The PDP-11

If IBM is known for their mainframe computers, Digital Electronics Corporation (DEC) was most famous for their contributions to the minicomputer industry.

By the 1970s, mainframe computers were being replaced by smaller yet more powerful computers known as miniframe or minicomputer (not to be confused with the microcomputer) became popular with businesses and governments.

IBM released several minicomputers – most notably the System/3x series. This included the System/32, System/34, System/36, and System/38. IBM also made other minicomputers, like the 1130, before the System/3x series was introduced. However, none of these would be as popular or as industry-changing as the DEC minicomputers.

While DEC released many minicomputers prior to the PDP-11 (including the PDP-8 and other PDP versions), the PDP-11 is probably their most notable computer. And for good reason. Similar to how the IBM System/360 changed the world in 1964, the PDP-11 changed the way we computed.

The PDP-11, or Programmed Data Processor-11, was introduced in 1970 and remained in production until the mid-1990s.

Major Contributions

The PDP-11 is known for introducing many “firsts” into the computing industry. At the time, it was the smallest machine that could run UNIX operating systems.

The PDP-11 also had an innovative instruction set that made it a popular system with programmers and design languages alike. The C programming language took advantage of the special instruction set.

The hardware was also changing, as many versions of the PDP-11 utilized LSI (Large Scale Integration) in its processors. Terminals with graphical displays were also more common with the PDP-11, even fairly early on, whereas most IBM System/360 machines relied on typewriter-based terminals. (Although IBM did have a terminal with a graphical display, it didn’t become popular until later in the S/360’s life.)

With being called a minicomputer, the PDP-11 consumed less space than the System/360 from IBM.

A DEC PDP-11 terminal in use. (Photo source:

The microcode featured in the PDP-11 also featured a debugging system that allowed the user to debug the system from terminals, rather than doing it manually at the control panel like IBM’s systems of the time.

A Decline

DEC’s PDP machines were all the rage in the 1970s, much like IBM during the 1950s and 1960s. However, the PDP-11 quickly faded out of the scene for most people in the late-1970s/early-1980s while smaller microcomputers were showing up on the scene.

In 1984, BYTE! magazine stated that the processor of many IBM PC and PC-clones could execute UNIX operating systems with more speed and power than the PDP-11 computer could. The Intel 80386 featured in later PC-clones, as well as the Motorola 68k microprocessors, could logically address up to 32-bits, whereas the DEC was limited to only 16-bits.

Speaking of 16-bits, this really hurt the PDP-11. While DEC addressed this problem with a later system, the VAX, sales of the PDP-11 hurt and companies replaced the PDP-11 with newer systems that could process 32-bits, such as other minicomputers.

In the 1990s, DEC themselves were sharply declining. DEC, along with other New England-based computing firms, weren’t doing so hot. While the rest of the industry wasn’t doing so hot, New England firms like DEC and IBM were hurting in the 1990s as most of the innovators – like Apple and Dell – were in other parts of the country, such as California and Texas (respectively.) In 1994, DEC sold rights to PDP software to an Irish manufacturer of hardware cards for the PDP-11.

In 1998, Compaq purchased DEC. In turn, Hewlett-Packard purchased Compaq in 2002.

Inspiration to the Industry

Much like SAGE, many great innovations and things that we take for granted to this day were children of the PDP-11 computer system.


A VAX-11 superminicomputer, which was an extremely popular computer during the 1980s and early 1990s with businesses. The VAX served as the successor to the PDP-11 machines. (Source:

In 1977, DEC released the VAX superminicomputer. The VAX, or Virtual Address eXtension, was extremely popular with businesses and large organizations – including the US Government. (Rumor has it that VAX computers still control the Minutemen ICBM missiles.)

Unlike the PDP-11, VAX equipment could address up to 32-bits. The computer featured a specialized instruction set which allowed programmers to easily create assembly programs, as most programmers of the time were working in Assembly language and similar low-level languages.

Other Inspirations

Besides directly influencing the creation of the VAX, the PDP-11 also inspired the design of many microprocessors and computer operating systems.

The microprocessor designs which were inspired by the PDP-11 included the Intel x86 (8086) and Motorola 68k microprocessor architectures. The PDP-11 also inspired the creation of operating systems like CP/M and MS-DOS.

The PDP-11 also had a role in electronic engine control systems used in automobiles. The PDP-11 was used by Ford Motor Company as a base to create the EEC  and EEC-II control modules in the late 1970s. This would later turn into the EEC-IV and EEC-V control modules that are found in almost any Ford car made after 1984. (Learn more about the EEC-IV and the history of the EEC here.)

The PDP-11 was an important machine and laid the foundation for many of the things we take for granted today. While the System/360 and IBM 1401 both hold their places in museums and textbooks, the PDP-11 also deserves a spot in those areas too.

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