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

Eric Smith spacewar at
Fri Oct 23 14:35:38 CDT 2020

On Fri, Oct 23, 2020 at 11:57 AM Eric Smith <spacewar at> wrote:

> On Wed, Oct 21, 2020 at 6:22 AM Pontus Pihlgren via cctalk <
> cctalk at> wrote:
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 [H215/H216], 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.

The PDP-15 print set on Bitsavers shows that the original PDP-15 memory was
the MM15, which was in fact very similar to the PDP-8/I memory, and was
available with or without parity.

The ME15 must have been developed for later PDP-15 systems, and used the
newer H215 core stack. I can't find any evidence of a parity version of
that, which if it existed would have used the H216.

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