual) and resistant to cosmic rays and similar
particles. Having no moving parts it is more rugged than disk memory. Bubble
memory has found limited application where these properties are of value, e.g.
in portable computer terminals and in satellites. However the initial promise of
the technology (pioneered by Bell Telephone Laboratories) has not been realized
in full since manufacturing costs have proved higher than expected.
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