> On Apr 18, 2019, at 11:47 AM, Jon Elson via cctalk <[email protected]>
> 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
