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The famous PDP-7 comes to the rescue

While mulling over the problems of operating systems in 1969, Thompson in his spare time developed a computer game called "Space Travel." The game simulated the motion of the planets in the solar system. A player could cruise between the plants, enjoy the scenery, and even land the ship on the planets and moons.

The game, first written on Multics and then transliterated into Fortran for the GECOS operating system, ran on a GE 635 computer. The game's display was jerky and hard to control because the player had to type commands to control the ship. Also, it cost about $75 in CPU time on the big GE 635, a cost that hardly endeared it to management.

"It did not take long, therefore, for Thompson to find a little-used PDP-7 computer with an excellent display terminal," Ritchie explained. "He and I rewrote 'Space Travel' to run on this machine." Their effort included a floating-point arithmetic package, the pointwise specification of the graphics characters for the display, and a de-bugging subsystem that continuously displayed the contents of typed-in locations in the corner of the screen.

"All this was written in assembly language for a cross-assembler that ran under GECOS and produced paper tapes to be carried to the PDP-7," Ritchie said. "'Space Travel,' though it made a very attractive game, served mainly as an introduction to the clumsy technology of preparing programs for the PDP-7."

"It was the natural candidate as the place to put the file system," Thompson said. "When we hacked out this design, this rough design of the file system on the dictation machine that day in Canaday's office, I went off and implemented it on the PDP-7."

Next: The UNIX system begins to take shape

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