Plane of core memory

dwight dkelvey at
Thu Apr 18 23:18:26 CDT 2019

Although, after written, there is little magnetism lost out side of the ring, while being magnetized, there is quite a bit of stray magnetism. By placing the the rings at 90 degrees, it minimizes the magnetism induced in the adjacent ring. The fields follow the inverse square law so the effect drops off quite quickly. Also the ring tend to pull the magnetic field into the ring, at least until saturated. At that time the field can leak into a neighbor and flip its state. Not being aligned with the direction of the ring also minimizes this stray field.

From: cctalk <cctalk-bounces at> on behalf of Anders Nelson via cctalk <cctalk at>
Sent: Thursday, April 18, 2019 6:01 PM
To: paulkoning at; General Discussion: On-Topic and Off-Topic Posts
Subject: Re: Plane of core memory

I believe I read they weaved the planes this way to minimize crosstalk, EMI
or heat.


On Thu, Apr 18, 2019, 1:13 PM Paul Koning via cctalk <cctalk at>

> > On Apr 18, 2019, at 11:47 AM, Jon Elson via cctalk <
> cctalk at> wrote:
> >
> > On 04/18/2019 04:49 AM, Brent Hilpert via cctalk wrote:
> >> It's a 4-wire 3D planar array. By topology and construction I would
> guess it date it from the 60s.
> > Make that EARLY '60s.  As soon as somebody figured out that you could
> combine the sense and inhibit wires, everybody immediately went to 3-wire
> planes.
> >
> > Jon
> Is that true even for the highest speed designs?
> CDC 6000 series memory is unusual in that it has 5 wires per core.
> Instead of the classic X, Y, Inhibit, Sense it has two inhibit wires,
> routed in the X and Y direction.  There are four X and four Y inhibit
> wires, each of which run through 1/4th of the cores, so a given inhibit
> pair acts on 1/16th of the cores.
> The documentation doesn't spell out why this is done.  My guess is that it
> makes the various driven wires more alike in how many cores they pass
> through.  X and Y, in the 12 bit stack, pass through 64 * 12 cores.  Each
> inhibit wire passes through 64 * 16 cores, i.e., nearly the same number.
> And the driver circuits for all these wires are the same.
> A regular full-plane inhibit wire would pass through 4k cores, meaning the
> inductance is far higher than that of the X and Y wires.  So either the
> drive circuit would require a lot more power, or it would be significantly
> slower than the X/Y drive.
> As for separate sense, split inhibit obviously requires that, but even
> with conventional inhibit, keeping sense separate avoids the overhead of
> switching the signal path between two very different bits of circuitry.
> Compared to many other core memory designs of that same era, the 6000
> memory is quite fast, with access times of a few hundred nanoseconds and
> full cycle (read plus restore) in one microsecond.  Actually, comfortably
> under 1 microsecond, allowing for magic like read and update in one cycle
> (for the exchange instruction in the CPU) or read and write new data in one
> cycle via the ALU data path (in the PPUs).  I suspect the unusual core
> plane design was a factor in making this speed possible.
>         paul

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