Programmed Data Processor

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PDP-15 (partial)
PDP-15 graphics terminal with light pen and digitizing tablet

Programmed Data Processor (PDP) is a series of minicomputers made and marketed by the Digital Equipment Corporation from 1957 to 1990. The name "PDP" intentionally avoids the use of the term "computer" because, at the time of the first PDPs, computers had a reputation of being large, complicated, and expensive machines, and the venture capitalists behind Digital (especially Georges Doriot) would not support Digital's attempting to build a "computer"; the word "minicomputer" had not yet been coined.[citation needed] So instead, Digital used their existing line of logic modules to build a Programmed Data Processor and aimed it at a market that could not afford the larger computers.

The various PDP machines can generally be grouped into families based on word length.

PDP series[edit]

Members of the PDP series include:

The original PDP, an 18-bit machine used in early time-sharing operating system work, and prominent in MIT's early hacker culture, which led to the (Massachusetts) Route 128 hardware startup belt (DEC's second home, Prime Computer, etc.). What is believed to be the first video game, Spacewar!, was developed for this machine, along with the first known word processing program for a general-purpose computer, "Expensive Typewriter".
A number reserved for an unbuilt, undesigned 24-bit design.
First DEC-designed (for US "black budget" outfits) 36-bit machine, though DEC did not offer it as a product. The only PDP-3 was built by the CIA's Scientific Engineering Institute (SEI) in Waltham, MA to process radar cross section data for the Lockheed A-12 reconnaissance aircraft in 1960.[1][2] Architecturally it was essentially a PDP-1 controlling[citation needed] a PDP-1 stretched to 36-bit word width.[3]
18-bit machine intended to be a slower, cheaper alternative to the PDP-1; it is not considered commercially successful. All later 18-bit PDP machines (7, 9 and 15) are based on a similar, but enlarged instruction set, more powerful, but based on the same concepts as the 12-bit PDP-5/PDP-8 series. One customer of these early PDP machines was Atomic Energy of Canada. The installation at Chalk River, Ontario included an early PDP-4 with a display system and a new PDP-5 as interface to the research reactor instrumentation and control.
DEC's first 12-bit machine. Introduces the instruction set later expanded, in the PDP-8, to handle more bit rotations and to increase the maximum memory size from 4K words to 32K words. It is the first computer series with more than 1,000, then 10,000 built, which was a large number in the decade after ENIAC/UNIVAC builders predicted that 3 computers would serve the nation's computing needs.
36-bit timesharing machine. Introduces the instruction set later used in the PDP-10 and DECSYSTEM-20. It was considered by its detractors a large minicomputer or, by DEC fans especially, Big Iron - a mainframe As a timesharing machine, it constantly outran the batch-oriented IBM System/360 and even IBM System/370-series mainframes.[citation needed]
Replacement for the PDP-4; DEC's first wire-wrapped machine. The first version of Unix, and the first version of B, a predecessor of C, were written for this machine at Bell Labs, as was the first version (by DEC) of MUMPS.
12-bit machine with a tiny instruction set; DEC's first major commercial success and the start of the minicomputer revolution. Many were purchased (at discount prices, a DEC tradition, which also included free manuals for anyone who asked during the Ken Olsen years) by schools, university departments, and research laboratories. Later models are also used in the DECmate word processor and the VT-78 workstation. It is reported that Edson de Castro, who had been a key member of the design team, left to form Data General when his design for a 16-bit successor to the PDP-8 was rejected in favour of the PDP-11; the "PDP-X" did not resemble the Data General Nova,[4] although that is a common myth.[citation needed]
A hybrid of the LINC and PDP-8 computers; two instruction sets. Progenitor of the PDP-12.
Successor to the PDP-7; DEC's first micro-programmed machine. It features a speed increase of approximately twice that of the PDP-7. The PDP-9 is also one of the first small or medium scale computers to have a keyboard monitor system based on DIGITAL's own small magnetic tape units (DECtape).[5] The PDP-9 established minicomputers as the leading edge of the computer industry.
36-bit timesharing machine, and fairly successful over several different models. The instruction set is a slightly elaborated form of that of the PDP-6.
The archetypal minicomputer; a 16-bit machine and another commercial success for DEC. The LSI-11 is a four-chip PDP-11 used primarily for embedded systems. The 32-bit VAX series is descended from the PDP-11, and early VAX models have a PDP-11 compatibility mode. The 16-bit PDP-11 instruction set has been very influential, with processors ranging from the Motorola 68000 to the Renesas H8 and Texas Instruments MSP430, inspired by its highly orthogonal, general-register oriented instruction set and rich addressing modes. The PDP-11 family was extremely long-lived, spanning 20 years and many different implementations and technologies.
12-bit machine, descendant of the LINC-8 and thus of the PDP-8. It can execute the instruction set of either system. See LINC and PDP-12 User Manual. With slight redesign, and different livery, officially followed by, and marketed as, the "Lab-8".[citation needed]
Designation was not used.
A machine with 12-bit instructions, intended as an industrial controller (PLC). It has no data memory or data registers; instructions can test Boolean input signals, set or clear Boolean output signals, jump conditional or unconditionally, or call a subroutine. Later versions (for example, the PDP-14/30) are based on PDP-8 physical packaging technology. I/O is line voltage.
DEC's final 18-bit machine. It is the only 18-bit machine constructed from TTL integrated circuits rather than discrete transistors, and, like every DEC 18-bit system (except mandatory on the PDP-1, absent on the PDP-4) has an optional integrated vector graphics terminal, DEC's first improvement on its early-designed 34n where n equalled the PDP's number. Later versions of the PDP-15 run a real-time multi-user OS called "XVM". The final model, the PDP-15/76 uses a small PDP-11 to allow Unichannel peripherals to be used.[6]
A "roll-your-own" sort of computer using Register Transfer Modules, mainly intended for industrial control systems with more capability than the PDP-14. The PDP-16/M was introduced as a standard version of the PDP-16.

