The weird stuff I keep finding: 19 bit core memory?

Eric Smith spacewar at
Fri Oct 23 12:57:01 CDT 2020

On Wed, Oct 21, 2020 at 6:22 AM Pontus Pihlgren via cctalk <
cctalk at> wrote:

> On Tue, Oct 20, 2020 at 11:26:22PM -0400, Chris Zach via cctalk wrote:
> > > Or maybe a PDP-15?  18 bits plus parity.
> >
> > Possible, did the pdp15 use that type of board?
> >
> I have seen PDP-15 core memory and it is not that format. It looks like
> the memory modules from a PDP-8/I or -8/L

The ME15 memory for the PDP-15 used that kind of memory, but it normally
used an 18-bit  H215. I think there was another variant that offered parity
that would have used the 19-bit H216.

There might have been PDP-15 memory earlier than the ME15, but at the time
of the PDP-15 introduction (1970), H21x core planes were used in most DEC
machines, so it would have been surprising for them to use something else.
Perhaps some PDP-15 systems were upgrades from the PDP-9, which used memory
modules similar to the PDP-8/I.

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