LSI-11

The PDP-11 is a series of 16-bit minicomputers sold by Digital Equipment Corporation (DEC) from 1970 into the 1990s, one of a succession of products in the PDP series. The PDP-11 had several uniquely innovative features, and was easier to program than its predecessors through the additional general-purpose registers. The PDP-11 replaced the PDP-8 in many real-time applications, although both product lines lived in parallel for more than 10 years. In total, around 600,000 PDP-11s of all models were sold, making it one of DEC's most successful product lines. Its successor in the mid-range minicomputer niche was the 32-bit VAX-11, named as a nod to the PDP-11's popularity.

The PDP-11 is considered by some experts to be the most popular minicomputer ever. Design features of the PDP-11 influenced the design of late-1970s microprocessors including the Intel x86 and the Motorola 68000.

Design features of PDP-11 operating systems, as well as other operating systems from Digital Equipment, influenced the design of other operating systems such as CP/M and hence also MS-DOS. For a decade, the PDP-11 was the smallest system that could run Unix; the first officially named version ran on the PDP-11/20 in 1970. It is commonly stated that the C programming language took advantage of several low-level PDP-11–dependent programming features, albeit not originally by design.

In 1967–68, DEC engineers designed a 16-bit, word-addressed machine. Management cancelled the project and some of the engineers later left DEC and produced it as the Data General Nova. A subsequent effort, code-named "Desk Calculator", looked at a variety of options before choosing what became the 16-bit PDP-11; DEC's previous PDP-8 and PDP-9 had 12- and 18-bit words, respectively. The PDP-11 family was announced in January 1970 and shipments began early that year. DEC sold over 170,000 PDP-11s in the 1970s. Initially manufactured of small-scale transistor–transistor logic, a single-board large scale integration version of the processor was developed in 1975. A two-or-three-chip processor, the J-11 was developed in 1979. The last models of the PDP-11 line were the PDP-11/94 and -11/93 introduced in 1990.

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