Related computers[edit]

  • TX-0 designed by MIT's Lincoln Laboratory, important as influence for DEC products including Ben Gurley's design for the PDP-1
  • LINC (Laboratory Instrument Computer), originally designed by MIT's Lincoln Laboratory, some built by DEC. Not in the PDP family, but important as progenitor of the PDP-12. The LINC and the PDP-8 can be considered the first minicomputers, and perhaps the first personal computers as well. The PDP-8 and PDP-11 are the most popular of the PDP series of machines. Digital never made a PDP-20, although the term was sometimes used for a PDP-10 running TOPS-20 (officially known as a DECSYSTEM-20).
  • SM EVM series of computers in the USSR
  • DVK personal computers series are PDP clones developed in USSR in 1970s.
  • Elektronika BK
  • UKNC


  1. ^ "Announcements from The DEC Connection". The DEC Connection. 2007-02-14. Retrieved 2008-11-24. 
  2. ^ "PDP-8 Frequently Asked Questions". 2001-04-08. Retrieved 2008-11-24. 
  3. ^ "Preliminary Specification - Programmed Data Processor Model Three (PDP-3)" (PDF). Digital Equipment Corporation. October 1960. 
  4. ^ Burkhardt, H.; Selighman, L. (July 24, 1967). "PDP-X Processor Description" (PDF). 
  5. ^ "PDP-9". Digital Computing Timeline. Computer History Museum. Retrieved 6 March 2013. 
  6. ^ Bell, C. Gordon; Mudge, J. Craig; McNamara, John E. (May 12, 2014). Computer Engineering: A DEC View of Hardware Systems Design. Digital Press. p. 162. 

External links[edit]

Various sites list documents by Charles Lasner, the creator of the alt.sys.pdp8 discussion group, and related documents by various members of the alt.sys.pdp8 readership with even more authoritative information about the various models, especially detailed focus upon the various members of the PDP-8 "family" of computers both made and not made by DEC.