What is a PDP-11?

Evolution of the PDP-11 family of computers.

During the late 1960s DEC had built a successful business based mainly on the success of its 12-bit PDP-8 series of minicomputers. However, the time had come for the introduction of a 16-bit machine to replace/supercede the -8. Edison DeCastro designed a 16-bit machine, but after a disagreement left DEC, formed DG, and built the initial success of that company on his 16-bit NOVA. There was much rancor surrounding the events of DeCastro leaving DEC and setting up DG. Stories abound as to the reasons of the bitterness, but I have never been able to get an authoritative source to confirm what really happened.

DEC eventually countered with its entry into this market segment - the PDP 11/20. This machine was to father a whole industry; lay the foundations on which the VAX would later be built; and virtually launch the OEM industry attracted to the connectability and extendability of the UNIBUS, and later, Q-Bus. Indeed, Computer Engineering by Bell/Munge/McMcNamara refers to the VAX-11/780 as being another -11 rather than the start of a new and different family. The early VAXes, of course, had PDP-11 compatability mode built in, and many VMS utilities were taken from RSX and run in compatability mode for a number of years until eventually replaced by native mode utilities sometime around VMS V3.0.

Since then, the PDP-11 had 16 to 22 implementations, depending on how you count them, many with variants. The following attempts to briefly track the evolution and progression.

In 1969 the -11 family was projected as follows:

   Model  CPU   Comments
   ------ ----- ----------------------------------------
   11/20  KA11  Origin of the species 1x performance.
   11/10   -    .7 of the 11/20, technologically cost reduced 11/20 in
                MOS. [This obviously became the 11/05, 11/10]
   11/30  KA11  [Seems to have been the same as an 11/20 packaged with a
                little more memory, etc. I believe this is what
                eventually became the 11/20 that actually shipped]
   11/40  KB11  2x performance. 
   11/45  KB11  2x performance. [Seems to have been intended to be an 11/40
                with MMU. [Looks like this became the 11/40 that
                eventually shipped.]
   11/50  KC11  2x performance. Hardware floating point 32 bit
                processor.  [I believe the 32 bit refers to the FPU!]
   11/55  KC11  2x performance. With MMU. [It looks like the 11/50 plus 11/55
                became the eventual 11/45]
   11/65  KD11  4x performance. 32 bit seperate memory bus, 32 bit processor.

It is interesting to note how things progressed in reality. Technology allowed, and the market drove, the production, within a couple of years, of machines considerably in advance of what was envisaged on April 3 1969. It is also interesting to note that some people were already thinking 32 bits!

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