What is a PDP?

Extracted from the PDP-8 FAQ By Doug Jones. Used with his permission.

Digital Equipment Corporation (DEC) was founded in 1957, with facilities in an old woolen mill in Maynard Massachusetts; at first, they sold transistorized "systems modules", plug-in circuit boards with a few transistorized logic gates per board. Starting in 1960, though, DEC began to experiment with selling computers; by 1961, they had sold enough that DECUS, the Digital Equipment Computer User's Society was founded.

DEC's first computer, the PDP-1, sold for only $120,000 at a time when other computers sold for over $1,000,000. Everyone (the government and DEC's stockholders included) knew that computers were big and expensive and needed a computer center and a large staff; DEC chose to avoid dealing with these stereotypes by entirely avoiding the term "computer"; thus, for over a decade, all digital computers sold by DEC were called Programmed Data Processors (PDPs). In early DEC documentation, plural form "PDPs" is used as a generic term for all DEC computers. [Ken Olsen claimed that the board of directors would not let him call the machines computers because some contemporary study had predicted that the world market for computers would be very small - less than 100 if memory serves. They were mollified with Programmed Data Processor, however, and PDP lives to this day. -MMcC]

DEC built a number of different computers under the PDP label, with a huge range of price and performance. The largest of these are fully worthy of large computer centers with big support staffs. Many early DEC computers were not really built by DEC. With the PDP-3 and LINC, for example, customers built the machines using DEC parts and facilities. Here is the list of PDP computers:

    =====  ====  ========  ====  =====
    PDP-1  1960  $120,000  18    DEC's first computer
    PDP-2            NA    24    Never built?
    PDP-3                  36    One was built by a customer, none by DEC.
    PDP-4  1962            18    Predecessor of the PDP-7.
    PDP-5  1963   $27,000  12    The ancestor of the PDP-8.
    PDP-6  1964  $120,000  36    A big computer; 23 built, most for MIT.
    PDP-7  1965  ~$60,000  18    Widely used for real-time control.
    PDP-8  1965   $18,500  12    The smallest and least expensive PDP.
    PDP-9  1966   $35,000  18    An upgrade of the PDP-7.
    PDP-10 1967  $186,500  36    A PDP-6 successor, great for timesharing.
    PDP-11 1970   $10,800  16    DEC's first and only 16 bit computer.
    PDP-12 1969   $27,900  12    A PDP-8 relative.
    PDP-13           NA          Bad luck, there was no such machine.
    PDP-14                       A ROM-based programmable controller.
    PDP-15 1970   $16,500  18    A TTL upgrade of the PDP-9.
    PDP-16 1972 $.8-$4,000 NA    8/16  A register-transfer module system.

Corrections and additions to this list are welcome! The prices given are for minimal systems in the year the machine was first introduced. The bits column indicates the word size. Note that the DEC PDP-10 became the DECSYSTEM-20 as a result of marketing considerations, and DEC's VAX series of began as the Virtual Address eXtension of the never-produced PDP-11/78.

It is worth mentioning that it is generally accepted that the Data General Nova was originally developed as the PDP-X, a 16-bit multi-register version of the PDP-8. A prototype PDP-X was built at DEC before the design was rejected. This and a competing 16-bit design were apparently submitted to Harold McFarland at Carnegie-Mellon University for evaluation; McFarland (and perhaps Gordon Bell, who was at C-MU at the time) evaluated the competing designs and rejected both in favor of what we know as the PDP-11. (A less common version of this story is that the reason that DEC never produced a PDP-13 was because the number 11 was assigned to what became the Nova; this is unlikely because the PDP-X prototype came before the PDP-11.) Both DEC and Data General are quiet about these stories.

Today, all of the PDP machines are in DEC's corporate past, with the exception of the PDP-11 family of minicomputers and microprocessors.

Of course, occasionally, some lab builds a machine out of DEC hardware and calls it a PDP with a new number. For example, the Australian Atomic Energy Commission once upgraded a PDP-7 by adding a PDP-15 on the side; they called the result a PDP-22. See Mark Crispin's 1986 list of PDP's elsewhere in this document.

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