Great Ideas Never Die: They just get improved upon

On Pi Day this week, Ars Technica published an item on the PDP-11 Minicomputer, arguably the most influential minicomputer in history (or most influential computer, period), as well as longest in production (more than 20 years), outliving the Apple II (1977-1993).  The PDP-11 cost US$20,000 in 1970, which is US$146,000 today.

The PDP-11 sales numbers of 600,000 look unimpressive only if you think in terms of microcomputers that sold millions.  This was a minicomputer used by universities, governments, businesses, militaries, etc. As with the Apple II and ZX Spectrum, the Soviets and East Bloc countries tried to clone or steal the PDP-11.  There’s an apocryphal tale from the 1970s/1980s of a VAX headed for Berlin going missing on the Autobahn, but I can’t find a confirmed source for the story.

The AT item talks not just about the history of the PDP-11, how it led to the Unix operating system (and its descendants) and the C programming language, it provides the basics of its architecture, programming in assembler.  I never used a PDP-11, but its assembly language looks near identical to its descendant, the VAX 11/780 and 4500 that I used in college.  Reading this was like hearing a dialect that you understand enough to pick out many phrases, but not a whole conversation.

A brief tour of the PDP-11, the most influential minicomputer of all time

The history of computing could arguably be divided into three eras: that of mainframes, minicomputers, and microcomputers. Minicomputers provided an important bridge between the first mainframes and the ubiquitous micros of today. This is the story of the PDP-11, the most influential and successful minicomputer ever.

In their moment, minicomputers were used in a variety of applications. They served as communications controllers, instrument controllers, large system pre-processors, desk calculators, and real-time data acquisition handlers. But they also laid the foundation for significant hardware architecture advances and contributed greatly to modern operating systems, programming languages, and interactive computing as we know them today.

[. . .]

The PDP-11 was introduced in 1970, a time when most computing was done on expensive GE, CDC, and IBM mainframes that few people had access to. There were no laptops, desktops, or personal computers. Programming was done by only a few companies, mostly in assembly, COBOL, and FORTRAN. Input was done on punched cards, and programs ran in non-interactive batch runs.

Although the first PDP-11 was modest, it laid the groundwork for an invasion of minicomputers that would make a new generation of computers more readily available, essentially creating a revolution in computing. The PDP-11 helped birth the UNIX operating system and the C programming language. It would also greatly influence the next generation of computer architectures. During the 22-year lifespan of the PDP-11—a tenure unheard of by today’s standards—more than 600,000 PDP-11s were sold.

[. . .]

The UNIX operating system started life on a PDP-7 but was perfected on a PDP-11. The first version of UNIX was written in PDP-11 assembler; it had 34 system calls, it was written in 4,200 lines of code, and it ran on 12KB of main memory. Files were limited to 64K in size. It provided a hierarchical file system, the roff text formatter, the ed editor, system administration tools for dealing with disks, magnetic tape, and paper tape, and it included Blackjack, Chess, and tic-tac-toe.

Most importantly, UNIX provided an interactive, time-shared system that was accessible from inexpensive terminals. PDP-11 with UNIX opened the floodgates for inexpensive interactive computing, which then led to an explosion of office productivity. People finally had a means of editing, storing, and printing office documents. This was big in the corporate world, obviously, but things were just getting started.

According to History Computer, many PDP-11s are still in use today (US Navy, Airbus, et al) because they are so robust and reliable.  Now defunct Irish company Mintec continued to service hardware and software after Digital Equipment Corporation (DEC) went out of business, but that doesn’t mean you can’t still get help for the PDP-11.  Multiple vendors (ComWareTech, StromaSys, StrobeData) provide migration to emulation on Intel hardware.  User and service manuals for the PDP-11 (in PDF form) can be found easily.

There’s also a PDP-11 emulator for the Raspberry Pi called the PiDP-11.  This writer explains his assembly of the kit, plus there are others talking about it (e.g. Hackaday).

There is (was?) a clone called the PDPii in mini-ITX form factor.


  1. Some Old Programmer says

    Some readers may not know that the PDP-11 is an architecture. I worked on PDP 11/45 and 11/70 incarnations, and they had different form factors and capabilities, all based on the architecture. A 1970s minicomputer configuration was in refrigerator-size cases with attendant large disk media and reel-to-reel tape drives.

    While I was a lowly summer hire (“Data Aide”), I formed the impression that a lot of the power of the architecture came from one of the advantages that propelled the IBM PC to success–an open, simple specification for I/O. The company I worked for made some hardware to attach to the PDP 11 Unibus, allowing them to manufacture interfaces to Remote Telemetry Units (RTUs) to run a SCADA system for electrical utilities. The combination of off-the-shelf computer, a small bit of custom hardware, and a large bit of custom software made for a successful company. There was a large secondary market for Unibus peripherals to do things that DEC couldn’t or wouldn’t.

  2. xohjoh2n says

    Hmm. They did break and fall over quite a lot, so “robust and reliable” is at least a little bit questionable, but there’s probably worse out there. If you were lucky you had enough spare/replacement machines on site to swap parts and fix immediate failures – they were simple enough machine that you could do that, for any particular part that failed. I’ve no idea how current users manage to keep them going – perhaps there are still companies who can repair/replace running PDP-11 hardware. (Making the CPU out of wire-wrapped 7400 logic makes it at least possible you could repair it yourself!)

    But of course the most awesome thing about any 11/ system was the physical console, you just can’t beat that.

  3. Some Old Programmer says

    xohjoh2n @2, there was the old joke “How many DEC technicians does it take to change a flat tire? A: It depends on how many flat tires they brought with them.”

  4. mikey says

    I installed and maintained security systems (campus-wide, fire/security/access control) based on the 11/23 all through the 80s running on RSX-11M. The last ones in service only got changed out in ’99 for Y2K issues. Kinda wish I’d saved an old RL01/2 disk for a decoration; I’ve seen the platters used as cool clock faces.

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