Core memory

core memory
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].
core drawing
Core diameter varied from several millimeters in the early days to barely visible dimensions in the 80's. Typically, a core memory consisted of a stack of 64*64 or 128*128 matrix planes ('mats'). However essentially two-dimensional core memories often much larger than 128*128 were also built. Examples of both techniques are shown (both in vivo and in vitro) in the UvA Computer Museum.
large cores

Top illustration: detail of a 64*64 cores matrix. The core diameter is 2.54 mm. Each core is traversed by four wires in this design. Twelve of these planes were stacked in a memory system of Ferroxcube Inc. The stack was contained in a heavy 19" rackmount box, together with power supply and driver electronics.
Similar stacks, however built from 1.2 mm cores with 5 read/write wires were used in the Control Data Company's 6000 mainframes, around 1968. About 80,000 of these compact 4k*12 modules (measuring 17*17*10 cm) were built by CDC.
Bottom illustration: Detail of early core memory module, probably of British origin.

References:

  1. Middelhoek, George and Dekker: Physics of Computer Memory Devices. Academic Press 1976
  2. Haley and Scott (eds): Analogue and Digital Computers. George Newnes, London 1960
  3. MF11-U/UP core memory system maintenance manual. Digital Equipment Corporation 1973
  4. Redmond and Smith: Whirlwind, the History of a Pioneer Computer. Digital Equipment Corporation 1980

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