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Tracing the History of the Computer - DEC PDP-1



Digital Equipment Corporation's first computer was the PDP-1

The PDP-1 (Programmed Data Processor-1) was the first computer in Digital Equipment'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 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 just starting to become popular. 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.[1] 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.

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 Pete 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.


It had an 18-bit word and had 4K words as standard main memory (equivalent to 9 kilobytes), upgradable to 64K words (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.

The PDP-1 was built mostly of DEC 1000-series system modules, using Micro-Alloy and Micro-Alloy-Diffused Transistors. Rated switching speed: 5 MHz. It is emulated by M.E.S.S.


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Steve Russell

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