I saw the original Spacewar! on a PDP-1 today

I went to the Computer History Museum today. I saw the Visual Storage exhibit, which is a collection of famous computers, the Babbage Difference Engine, which is a very elaborate reproduction of a never-actually-built Victorian era mechanical calculator, and the PDP-1 demo. This last demo was very special to me, because I finally got to play the original Spacewar! game, and meet and chat with Steve Russell, the main developer. (Perusing Wikipedia I now realize that Steve was also an early Lisp hacker. D’Oh!, I was going to ask a question about Lisp on the PDP-1, but I got distracted.)

There’s a Java Spacewar! emulator, but it doesn’t properly convey the look of the PDP-1 radar-scope-based display. The scope displays individual dots, 20,000 times per second. Each dot starts as a fuzzy bright blue-white dot, but then fades quickly to a dim yellow-green spot, which takes another 10 seconds to fade to black. This means that dim yell0w-green trails form behind the ships as they fly around. These trails add a lot to the game’s distinctive look. (In addition, due to time multi-plexing, the stars of the starfield are much dimmer than the space ships or the sun.) The fuzzyness of the dots means that the spaceships look much smoother on the PDP-1 scope than they do in the Java simulator.

According to Steve Russel and the other docents, the Java version also runs faster than a real PDP-1.

I also got to see serveral other cool PDP-1 hacks, including the original Munching Squares, 4-voice square-wave computer synthezed music, and the famed Minskeytron. The author of the music synth program, Peter Sampson, was present, and explained how he carefully patched into four of the console lights to make a four-voice D/A converter to get music out of the machine.

They keep all the hacks loaded into the PDP-1 core at the same time, and just use the front panel to decide which one to jump to. The core memory is non- volitile. The PDP-1 even booted in a few seconds – just the time it took the power supply to come up to speed.

The PDP-1 demo is given twice a month, on the second and fourth Saturdays. I highly recommend it for adults and children over 12. (It’s 45 minutes long, so younger kids might get bored.)