Core memory Re: Old farts and new Linux

Jeff Vian jvian10 at
Wed May 5 00:16:13 UTC 2004

jludwig wrote:

>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.  
yeah, things were big and expensive.
In 1990 the military base where I was stationed finally upgraded the IBM 
mainframe they used to 64 mb memory.  They had been running on 16 mb and 
that was almost impossible to get approval for the expense.

And in one PC we needed more drive space.  A 400MB hard drive was $1600. 
 Now we cannot even load an OS on a drive that size.


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