There is a broad range of types of computer memory.
Most of them are obsolete as computer technology has advanced rapidly.
One key in of memory technology is core memory, also known as magnetic core memory.
Magnetic core memory is a bang out of random-access memory or RAM first commercialized in 1951.
Each metal ring represents a single bit of storage.
The value of the bit is stored in the rings magnetic field.
This magnetic field can be driven by charging specific wires.
Data can be read from a core by setting it to 0.
If it was already set to 0, nothing happens.
As it’s possible for you to probably imagine, memory density wasnt great.
The metal rings were each 2.5mm in diameter when magnetic core memory was first invented.
The rings also needed to be physically separated from each other.
By 1966, the rings were down to just 0.33mm in diameter.
This decreased manufacturing time and material costs.
It also reduced the power consumption, based on the rings volume, by a factor of 125.
One example was the 1.2MiB of magnetic core memory included in MITs PDP-6 computer in 1964.
At the time, this was considered an unimaginably large amount of memory.
It costs $380000, or four cents per bit.
Early work on the concept began in the 1940s.
Magnetic core memory uses a 2D array of cores.
Each row and column of cores could use a single cable to drive the cores.
Another two wires were required, a sense and inhibit wire.
These went diagonally through all of the cores in the array.
Multiple arrays could be stacked in a third dimension, though they were electrically independent.
This could significantly increase performance by spreading each byte across various banks.
The array structure and the wires need to be run through the cores made manufacturing difficult.
Part of the difficulty came from the cores needing alternately angled at 45 degrees.
An early advancement was a machine that could thread straight-charge wires.
Later, economics was further enhanced by combining the sense and inhibit wires that were never needed simultaneously.
It was also unaffected by EMP events and radiation and relatively resilient to physical shock.
This saw its use in military aircraft, especially those designed to carry nuclear weapons.
Magnetic core memory was temperature sensitive when writing data.
Some systems changed the voltage based on a temperature sensor to allow this.
Hardware issues were tricky to diagnose; software diagnostics were slow but thorough.
Due to the general reliability of core memory, though, this wasnt a common issue.
Magnetic core memory was superseded in 1975 by semiconductor DRAM chips.
