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 (solderpunk@sdf.org) 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.

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