Saturday, 7 November 2020

DEC PDP-1 (1960)

First unit shipped November 1960

In 1960, the Massachusetts firm Digital Equipment Corporation (typically known as “DEC” or just “Digital”) was just three years old. Founded by two ex-MIT engineers, DEC’s earliest products were small computing modules on cards that could be used in labs, the design of which was based on work originally done at MIT.

The company advanced quickly, and in 1959 it announced a minicomputer based on these modules, which they called the PDP-1. “PDP” stood for “Programmable Data Processor” – according to legend this was because DEC was afraid that IBM might sue if they called it a “computer”, but in reality it seems that this approach was because computer companies were regarded as high-risk for investors, where lab equipment companies were less so.


Although clever and flexible in approach, the modules still consisted of individual transistors and other components soldered to a board. This meant that the overall system was quite large (weighing hundreds of kilograms). It was 36-bit computer with the equivalent of just 9.2Kb of memory, mass storage was provided by paper tape and output could be to a printer or weird circular CRT that had basically been transplanted from a radar system.

Prices started at $130,000 for a basic system (around a million dollars today) which made it a feasible purchase for well-funded labs, universities and tech-oriented companies. It was essentially a single-user system until the development of timesharing OSes such as the one from BBN. As a comparison, an IBM 7090 mainframe computer of the same era cost around $2.9 million ($20 million today), and although the PDP-1 was much less powerful it was much more flexible and it could be programmed to do things that were impossible on a tightly buttoned-down mainframe.

Things that you couldn’t do on a mainframe but could do on a PDP-1 included playing synthesised music and a key early video game – Spacewar!. A precursor of much later video games such as Asteroids, the game used the Type 30 vector display of the PDP-1 to show two opposing spacecraft fighting each other around the gravitational field of a sun. The Type 30’s long persistent phosphor added to the space-age gameplay and of course it became a major reason for people to book time on their department’s PDP-1 for (ahem) “research”.


DEC were still a fledgling company, and although the PDP-1 was a success it only shipped 53 units. However, just five years later DEC was growing quickly and the 12-bit PDP-8 launched in 1965 shipped an astonishing 50,000 units over its lifetime, and five years after that the 16-bit PDP-11 eventually shipped 600,000 units. As an aside, DEC also made 36-bit and 32-bit computers in the 60s and 70s, to add to the confusion.

The architecture used in the PDP-1 turned out to be a dead end, ending in 1970 with the PDP-15. However, it helped start a revolution in small-scale computing which – after nearly two decades – allowed everybody who wanted one to have a computer on their desk. Unfortunately for DEC, they never could successfully transition from minis to micros, but that is another story.

Image credits:
M.Hawksey via Flickr – CC BY 2.0
Kenneth Lu via Flickr – CC BY 2.0

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