PDP-12 Restoration at the RICM

Lyle Bickley lbickley at bickleywest.com
Thu May 14 10:28:03 CDT 2015

On Wed, 13 May 2015 19:37:57 -0600
Eric Smith <spacewar at gmail.com> wrote:

> >From the web page:
>     "so it actually has two processors"
> Is that true? I haven't had time to make a detailed study of the
> PDP-12 logic design, but my understanding was that unlike the LINC-8,
> which basically was two separate processors mashed together, the
> PDP-12 was a single integrated processor with two modes (PDP-8 and
> LINC). I was told that there were some commonalities that were taken
> advantage of, such that merging it resulted in reduction of the total
> logic necessary as compared to the LINC-8.

I owned a PDP-12 for years. I bought it directly from DEC shortly after
it went out of production for $3,000!

My PDP-12 had 12Kw of RAM, an RF08 (2x256K) disk, dual TU56 Linctapes,
TC08 and TU55, ASR-33 and ADM-3 terminals, 9-track reel to reel mag
tape. In addition, because I wanted really high speed paper tape, I
created an interface to a Tally 1000cps PT reader and high speed punch.

I was the last official DECUS Coordinator for the PDP-12, so my PDP-12
was used to copy the contents of the DECUS PDP-8 and PDP-12 libraries to
all of the different media using my PDP-12. I still have some of the
"DECUS dollars" that were used to make refunds to users. They could
subsequently use those "DECUS dollars" to purchase future copies. (DECUS
had standard copy charges for different types of media).

I donated my PDP-12 with all it's peripherals and the complete PDP-8 and
PDP-12 DECUS libraries to the Computer History Museum around 1996.

That library also contains the only known master copy of the DIAL OS
source code. DIAL was an OS that ran in LINC mode on the PDP-12.
Shortly after DEC shutdown production of the PDP-12, they offered me
the master source, since they were not planning to continue support of

Regarding the question about number of "processors" in the PDP-12:

The PDP-12 does NOT have two processors. As Eric suggested, it was a
single integrated processor with two instruction sets - PDP-8 and LINC.
Much of the logic was shared between the two.

When in PDP-8 mode, a single instruction would put the 12 into LINC mode
and vice versa.

Bickley Consulting West Inc.

"Black holes are where God is dividing by zero"

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