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The PDP-11 is a series of minicomputers introduced in 1969 [1] by the Digital Equipment Corporation (DEC), in production by them from 1970-1990. Their life-time spanned a period of momentous changes in the computer world: when they were first introduced integrated circuits had just been introduced (small ones, containing only a few simple gates); and for a little less than half their life, core memory was still the standard main memory technology. By the end, the now-ubiquitous microprocessors and dynamic RAM had completely taken over.

Front panel of the KA11, the first PDP-11 model

It was the machine which made UNIX, which now seems to have taken over the world (in the form of Linux) widely known and popular.

The machine word size was 16 bits, and it was a general register architecture. Although it was not the first to feature the latter, its wide distribution (in 1980, it was the world's best-selling computer) and its novel addressing modes (below) have helped influence almost all later machines to follow that path.

(For more information about the architecture, including its innovative and much-copied stack-oriented addressing modes, see here.)

PDP-11's came in two groups: those which used the UNIBUS for a bus, and the later ones which used the QBUS. Eventually DEC stopped producing UNIBUS PDP-11's (the last were the PDP-11/44 and PDP-11/24); later 'UNIBUS' machines (the PDP-11/84 and PDP-11/94) actually contained QBUS processors with a QBUS<->UNIBUS adapter board.

Towards the end of its life, there were several microprocessor implementations of the PDP-11, as chips. After DEC discontinued production of PDP-11's, the line was sold to Mentec, who produced a few newer models.

PDP-11 Models and notes

Model Introduced Bus Type Addressing Split I & D Notes Speed (VUPS)
11/20 1969[1] UNIBUS 16-bit no
11/05 1972 UNIBUS 16-bit no OEM model
11/10[*] 1972 UNIBUS 16-bit no
11/15 1972 UNIBUS 16-bit no OEM model
11/35 1973 UNIBUS 18-bit no OEM model
11/40 1973 UNIBUS 18-bit no
11/45 1973 UNIBUS 18-bit yes core memory
11/50 1975 UNIBUS 18-bit yes MOS memory
11/55 1976 UNIBUS 18-bit yes fast bipolar memory
11/70 1975 UNIBUS 22-bit yes 0.6
11/03 1975 QBUS 16-bit no first QBUS model 0.05
11/04 1976 UNIBUS 16-bit no 0.11
11/34 1976 UNIBUS 18-bit no 0.21
11/60 1977 UNIBUS 18-bit no writable control store
11/23 1979 QBUS 18-bit or 22-bit no first F-11 0.12
11/24 1979 UNIBUS 22-bit no only UNIBUS model to use F-11 chip 0.18
11/44 1979 UNIBUS 22-bit yes last non-LSI PDP-11 0.42
11/23+ 1981 Nov QBUS 22-bit no 0.18
11/73 1983 QBUS 22-bit yes first J-11 machine, 15MHz, integrated FPU, also first PMI PDP-11 0.45
11/53 1984 QBUS 22-bit yes S-box or standard QBUS, integrated FPU, 768KiW memory 0.29
11/83 1988 QBUS 22-bit yes J-11 at 18MHz, integrated FPU 0.72
11/84 1988 UNIBUS 22-bit yes J-11 at 18MHz, integrated FPU 0.72
11/93 1990 QBUS 22-bit yes J-11 at 18MHz, integrated FPU, 2MiW onboard memory 1.0
11/94 1990 UNIBUS 22-bit yes J-11 at 18MHz, integrated FPU, 2MiW onboard memory 1.0

[*]The name PDP-11/10 was recycled by DEC from an earlier KA11 CPU-based 11/10 from 1969, or at least it existed in advertisements[1]

Operating Systems

There were a number of well-known operating systems for the PDP-11. Many were produced by DEC themselves, but several were produced by third-parties. Often DEC would purchase or re-brand such an OS and re-sell it as their own product; for example, UNIX sold as Ultrix by DEC.

UNIX was not originally a PDP-11 system (it was born on the PDP-7), but it moved to the PDP-11 shortly after it was first created, and spread widely there (since PDP-11's were common machines, due to their relatively low cost and good engineering), with the real explosion of use starting with UNIX Sixth Edition.


These are the original Bell Laboratories releases of Unix; the first 4 were only internal to Bell, the Fifth saw limited distribution outside it, and the Sixth took over the world.

For PDP-11's without memory management.

This was the first shipping Unix distro by AT&T. It only supported the PDP-11 and VAX computers.

This version was a port of the 4.3 BSD feature set to the PDP-11. Although considered an impossible goal by many, it accomplished this by using overlays for portions of the kernel, and to allow for user programs larger than 64KB.

This last version is still supported, and if one really felt the need to load a Unix for use on a PDP-11 this would be the best fit. It has support for TCP/IP, large memory space and is the best UNIX experience one can get going to get on a 16-bit mini.


Technically also an OS is XXDP, which is an 'overseer' for running the PDP-11 diagnostics produced/provided by DEC.

Real-time OS's

PDP-11's were very popular in real-time and data networking uses; a number of OS's were developed for such environments:


  • SITS - "Small ITS" for running Logo
  • Camexec - hosted the Camex typesetting system
  • Trantor - written for the MIT Applied Mathematics Department


There a number of good simulators for the PDP-11:

The latter is less well known than the ubiquitous SIMH, but is very fast; simulated systems running on Ersatz-11 are usually faster than real PDP-11's.

New Emulator Hardware

See also

External links