A small ring, or core, of ferrite (a ferromagnetic ceramic) can be magnetized in either of two opposite directions (clockwise or counterclockwise). Therefore such a core can be used for storing a bit of information. For almost 15 years, 'core' has been the most important memory device.
Around 1950, memory was implemented by mercury and nickel-wire
delay lines, magnetic drums and 'Williams
tubes' (modified CRT's). The invention of core memory in 1949
was a leap forward in cost-effectiveness and reliability.
The invention of core memory is ascribed both to A. Wang (working
at the Harvard Computing Laboratory with H. Aiken) and W. Papian
(working with J.W. Forrester at MIT in Project Whirlwind). At least
the coincident current selection method of memory cores (see below)
is ascribed to the latter.
Core memory was used first in the Whirlwind computer besides
Williams tubes. Whirlwind [4], in a sense the first general-purpose
computer, was a U.S.Navy/MIT project and became operational in
1951.
The first commercial computer with a core memory (100.000 bits,
cycle time 17 microseconds) was the IBM 705, a vacuum tube based
computer delivered in 1955. In 1976, 95% of all computer main
memories consisted of ferrite cores, 20-30 billion of which were
yearly produced worldwide [1]. The price per bit of core memory was
20 dollarcent in 1960 and decreased from there with 19% per year.
In 1974 was the 'turn-over' to semiconductor (transistor) memory
with the advent of the 4 kbit chip; the cost was 1 cent per bit for
both techniques by then.
Core memory has been in use until recently for special purposes,
because it retains the information when the power is switched off,
and it is resistant against radiation.
The physical basis of core memory is the fact that a current
sent along a wire passing through a ferrite core sets a persisting
core magnetization, if the current exceeds a certain threshold. A
current in the opposite direction will reverse the direction of the
magnetization. In this event a voltage pulse will be induced in
another wire threaded through the core, the 'sense wire'; the
polarity of this pulse is determined by the original magnetization
direction. Clearly reading is a destructive operation, and as part
of the read cycle the original state of each core must be
restored.
Core memories were often organized as a planar matrix, the 'write'
wire being split up into two wires (row, column) each carrying half
of the threshold switching current. This made it possible to
address a specific core in the matrix for reading or writing. (Accordingly, this type of core memory is often referred to as 'coincident current memory'; for a different type of core memory, see our description of the HP9100 calculator). See
the illustration below, taken from [2].
