The PDP-1 (Programmed Data Processor-1) was the first computer in Digital Equipment Corporation's PDP series and was first produced in 1960. It is famous for being the computer most important in the creation of hacker culture, at MIT, BBN and elsewhere. The PDP-1 was also the original hardware for playing history's first computerized video game, Steve Russell's Spacewar!.


It has an 18-bit word and had 4 kilowords as standard main memory (equivalent to 9 kilobytes, or 9,000 bytes), upgradable to 64 kilowords (144 KB). The magnetic core memory's cycle time was 5 microseconds (corresponding very roughly to a "clock speed" of 200 kilohertz; consequently most arithmetic instructions took 10 microseconds (100,000 operations per second) because they had two memory cycles: one for the instruction, one for the operand data fetch. Signed numbers were represented in one's complement.

The PDP-1 was built mostly of DEC 1000-series System Building Blocks, using Micro-Alloy and Micro-Alloy-Diffused Transistors. Rated switching speed: 5 MHz.


The PDP-1 used punched paper tape as its primary storage medium. Unlike punched card decks, which could be sorted and re-ordered, paper tape was difficult to physically edit. This inspired the creation of text-editing programs such as Expensive Typewriter and TECO. Because it was equipped with online and offline printers that were based on IBM electric typewriter mechanisms, it was capable of what, in eighties terminology, would be called "letter-quality printing" and therefore inspired TJ-2, arguably the first word processor.

The console typewriter was the product of a company named Soroban Engineering. It was an IBM Model B Electric typewriter mechanism modified by the addition of switches to detect keypresses and solenoids to activate the typebars. It used a traditional typebar mechanism, not the "golfball" IBM Selectric typewriter mechanism which was not introduced until the next year. Case shifting was performed by raising and lowering the massive type basket. It was equipped with a two-color red-and-black ribbon, and the interface allowed color selection. Programs commonly used color coding to distinguish user input from machine responses. The Soroban mechanism was unreliable and prone to jamming, particularly when shifting case or changing ribbon color, and was widely disliked.

Offline devices were typically Friden Flexowriters that had been specially built to operate with the FIO-DEC character coding used by the PDP-1. Like the console typewriter, these were built around a typing mechanism that was mechanically the same as an IBM Electric typewriter. However, Flexowriters were highly reliable and often used for long unattended printing sessions. Flexowriters had electromechanical paper tape punches and readers which operated synchronously with the typewriter mechanism. Typing was performed about ten characters per second. A typical PDP-1 operating procedure was to output text to punched paper tape using the PDP-1's "high speed" (60 character per second) Teletype model BRPE punch, then carry the tape to a Flexowriter for offline printing.

Computer music

MIT hackers also used the PDP-1 for playing music in four-part harmony, using some special hardware—four flip-flops directly controlled by the processor (filtered with simple RC filters). Music was prepared via Peter Samson's Harmony Compiler, a sophisticated text-based program with some features specifically oriented toward the efficient coding of baroque music. Several hours of music were prepared for it, including Bach fugues, all of Mozart's Eine kleine Nachtmusik, Christmas carols, and numerous popular songs.

Current status

Only three PDP-1 computers are still known to exist, and all three are in the collection of the Computer History Museum. One was a prototype, and the other two are production PDP-1C machines. One of the latter, serial number 55 (the last PDP-1 made) has been restored to working order, is on exhibit, and is demonstrated two Saturdays every month. The demonstrations include:

  • the game Spacewar!
  • graphics demonstrations such as Snowflake
  • playing music

The restoration is described on a special web page of the Computer History Museum

Simulations of the PDP-1 exist in SIMH and MESS, and paper tapes of the software exist in the archives

BBN was DEC's first customer for the PDP-1. MIT's PDP-1, donated by DEC in 1961, occupied the room next door to the TX-0 which was on indefinite loan from Lincoln Laboratory.

At the Computer History Museum TX-0 alumni reunion in 1984, Gordon Bell said DEC's products developed directly from the TX-2, the successor to the TX-0 which had been developed at what Bell thought was a bargain price at the time, about USD $3 million. At the same meeting, Jack Dennis said Ben Gurley's design for the PDP-1 was influenced by his work on the TX-0 display.

At the museum's PDP-1 restoration celebration in May 2006, Alan Kotok said his Mac G4 laptop was 10,000 times faster, came with 100,000 times the RAM and 500,000 times the storage, was 1/2000 the size, and cost 1/100 as much.

See also


External links

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