A lunar landing program helps set Unix in motion

What happened

Thompson wrote the game on a large time-shared machine, where each session was billed against a research budget at a rate that made playing hard to defend. The simulation drew planets and moons and asked the player to manage thrust and orbits, with a successful landing as the object. When an idle PDP-7 with a display turned up in a corner of the laboratory, moving the program there looked like a way to keep playing without an invoice, and the port meant writing arithmetic and graphics routines from scratch.

The PDP-7 had no software worth the name, so getting anything to run meant building tools: an assembler, then a way to organize files on a disk, then the other pieces of a working environment. Thompson and Dennis Ritchie were already turning over operating system ideas after their group withdrew from a large collaborative project, and the small machine gave them somewhere to try them. The file system work in particular carried straight into the system that acquired the name Unix the following year.

The relationship is often compressed into a slogan, that a video game caused Unix, which overstates it. Thompson's own accounts describe the game as an occasion and a testbed rather than a motive, and the design ideas came from years of thinking about time-sharing. What the episode does show accurately is how often foundational software has been shaken out by somebody trying to make something enjoyable work properly on hardware that was not ready for it.

The world at the time

Bell Labs in 1969 was recovering from its part in an ambitious time-sharing project that had grown unwieldy, leaving a few researchers with strong opinions and no machine to test them on. Computer time was accounted for carefully. Displays able to draw moving pictures were uncommon, and a discarded PDP-7 with one attached was a genuine opportunity for anyone willing to write software.

What it changed

Unix spread through universities in the 1970s, brought the C language along with it, and shaped how software has been written ever since, including the tools used to build games. Space Travel itself was never distributed and is remembered only through the recollections of the people who wrote and played it, which is a fair summary of how much of this history survives.