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The calculating part of
it and the memory were solid state, but the power supplies were all
tube amplifiers and huge racks of equipment and it took up a lot of
space. The PDP-1 on the other hand, was entirely solid state and it
took up about as much space as two large refrigerators. The principal
difference was you could start it up yourself.”
An informal group of students, faculty, and research staff gathered in the halls of the Research Lab during the off-hours, eager to grab some time on the computers. “The PDP-1 was available pretty much at any time,” Graetz said. “Jack Dennis wrote out a schedule and people booked time on it.” Circulating through the mix were members of the Tech Model Railroad Club.
Ostensibly they were a group of gear heads devoted to model trains but they had become increasingly preoccupied with designing “hacks” or clever improvisations that created new configurations out of scavenged technology.
Russell and Graetz, among others, had plans for the machine as well. “One of the things we knew was coming was this CRT that was going to be interactive, something that was not the case with the big mainframe computers,” Graetz remembered.
“We thought how could we show off
what this thing can do and it didn’t take long to realize the best
way to show it off was with a game. It just seemed like a natural
tendency. We were still thinking about E.E. Smith in a movie and we
thought we could we do something like that. It didn’t take very
long for us to figure out that the right kind of game would be a
two-person game in which you tried to shoot each other out of space,”
“When we told Jack Dennis that we wanted to do this thing called Spacewar and could we have time on the computer he said, ‘I’ll give you a trade. If you can develop essentially the same assembly and diagnostic software, debugging software, that we had on the TX-0 for the PDP-1 over the weekend then, yeah, you can do this’,” Graetz remembered.
Enthusiastic volunteers immediately set to work creating the software tools needed to make the machine dance. “What they did was they took the pieces of what amounted to an assembly and debugging program that we used on the TX-0 and they wrote the MACRO assembler and [the] DDT [debugging program],” Graetz said.
“Having those things in hand, then we were allowed to have time on the machine to develop [Spacewar]. It started really with Slug programming the control program and then all of us pitching in to write different pieces of it and it went from there. We needed a few sub-routines that Alan Kotok got from Digital, a couple of multiplication and division sub-routines,” Graetz recalled.
Initially, Russell was slow to start work on the game and the hackers were getting impatient. In Steven Kent’s book The Ultimate History of Video Games, Russell said, “Eventually, Allen Kotok came to me and said, ‘Alright, here are the sine-cosine routines. Now what’s your excuse?’”
Spacewar began to take shape in January of 1962 with a simple program that allowed controller switches to change the direction and acceleration of a dot moving on the computer’s CRT. Within a month Russell had refined the program, creating two spaceships moving independently across the screen.
In Kent’s book, Russell further states, “They [the rockets] were rather crude cartoons. But one of them was curvy like a Buck Rogers 1930s spaceship. And the other one was very straight and long and thin like a Redstone rocket.”