Core memory Re: Old farts and new Linux

jludwig wralphie at
Tue May 4 16:59:07 UTC 2004

Keith corrected me on this which is ok since I had an Old (brain) fart!
It's nice to see someone who knows or remembers, and I find this
reminder very interesting and educational.
On Mon, 2004-05-03 at 19:23, Keith Lofstrom wrote:
> You wrote:
> > For those who don't know what core memory is, it's miniature ferrite
> > beads strung together with three wires through them to read/write
> > [magnetize and demagnetize], and reset if I remember correctly. I have
> > seen 16 MB of core memory, it was the size of a full size tower case.
> Some corrections of fact, which I will share with you privately.
> You can research and post;  it is always better to correct yourself
> than have someone else do it publically.   
> The three wires in a core memory are X select, Y select, and sense.
> The cores work by pushing an X current and a Y current through an
> intersection; two currents pushed the hysteretic toroidal magnetic
> core past a threshold, either causing it to flip magnetic polarity
> (generating a little voltage blip on the sense line) or not, depending
> on previous magnetization.  After flipping a core during a read, you
> had to schedule a reverse set of currents on the same X and Y wires to
> put the core back.  The size of the arrays were limited by the number
> of cores you could put on a sense wire before accumulated noise made
> reading unreliable.
> A friend in high school wanted to build a core memory, and wrote a letter
> to Honeywell.  They sent him a one pound jar of 15mil cores.  All out
> of spec, as near as we could measure.
> What you probably saw was a 16*K* byte memory, the boards from the late
> 60s were typically around 16K bits, 8 boards to a byte, and about as big
> on a side as a tower case with the small scale integration driver chips
> around the sides of the core fabric.  A 16MB machine would have been
> gargantuan, indeed;  I would guess 100 or more 19inch racks, 8 feet tall,
> and probably costing on the order of $50M in current dollars.  
> Keith
jludwig <wralphie at>

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