Today in History – March 13,1970 – PDP-11 minicomputer introduced by DEC (Digital Equipment Corporation) and remained in active production until 1996. It was one of the most popular 16-bit minicomputers ever produced.

PDP-11 and its successor, the VAX-11, lived over almost three decades during the time DEC rose to number two computer manufacturer, and then was purchased by Compaq, who in turn was purchased by HP. The architecture was done by my former CMU student, Harold McFarland. (I was a consultant to DEC while on the CMU faculty 1966-1972). The PDP-11 was known for at least two innovations: the Unibus for interconnecting the computer’s components and the architecture using General Registers. Both concepts were derived while writing the book Computer Structures with Allen Newell.

Andy Knowles was in charge of the group. The marketing ads featured the Unibus, a 56 conductor ribbon cable to which the processor, memories, and various I/O control units attached. The initial ads for the PDP-11 featured the ribbon cable bus and a scissors and the notion that anything could be connected to it – making it ideal as a component of a larger system. This is what DEC said about it: “The PDP-11/20 was the first minicomputer to interface all system elements — processor, memory and peripherals — to a single, bi-directional, asynchronous bus. The UNIBUS enabled fast devices to send, receive or exchange data without intermediate buffering in memory. The PDP-11 became the world’s most successful family of minicomputers.“ The manual has a description in ISP, the language that Allen Newell and I developed to describe computer instruction sets aka instruction set architectures (ISAs).

My web site has several articles and comments that may be of interest on my web site.

1. Bell, C. G., R. Cady, H. McFarland, B. Delagi, J. O’Laughlin, R. Noonan and W. Wulf, “A New Architecture for Mini-Computers — The DEC PDP-11″, Sprint Joint Computer Conference, pp. 657-675 (1970). This is the conference where it was announced.
2. What we learned from the PDP-11, published by myself and Bill Strecker in 1975 just before the VAX project was started. This has become a ‘classic paper’ in architecture that starts with: “There is only one mistake that can be made in a computer design that is difficult to recover from: not providing enough address bits for memory addressing and memory management . The PDP-11 followed the unbroken tradition of nearly every known computer”.
3. Retrospective on the PDP-11 written by Bill Strecker with a retrospective about VAX and Alpha and published in 1995.
4. Bell, C. G., “The Mini and Micro Industries“, Computer (17) no. 10, pp. 14-30 (October 1984) provides an overview of the entire industry that was minicomputers from the beginning until 1984 when workstations and distributed computers and clusters were emerging. It showed that there were about 100 computer companies formed and died.
5. Gordon Bell 3 hour class lecture on the Formation of the Minicomputer Industry from a DEC (Digital Equipment Corporation) Viewpoint, including Bell’s Law of Computer Classes. The talk was given to the History of Computers Seminar at UC Berkeley and included UC San Diego, U of Washington, and Microsoft on October 11, 2006.
6. Bob Supnik has written simulators and papers on the PDP-11, VAX, and Alpha.

Check out the Engineering Pathway’s educational resources on the PDP-11 and history of computing. For more educational resources, see our electrical engineering education, computer science education and computer engineering education community pages. The Engineering Pathway also hosts Engineering Education communities in all ABET-accredited disciplines.