My high school had a couple of computers. The main one was a PDP-8e, and the lesser-known one was a PDP-4 (which was kept in one of the teacher's offices; most people didn't know it was there). The PDP-4 was very interesting. It was an 18-bit machine. I don't think it had any integrated circuits at all - just transistors, resistors, capacitors, etc. It came with a full set of prints which I never really looked at, but a friend of mine did. My understanding was that it used "negative logic", which I believe meant low for 1 and high for 0. It had a variable speed control, which was fun to play with. You'd enable it, then you could turn it down all the way to a point where there was a second or so between cycles. It also had a monstrous looking baudot console, with "figure shift" and "letter shift". The console was seriously prehistoric looking. It supported both 1's and 2's complement arithmetic - I guess the jury was still out on which was better, so they implemented both. I believe it was a successor of, and architecturally similar to, the PDP-1. Here's a PDP-4 manual: http://archive.computerhistory.org/resources/text/DEC/pdp-4/DEC.PDP4.1963.10... Tom