PDP-11 – Everything You Need to Know

6 Facts about The DEC PDP-11 Computer

• If the DEC PDP-11 computer were to be manufactured today, it would cost around $140,000.
• The DEC PDP-11 computer was so successful it was cloned and copied by Eastern European Socialist Countries.
• The DEC PDP-11 computer is one of the world’s longest-serving computers as it is still in use today.
• The DEC PDP-11 computer was the world’s best-selling computer in 1980. Some experts consider it the most successful computer of all time.
• In the year 1994, DEC sold the PDP-11 product line to an Irish Company called Mentec Inc.
• The influence of the PDP-11 is still felt today as it has influenced tons of other machines.

The History of The DEC PDP-11 Computer: What to know

The first commercial minicomputer introduced to the market was the PDP-5 which used a 12-bit design based on the LINC Machine. The product was a success as it sold around 50,000 copies. Around this period, the demand for smarter computers was increasing.

During the late 1960s, DEC started work on building a 16-bit machine. Edson de Castro happened to be the engineer saddled with creating the design for the product, which was codenamed PDP-X. However, the project was canceled in the spring of 1968 as the management did not consider it a major improvement over their already existing 12- and 18-bit platforms. Edson de Castro left DEC with some group of friends to establish Data General, where he built and launched the 16-bit NOVA computer in 1968.

Later on, Gordon Bell, who was the Vice President of Research in DEC, recommended one of the designs of Harold McFarland, who happened to be working on several computer designs at Carnegie-Mellon University at the time. That architecture became the PDP-11, being entirely different from Castro’s intended PDP-X.

The design of the PDP would prove to be relevant and helpful to the many other machines that towed the same part. The prototype was ready in 1969, and the final product was launched to the market in January 1970.

The DEC PDP-11 Computer included several innovative features that made it stand out from the other products before it as it was easier to program. In addition, the use of the innovative Unibus system made it easy for the minicomputer to interface with external devices using memory access. This made it possible for the PDP-11 to use a wide range of peripherals.

The first model of PDP-11 (priced $20,000), named PDP-11/20, was shipped in the spring of 1970. It had a word length of 16 bits, a speed of 800 nanoseconds, the cycle time was 1.5 microseconds, and the access time 0.75 microseconds.

The CPU had eight 16-bit registers, six general-purpose, the stack pointer, and the program counter. Primary memory was magnetic core, 56 Kbytes (28 KWords) maximum (some documentation referred to 32k max memory, but the top 4k was reserved for the I/O space). Initial software included symbolic editor, debugger, utilities. PAL Architecture, console, and typical I/O were UNIBUS, TTY ASR33, and a paper tape reader/punch, respectively.

The product went on to sell over 170,000 copies throughout the 1970s. More models of the PDP-11 were built over the coming years, and improvements were made time and time again. One significant change was from a small-scale transistor-transistor logic to a single-board large-scale integration version of the processor developed in 1975.

Also, in 1979, a two or three-chip processor, known as the J-11, was developed. Some of the final models of the PDP-11 line included the PDP-11/93 and PDP-11/94, which were introduced in 1990.

The PDP-11 is regarded as one of the most successful computers of all time. Some would even dare to call it the best. It was so successful that it was cloned and copied by the so-called Iron Curtain, i.e., Eastern European Socialist countries.

Several plants produced PDP-11 compatible systems in the Soviet Union (СМ-4, СМ-1420/xxx, СМ-1600/xxx, Електроника-xxx, etc.), Bulgaria (СМ-4, СМ-1420/xxx), DDR (SM-1420/xxx), Poland (Mera-xxx), Hungary (SM-4). Nobody knows how many of these clones were issued by many plants in the socialist countries, but some believe that the total number of units should be tens of thousands.

The golden age of the DEC PDP-11 Computer series was between the 1970s and 1980s, in which it sold around 600000 copies. However, the emergence of new and more functional technology led to a decline in the reign of the PDP-11 series.

The Unibus and the Q-Bus’s limited throughput and the 16-bit logical address made it impossible to develop larger software applications. Additionally, the emergence of other computers like the IBM PC and microprocessors like the Intel 8088(1984) and the Intel 80386 (1985) made things more difficult for the PDP-11 series.

The DEC released the DEC Professional Series based on the PDP-11 and intended to compete with the IBM PC. The product was a commercial failure, along with other DEC products which weren’t variants of the PDP-11.

Just before the end of its life, the PDP-11 computer was severally implemented as chips. While some of these were produced by the DEC, many other third-party companies were involved in such productions.

In 1994, after much struggle to win back the market, DEC sold the PDP-11 system software rights to Mentec Inc., an Irish-based company. DEC finally discontinued the production of the PDP-11 series. Mentec, on the other hand, produced new PDP-11 processors for several years. The PDP-11 was succeeded by the VAX-11, which is also regarded as one of the most successful and studied computers of all time.

The DEC PDP-11 Computer Versions: Each Edition

The DEC PDP-11 Computer had various models, which could all be categorized into two broad categories.

The Unibus Models

The computers under this category used the Unibus as their principal bus. This included the PDP-11/20, PDP-11/15, PDP-11/45 (1972), PDP-11/50 (1975), PDP-11/55 (1976), etc.

The Q-Bus Models

These models used the Q-Bus as their principal bus. They include the PDP-11/03, PDP-11/23, PDP-11/23+, MicroPDP-11/78, MicroPDP-11/53, MicroPDP/83, etc.

The Public Response

The entry of the DEC PDP-11 Computer series into the market caused a great stir. It was, in a sense, a pioneering product that came with great features that were not available on former PDP or DEC products. The product sold around 170,000 copies quickly and sold about 600,000 copies during its golden age. The product is still considered as one of the most successful computers of all time.

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