Mark Crispin's 1986 list of PDP's

With comments and spelling corrections by Charles Lasner, Bernd Ulmann, and Mike McCrohan

        Date: Wednesday, 20 August 1986  03:42-EDT
        From: Mark Crispin <MRC%PANDA@SUMEX-AIM.ARPA>
        Re:   DEC's PDP's

A number of people have requested my list of all the DEC PDP's, so I thought I'd bore you all with it.

The PDP-1 was an 18 bit machine. It was DEC's first computer, and some of the first timesharing systems were designed for it. It's also unique in being one's complement; all later DEC computers were two's complement. Some machines, such as one of MIT's PDP-1s, were in operation until the late '70s.

[Charles Lasner comments: The LINC portion of the LINC-8 and PDP-12 are one's complement!]

The PDP-2 was a designation reserved for a 24 bit machine, but as far as I can tell it was never even designed and definitely none were ever built.

The PDP-3 was a 36 bit machine that was designed but never built by DEC. However, Scientific Engineering Institute built one in 1960.

The PDP-4 was an 18 bit machine that was intended to be a cheaper, slower alternative to the PDP-1. It was so slow that it didn't sell well, although it was interesting for its auto-incrementing memory registers. It was not program-compatible with the PDP-1, but its instruction set was the basis of DEC's future 18 bit computers.

The PDP-5 was a 12 bit machine designed to be a small laboratory system. It used many of the ideas in the LINC (Laboratory Instruction Computer, designed by Lincoln Labs at MIT, some of which were built by DEC).

The PDP-6 was a 36 bit machine and the first machine to implement the most wonderful computer architecture known to man. It was rather expensive and difficult to maintain, and not many were sold. As a result, DEC cancelled 36 bit computers for what was to be the first of many times.

The PDP-7 was an 18 bit machine and the sucessor to the PDP-4. It was a major price/performance win over the PDP-4 and the first DEC computer to use wire-wrapping.

The PDP-8 was a 12 bit machine and the sucessor to the PDP-8.

[Charles Lasner comments: I assume he means -5!]

It basically defined the term "minicomputer", and went through several incarnations. The original PDP-8 was followed by the extremely slow PDP-8/S (as bad as the PDP-4 was to the PDP-1, but at least the /S was program-compatible). DEC recouped with the PDP-8/I (using MSI integrated circuits) and the smaller PDP-8/L, and somewhat later came out with the "Omnibus 8" machines -- the PDP-8/E, the PDP-8/F (a half-sized version of the PDP-8/E), the PDP-8/M (an OEM version of the PDP-8/F), and the final machine, the single board PDP-8/A. The PDP-8/A still exists after a fashion as a current DEC product.

The PDP-9 was an 18 bit machine and the sucessor to the PDP-7. It had a faster memory than the PDP-7 and was the first microprogrammed DEC computer. Modulo a 300 wire(!) ECO required in the first machines, the PDP-9 was a reliable machine and some are still in operation today. There was a short-lived PDP-9/L.

[Comment by Bernd Ullman: As far as I know the PDP-9 had a 64 word by 36 bit read-only control-store with an access time of 212 ns (it was a transformer coupled rope memory). The first intention was to build a horizontally programmed machine but this was dropped because of the resulting word length needed for the control-words. So some (most ?) of the control signals were encoded and this led to a typical diagonally microprogrammed machine I think.]

[Mike McCrohan comments: 212 ns seems fast to me for a PDP-9 in the mid 1960's when a core memory of 5 years later was cycling at 900ns, and PDP-11 instructions were at about 1 microsecond.]

[Paul Koning comments: [This speed] sounds reasonable. For one thing, 800 ns was available for read-write core memory in the early 60s (CDC had it). More to the point, core ROM has very different timing. It's not exactly my field but it sounds plausible.]

The PDP-10 was a 36 bit machine and the sucessor to the PDP-6. It is especially noted for its software, which represents the pinnacle of DEC software engineering and has never been equalled. The first KA10, largely installed in universities, created a whole generation of timesharing hackers. The follow-on KI10, with paging and using IC's instead of discrete components but otherwise unexciting, mostly was sold to commercial organizations. The KL10 went through several incarnations and is today the most representative of this marvelous machine. The KS10 was a small, low-speed (approximately KA10 performance) processor which was DEC's last successful implementation of this architecture.

The PDP-11 was a 16-bit machine that went through more implementations and operating systems than can be counted. Presently it superceded the less powerful PDP-8 as the representative minicomputer. While the PDP-11 used octal, it was in its deep heart of hearts a hexadecimal machine, and the first indicator of the creeping IBMification of DEC that took full fruit in the VAX. [I can hear the flames now...] Rather than fight it the customers loved it; more PDP-11's have been sold than any other DEC computer (possibly more than all the others combined).

The PDP-12 was a 12 bit machine and the sucessor to the PDP-8. It combined a LINC and a PDP-8 type processor in the same box and basically was a new model of the LINC-8 which was the same thing.

No PDP-13 was ever designed or built. Even DEC gets superstitious.

The PDP-14 was a 12 bit machine with a 1 bit register. It was used as a process control engine in applications that were felt to be too rugged for a PDP-8, and basically replaced a set of relays. Later DEC made PDP-8's suitable for this sort of thing, but it didn't stop them from the ultimate silliness of building a PDP-14 that used a PDP-8 as its console processor!

[Charles Lasner comments: I remember something about a KL8-JA-oriented diagnostic that was for checking out somesuch configuration!]

The PDP-15 was an 18 bit machine and the final one of this design built by DEC. More PDP-15's were built and sold than any of the others,

[Charles Lasner comments: I assume he means other 18-bit DEC machines!]

several incarnations including some which used a PDP-11 as a front end. Apparently the cancellation of the PDP-15 came as a great surprise to the "Tiger Team" who worked on it, although considering its general ungainliness compared to comparable performance PDP-11's it wasn't surprising. In many ways the PDP-15 died for the same reason the PDP-10 did.

The PDP-16 was a "roll your own" 16 bit machine based on various "building blocks". Every PDP-16 was essentially custom-designed by the customer. It got a fair amount of attention when it was announced but evidentally didn't sell very well.

There was no PDP-17 or any other designator. DEC apparently decided that "PDP" had a perjorative ring to it.

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