IAS - the PDP-11 Interactive Applications System

(This originally appeared on my web page).

DEC's approach to operating systems for the PDP-11 was anything but disciplined. New ones got invented every time some engineer or marketing person blinked. In the early days, there was a real-time kernel called RSX-11A, designed for memory-resident applications in what we now call embedded processors. Features got added to this rapidly - code bloat is nothing new. By the time it got to RSX-11D it had a complete disk-based file system, a program development environment, and support for every peripheral in the Small Computer Handbook (and there were plenty of them - peripherals on the PDP-11 obeyed the same strategic imperatives as operating systems - see above). At this time, a bright young engineer called Dave Cutler decided that enough was enough, and set out to create a small system that would do the same, which he called RSX-11M.

Meanwhile, the PDP-11 also had a timesharing system very loosely based on TOPS-10, called RSTS/E. A senior engineering manager, newly installed in Geneva, decided that it would be a smart move to develop a system that could do both real-time and timesharing, based on RSX-11D. It was specifically targetted at the planned PDP-11/70, which was a kind of super-11. Since he had newly moved to Europe (his name was David Stone, by the way) he gave the project to the European Software Engineering group that he had just invented in Reading, England. This was about the time that I joined DEC, and after a few misadventures I found myself assigned to the project.

The system was to be called IAS, which if I remember rightly stood for "Interactive Applications System". It added to the RSX-11D kernel a clever timesharing scheduler, a bunch of security features, and a new command language. These were the days of MCR, a command language which makes even the Unix shell look lucid. (To delete a file you typed "PIP file/D" for example). The then-boss of software decided we needed a Digital Command Language, which of course later become a feature of VMS, but IAS was the guinea-pig. In fact, all DCL commands were translated into the corresponding MCR and the fired off to the appropriate utility. The command interpreter that did this was thrown together in great haste, and remains to this day the nastiest piece of software I have ever encountered.

I had tremendous fun on IAS. Like V1 of any system, it lacked just about every feature anyone wanted, and they all had to be added for V2. It says something for the team that in fact they mostly got there in V3, and they mostly worked. The team by the way consisted of about six people - that's probably about the same number that Microsoft has doing quality control on the stupid paperclip in Office 97. My first job was to write the driver for the latest new disk, the RK06. It had about the same capacity as a couple of floppies; four of them would fit in a six-foot cabinet. I was duly sent on the course for writing device drivers, but on the first morning I finished reading the manual and by the end I had coded the driver. It did various nifty things that nobody had done before on the 11, like overlapped seeks, and ended up becoming the basis for all future IAS and RSX-11D drivers although I remained unhappy with a lot of it.

My next job was to write a new terminal driver. Despite what I said about the command interpreter, the old terminal driver was pretty special too. Support for new features and new hardware had been thrown in over several years, and it was impossible to figure out how it worked. One story that sticks in my mind: I changed it to suppress nulls on formatted output (because of a misfeature in the command interpreter). Thereafter, if you typed rapidly, it would drop the first character of every other line. I never figured out why, I just removed the fix.

The terminal driver was one of the most enjoyable bits of work I ever did. It was all table driven, and in fact was object-oriented 15 years before it became fashionable. Thus adding a new device just meant writing some standard routines and plugging them into the tables. It seems obvious now, but it was pretty revolutionary at the time! I cooperated with the guy who was writing the driver for the new VMS system, so when I invented "read with prompt" (mainly to make the output on hardcopy terminals look prettier) this found its way into VMS, and ten years later was used in a way that I certainly never thought of to double the performance of All-in-1.

All of this found its way into V2. But by then, Cutler had decided that RSX-11M was going to take over the world. Since he was at the heart of things in Maynard, and we were 3000 miles away in Reading, it was pretty easy for him to get the sales and support people to listen to him. IAS did get some very loyal customers, including Boeing and the US Navy, who stuck with it long after Digital had tried to kill it.

In fact, as the cost of developing and maintaining software soared, Digital did try to rein in the PDP-11 situation. As a result, we had to combine IAS and RSX-11D into a "unified product strategy". (I seem to have spent a lot of time over the years taking products that were never meant to be the same thing, and making it look as though they were).

IAS had many features that RSX-11M didn't, such as a proper timesharing scheduler. This led to RSX-11M+, which was RSX-11M with a bunch of features intended to match IAS. This was really a stretch for 11M, in complete conflict with the "size is the goal" philosophy which Cutler had made into a rubber stamp that appeared on all of the early 11M design documents. Nevertheless, M+ had the visibility in Maynard and IAS didn't, and got the development funding. This meant that several new features first appeared in M+ and then had to be retrofitted into IAS.

One of these was "PLAS" (I forget what it was supposed to stand for), which gave programs access to the memory management features and was a kind of do-it-yourself virtual memory. It fell to me to implement the linker support for this. Now the linker was Cutler's first ever piece of software at DEC. It was very clever; doing memory layout for a machine as constrained as this could never be easy. In addition, it supported PSECTS which were modeled on what the IBM/360 did for memory management, and of course found their way into VMS as well. Thus a memory section could be shared between overlays, or not, and could be marked for code or for data, and could be overlaid to support Fortran COMMON or not, and so on. There were about seven different PSECT attributes, and so over a hundred different ways that memory allocation could be handled. And overlays - who remembers those? The linker had a write-only language called ODL (Overlay Description Language), which allowed you to set up hugely complicated overlay structures. As the PDP-11 address space (64 kbytes!) become more and more of a constraint, ever-fancier overlay techniques were invented, and since the linker had to handle them all its own overlay structure was the most complex of the lot.

But by this time the writing was on the wall for the PDP-11 as a general-purpose machine. The VAX and VMS had been a huge success and all serious investment went on them, rightly so. Personally I moved on after we released V3, in about 1979, but IAS retained an engineering group for a few more years. I think DEC continued to support it, for the benefit of the handful of big customers (like the US Navy) who were still using it, up until 1988 or so.

The PDP-11 spawned several great operating systems, the most famous nowadays being Unix. But IAS had something the others never did, a unique ability to support both timesharing and real-time applications at the same time. A big 11/70 - which is to say a megabyte of memory and a few tens of megabytes of disk - could give decent timesharing support to 20 or 30 users, and there were people who ran it at the limit of 64 users and seemed happy with it. Try telling that to the youth of today!