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Core memory

Timothy van der Linden writes:
 > To Steve,
 > Thanks for explaining the core-memory so clearly (Your ascii-art looks great)
 > >Although it takes more current to read or write a core than it takes to
 > >update a semiconductor memory cell, no power is required to maintain the
 > >state of core memory.  It wasn't until about the mid-70s that
 > >semiconductor memory became all of faster, denser, less power-intensive,
 > >and cheaper than equivalent amounts of core memory, and core was still
 > >used for some time after that.
 > Why doesn't the magnetic field in the ferrite core degrade? (I assume the
 > ferrite has some resistance.)

Have you ever known other magnets to degrade?  Once magnetized, a
material won't demagnetize unless exposed to heat above its Curie point
or another sufficiently strong magnetic field.

Magnetic materials have a certain amount of resistance to being
magnetized.  An external magnetic field won't affect the material until
it exceeds a certain strength; then the material is magnetized to the
orientation of the externally-applied field.  This is why only the core
at the intersection of the two address wires is magnetized; the current
sent through the address wires creates a magnetic field around the wires
lower than this threshold, but the sum of the fields at the intersection
of the wires is greater than this threshold.

 > Timothy