The 32-bit successor

The PDP-11 was so successful that its limits eventually mattered: sixteen bits could address only so much memory. DEC's answer, in October 1977, was the VAX-11/780, the first of the VAX family, whose name stood for Virtual Address eXtension, literally a 32-bit extension of the 16-bit PDP-11, complete with a compatibility mode that could still run PDP-11 code. It shipped with the VMS operating system and became the canonical "one-MIPS" machine of its era. Through the 1980s the VAX and VMS dominated the mini and supermini market, the workhorses of engineering, science and research computing, and the direct institutional descendants of the PDP line.

The Unix thread

The most consequential software legacy of the PDP line is Unix. It began in 1969 on a cast-off PDP-7 at Bell Labs, was named and moved to the PDP-11 in 1970–71, and in 1973 was rewritten in C, the language Dennis Ritchie had developed on the PDP-11. That rewrite, an operating system written in a portable high-level language, is one of the hinges of computing history, and it happened on a DEC minicomputer. Unix then spread through the enormous PDP-11 install base and onward to the VAX; Berkeley's BSD line ran first on a PDP-11 and then on the VAX, carrying the tradition into the workstation era and, eventually, into Linux and macOS. The command line on your computer or laptop is, genealogically, a PDP program.

How the PDP-11 shaped what came after

The PDP-11's instruction-set architecture, its general-purpose registers, its rich and orthogonal addressing modes, its clean memory model, was widely admired and widely imitated. It is regularly cited as an influence on the microprocessors that defined the personal-computer era, including the Intel x86 and the Motorola 68000, and its operating-system conventions fed forward through CP/M into MS-DOS. The interactive, single-user, real-time machine the PDP-1 had prototyped became, by this route, the normalised shape of the personal computer.

The architectural influence is well attested in the secondary literature; "influence" rather than direct derivation is the careful word.

The company's later course

DEC carried on into the RISC era with the 64-bit Alpha architecture in 1992, but the minicomputer model that had been its making was overtaken by the very personal computers its own machines had helped inspire. Digital Equipment Corporation was acquired by Compaq in 1998, at the time the largest merger the computer industry had seen; Compaq merged with Hewlett-Packard in 2002, and the Alpha line was wound down. The company that coined "Programmed Data Processor" to avoid frightening its customers with the word computer had, by then, helped make the computer something anyone could sit down and use.

From Spacewar! to the machine on your desk

Spacewar! on the PDP-1's Type 30 display was an early, vivid proof that a computer could be interactive, real-time, and personal, something you played with at the console rather than queued jobs for. The PDP line carried that forward, through time-sharing and Unix and the VAX, into the architectures and habits of modern interactive computing.

← PDPs at universities   Spacewar! overview →

References

• Wikipedia, VAX-11, VAX, History of Unix, PDP-11, and Digital Equipment Corporation, cross-checked against the Computer History Wiki (gunkies.org).
• Computer History Museum, the Revolution exhibit on the PDP-11 and DEC's minicomputers.