[1][2] The PDP–11 included a number of innovative features in its instruction set and additional general-purpose registers that made it easier to program than earlier models in the PDP series.

In 1967–1968, DEC engineers designed a 16-bit machine, the PDP–X,[5] but management ultimately canceled the project as it did not appear to offer a significant advantage over their existing 12- and 18-bit platforms.

[6] The Nova sold tens of thousands of units and launched what would become one of DEC's major competitors through the 1970s and 1980s.

Ken Olsen, president and founder of DEC, was more interested in a small 8-bit machine than the larger 16-bit system.

Not long after, Datamation published a note about a desk calculator being developed at DEC, which caused concern at Wang Laboratories, who were heavily invested in that market.

[7] The team decided that the best approach to a new architecture would be to minimize the memory bandwidth needed to execute the instructions.

Larry McGowan coded a series of assembly language programs using the instruction sets of various existing platforms and examined how much memory would be exchanged to execute them.

Harold McFarland joined the effort and had already written a very complex instruction set that the team rejected, but a second one was simpler and would ultimately form the basis for the PDP–11.

It lacked single instruction-word immediate data and short addresses, both of which were considered essential to improving memory performance.

McGowan and McFarland were eventually able to convince them that the system would work as expected, and suddenly "the Desk Calculator project got hot".

[7] At a late stage, the marketing team wanted to ship the system with 2K of memory[a] as the minimal configuration.

[8] Initially manufactured of small-scale transistor–transistor logic, a single-board large-scale integration version of the processor was developed in 1975.

More complex instructions such as add likewise can have memory, register, input, or output as source or destination.

DEC openly published the basic Unibus specifications, even offering prototyping bus interface circuit boards, and encouraging customers to develop their own Unibus-compatible hardware.

Although input/output devices continued to be mapped into memory addresses, some additional programming was necessary to set up the added bus interfaces.

The CPU microcode includes a debugger: firmware with a direct serial interface (RS-232 or current loop) to a terminal.

This option allowed programming of the internal 8-bit micromachine to create application-specific extensions to the PDP–11 instruction set.

The WCS is a quad Q-Bus board with a ribbon cable connecting to the third microcode ROM socket.

The source code for EIS/FIS microcode was included so these instructions, normally located in the third MICROM, could be loaded in the WCS, if desired.

DEC's 32-bit successor to the PDP–11, the VAX–11 (for "Virtual Address eXtension") overcame the 16-bit limitation, but was initially a superminicomputer aimed at the high-end time-sharing market.

The 68000 in particular facilitated the emergence of a market of increasingly powerful scientific and technical workstations that would often run Unix variants.

Personal computers based on the 68000 such as the Apple Lisa and Macintosh, the Atari ST, and the Commodore Amiga arguably constituted less of a threat to DEC's business, although technically these systems could also run Unix derivatives.

In 1994, DEC[14] sold the PDP–11 system-software rights to Mentec Inc., an Irish producer of LSI-11 based boards for Q-Bus and ISA architecture personal computers, and in 1997 discontinued PDP–11 production.

By the late 1990s, not only DEC but most of the New England computer industry which had been built around minicomputers similar to the PDP–11 collapsed in the face of microcomputer-based workstations and servers.

The PDP–11 processors tend to fall into several natural groups depending on the original design upon which they are based and which I/O bus they use.

The DEC Professional series are desktop PCs intended to compete with IBM's earlier 8088 and 80286 based personal computers.

As the design was intended to avoid software exchange with existing PDP–11 models, the poor market response was unsurprising.

[citation needed] Ersatz-11, a product of D Bit,[49] emulates the PDP–11 instruction set running under DOS, OS/2, Windows, Linux or bare metal (no OS).