Plane of core memory
paulkoning at comcast.net
Thu Apr 18 12:13:26 CDT 2019
> On Apr 18, 2019, at 11:47 AM, Jon Elson via cctalk <cctalk at classiccmp.org> 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.
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.
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