Electrically, core memory is fairly simple. Each row and each column has a wire that goes through each bead in the row or column, obviously they cross at right angles at each bead. To read or write a bit into the bead one half of the current necessary to flip the magnetic state of the ferrite bead is passed through the appropriate row and column wires. Where they cross, and only there, the ferrite bead receives sufficient energy to set its state to ‘0’ or ‘1’ depending on the direction of the current. The magnetic flux in the bead was clockwise for one state and anticlockwise for the other.
A single ‘sense’ wire snakes its way through every bead, that’s the wire you can see on the left and the right with the loops in it. To read the state of a bit, the bit is reset to the ‘0’ state. If the bit is already ‘0’, no change occurs in the magnetic flux of the ferrite bead. If however, the bit had been ‘1’, resetting it to ‘0’ flips it’s state and this causes a change in the magnetic flux. The ‘sense’ wire, which passes through the centre of the ferrite bead, gets a small current induced in it by this change. That current is amplified and read as a ‘1’ by the logic surrounding the memory plane.
If the bit is a ‘1’ the logic creates a further memory cycle to write a ‘1’ back to the bit. Obviously this cycle is unncessary if the bit was a ‘0’.
Unlike RAM memory today, ferrite core memory does not require power to keep it’s data intact. It was quite common to turn a machine off for service or storage and some considerable time later (even months) on turn-on the core memory was still fully intact and the machine could be run without going through the ‘boot loader’ process.
A trade-off with core memory was the amount of heat generated from the relatively high currents required to switch the magnetic flux. The early core modules were bathed in oil to keep them cool. I am told by the previous owner, who used the machine these planes came from, that the original memory bank cost the customer over $1m USD.
My early experience was with DEC PDP 8/e processors. The system at the Reserve Bank in Wellington had 16 K words of core memory organised as 4 banks of 4k 12 bit words. It also had a drum memory, a very unusual storage medium even at the time. More on that elsewhere in due course, and no, I don’t have a real drum memory.
Back with IBM computers, and just to prove that not much really changes in the computer industry, there used to be a joke circulating that if the computer was slow, it just needed the bird call ‘mawcaw, mawcaw’ to fix it. (pronounced more-core, more-core).