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Home * Hardware * PDP-11

PDP-11/40 [1]

a series of 16-bit minicomputers manufactured and sold by Digital Equipment Corporation from 1970 into the 1990s. The PDP-11 was a little-endian machine, concerning the byte-order of 16-bit words in memory [2]. The first officially named version of Unix ran on the PDP-11/20 in 1970. The C programming language was written to take advantage of PDP-11 features to rewrite Unix in a high level language. Further, the chess machine Belle by Ken Thompson and Joe Condon was composed of a PDP-11/23 with several custom boards.


Ken Thompson (sitting) and Dennis Ritchie at PDP-11 (2876612463).jpg

Ken Thompson (sitting) and Dennis Ritchie working together at a PDP-11, ca 1972 [3]

Orthogonal Instruction Set

The PDP-11 processor architecture had a mostly orthogonal instruction set, and influenced the design of microprocessors, such as Motorola's 68000. Almost any operand could apply any of eight addressing modes to eight registers R0 to R7, where R0 to R5 were general purpose registers, R6 the Stack- and R7 the instruction pointer.


A second innovation was the memory bus called Unibus - input and output devices were mapped to memory addresses, and no special I/O instructions or buses were needed.

See also



External Links


  1. PDP-11/40 as exhibited in Vienna Technical Museum. The processor is at the bottom. A TU56 dual DECtape drive is installed above it. Photo by Stefan Kögl, December 18, 2005, CC BY-SA 3.0, Wikimedia Commons, PDP-11 from Wikipedia
  2. Endianness and hardware from Wikipedia
  3. 11.5 An amusing photo from Lecture "UNIX" by Jürgen Plate (German), CC BY-SA 2.0, Wikimedia Commons, Ken Thompson from Wikipedia
  4. Gordon Bell's Publications

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