Sorting out PDPs
----------------
Lately I've been doing a lot of reading on various old computer systems,
including but not limited to DEC's classic PDP minicomputers. Even though I've
read a lot about these machines, I can never manage to keep them all straight in
my head. This is not really surprising. They made a lot of different machines
in different "families" or "series", and as far as I can tell they numbered them
incrementally based on release date with no regard for the family. So the
PDP-10 is not a slightly upgraded PDP-9, which is itself not an incremental step
up from the PDP-8. Those 3 machines have nothing in common. So, here is some
kind of summary/breakdown, mostly for my own sake in order to get things clear
in my head, but I figure some other people here will probably enjoy it.
I am absolutely no expert and most of this is gleaned from an afternoon cruising
Wikipedia. If you know that anything below is factually incorrect, please drop
me a line (
[email protected]) and I will correct it.
The PDP machines are primarily grouped by word length. Let's get the smaller
families out of the way first.
16-BIT FAMILY
~~~~~~~~~~~~~
First up, strangely, is the PDP-11. I say "strangely" because, for whatever
reason, in my head the PDP-11 is the canonical PDP machine, it's the first one I
think of and the one I use as a kind of mental prototype for a typical PDP
computer. This is actually not appropriate because it's anything *but* typical.
As far as I can tell, the PDP-11 is DEC's first, last and only 16-bit computer.
Possibly this one sticks in my head because to me a 16-bit machine is perfectly
normal, and all other PDP machines use word lengths that I consider bizarre.
Anyway, not much of an introduction needed here. This is the canonical 1970s
minicomputer, much beloved because of its elegant and orthogonal instruction
set, which inspired many future processor designs, including DEC's own VAX but
also the Motorola 68000. This is also the machine on which Unix was essentially
born (after a brief incubation on a PDP-7), and an early target for the C
programming language. This is probably actually more the reason that this one
sticks in my head, rather than the conventional seeming word length. I think
this is the PDP to which hackers who grew up in the microcomputer era have the
strongest historical connection.
That's it for this "family of one"!
36-BIT FAMILY
~~~~~~~~~~~~~
The next small family is the 36-bitters. These were whopping big mainframe
machines, typically spread across multiple cabinets, taking up a good chunk of
the room. They were used for time sharing: a university or company bought
exactly one of these and used it for everything. The PDP-6 was the first of
these machines. DEC only sold 23 of them. They were hard to build and had
lots of reliability problems. In effect, they ended up being the prototype
for the PDP-10, which was far more successful and played a big role in the
early ARPANET. PDP-10s typically ran the TOPS-10 operating system, or its
third-party enhancement TENEX, which DEC later bought, improved and released as
TOPS-20, nicknamed "TWENEX" as a combination of 20 and TENEX.
Technically the PDP-6 and PDP-10 were the only two 36 bit PDP machines, but
DEC's follow up to the PDP-10, the DECSYSTEM-20, is sometimes unofficially
thought of as the PDP-20.
SDF operates a public access TOPS-20 system running on an emualted PDP-10 - see
twenex.org.
Now onto the bigger families...
12-BIT FAMILY
~~~~~~~~~~~~~
DEC's first 12-bit machine was the PDP-5, which was a lot of other first as
well. Arguably "the world's first commerciall produce minicomputer" (but of
course, this claim is subject to endless debate about exactly what constitutes a
minicomputer, in exactly the same way as two dozen machines claim to be the
world's first personal computer), some more objectively the first computer of
which over 1000 units were built. The PDP-5 was followed by the PDP-8, which
was the first PDP machine which was a definite commercial success and actually
made DEC some money. They sold over 50,000 of these. The PDP-8 is remembered
now for its extremely minimal architecture, with very few instructions. In some
way it anticipated the RISC concept of the 80s, although unlike most RISC
machines it had very few registers to match the very few instructions.
Whereas the PDP-11 and the PDP-10 were commonly used in multiuser, time-sharing
contexts, the PDP-8 was most often deployed in single user, real-time
applications in scientific and industrial applications, reading data from
laboratory equipment and controlling factory robots. The follow up machines in
DEC's 12 bit series focussed more directly on these markets. The PDP-12 was an
upgraded and specialised PDP-8 designed for scientific use, marketed as the
"Lab-8". The PDP-14 in contrast was specialised for industrial applications,
and would probably be called a PLC today. The PDP-16 was a more capable upgrade
of the PDP-14.
In short, this is basically the "embedded PDP family".
18-BIT FAMILY
~~~~~~~~~~~~
Finally, we have the 18-bit machines, starting way back in 1959 with the *first*
of these machines, the PDP-1. This is, of course, the machine which ran
"Spacewar!", the first videogame (or, given the inevitable quibbling over
definitions, *one of* the first videogames). This was followed by the PDP-4,
PDP-7, PDP-9 and finally PDP-15. This *appears* to be the least successful of
the families. I say this based in part on sales figures I've seen (DEC sold
about 50 PDP-1s, 50 PDP-4s, 120 PDP-7s and 445 PDP-9s) and the fact that the
Wikipedia pages for these machines seem to have received far less effort from
the community than the other families.
These systems seem to fit inbetween the 12- and 36-bit families. Not quite as
big as typical DEC-10 installations, but much bigger than a PDP-8 or any of its
descendants. Certainly too big and too expensive to easily add onto your average
assembly line. I initially wrote here "I guess this is what you bought if you
couldn't affor a PDP-10", but then I realised that, of course, there *was* no
PDP-10 during the availability of most of this series. It pays here to keep the
timline in mind (which, thankfully, is easy, because the different PDP systems
are numbered by release date). At the time the PDP-7 and PDP-9 were selling,
neither the PDP-10 nor PDP-11 existed. The PDP-6 did, but remember that was an
expensive, unreliable machine that sold poorly. I guess if you desperately
needed a big 36-bit mainframe, you bought the PDP-6, if you needed to do
scientific or industrial computing you bought one of the 12-bit machines and if
you were inbetween these two niches you bought one of these 18-bitters. The
PDP-11 probably effectively killed off this family, offering a smaller and
cheaper machine for those who needed more than an embedded controller but less
than a mainframe. I guess the 18-bit PDPs were aimed at a market that didn't
really take off until minicomputers became a bit more affordable, and that's why
we don't pay as much attention today to these early machines which sold in
limited numbers to early adopters.
MIA?
~~~~
Some of you may be wondering what happened to the PDP-2 or PDP-3, which have not
been mentioend above. Nothing too exciting here. The PDP-2 designation was
reserved for a 24-bit machine which was designed but never built. The PDP-3
was a one-off 36-bit machine designed by DEC for the CIA, who built it
themselves and used it to process radar data from spyplanes. It was basically
two PDP-1s hacked together.
