and DEC, and as they did they spread the game to other universities and institutions with a PDP-1 computer. As a result, Spacewar! was perhaps the first video game to be available outside a single research institute. Over the next decade, programmers at these other institutions began coding their own variants, including features such as allowing more ships and players at once, replacing the hyperspace feature with a cloaking device, space mines, and even a first-person perspective version played on two screens that simulates each pilot's view out of the cockpit. Some of these Spacewar! installations also replicated Saunders' gamepad. DEC learned about the game soon after its creation, and gave demonstrations of it running on their PDP-1, as well as publishing a brochure about the game and the computer in 1963. According to a second-hand account heard by Russell while working at DEC, Spacewar! was reportedly used as a smoke test by DEC technicians on new PDP-1 systems before shipping because it was the only available program that exercised every aspect of the hardware. Although the game was widespread for the era, it was still very limited in its direct reach: while less expensive than most mainframe computers, the PDP-1 was priced at and only 53 were ever sold, most without a monitor and many of the remainder to secure military locations or research labs with no free computer time, which prevented the original Spacewar! from reaching beyond a narrow, academic audience. Though some later DEC models, such as the PDP-6, came with Spacewar! pre-loaded, the audience for the game remained very limited; the PDP-6, for example, sold only 23 units. Distribution and legacy Spacewar! was extremely popular in the small programming community in the 1960s and was widely recreated on other minicomputer and mainframe computers of the time before migrating to early microcomputer systems in the 1970s. Just as it was during development, the game was public domain and the code was available to anyone with access to it or who contacted Russell; no attempt was made to sell it commercially, as the programming community was too small to support any commercial industry. It spread initially both by people bringing copies of the code to other installations as well as by programmers recreating the game with their own code. Early installations included the PDP-1 at Bolt, Beranek, & Newman, which also recreated the gamepads; an installation by Russell