The Jargon File (version 4.4.7, 29 Dec 2003):
Possibly the single most successful minicomputer design in history, a
favorite of hackers for many years, and the first major Unix machine, The
first PDP-11s (the 11/15 and 11/20) shipped in 1970 from DEC; the last
(11/93 and 11/94) in 1990. Along the way, the 11 gave birth to the VAX,
strongly influenced the design of microprocessors such as the Motorola 6800
and Intel 386, and left a permanent imprint on the C language (which has an
odd preference for octal embedded in its syntax because of the way PDP-11
machine instructions were formatted). There is a history site.
The Free On-line Dictionary of Computing (18 March 2015):
Programmed Data Processor model 11.
A series of minicomputers based on an instruction set
designed by C. Gordon Bell at DEC in the early 1970s (late
60s?). The PDP-11 family, which came after, but was not
derived from, the PDP-10, was the most successful computer
of its time until it was itself succeeded by the VAX.
Models included the 11/23 and 11/24 (based on the F11
chipset); 11/44, 11/04, 11/34, 11/05, 11/10, 11/15, 11/20,
11/35, 11/40, 11/45, 11/70, 11/60 (MSI and SSI); LSI-11/2
and LSI-11 (LSI-11 chipset). In addition there were the 11/8x
(J11 chipset) and SBC-11/21 (T11 chip) and then there was
compatibility mode in the early VAX processors.
The B and C languages were both used initially to
implement Unix on the PDP-11. The microprocessor design
tradition owes a heavy debt to the PDP-11 instruction set.
See also SEX.