The earliest distributions of Unix from Bell Labs in the 1970s included the source code to the operating system, allowing researchers at universities to modify and extend Unix. The first Unix system at Berkeley was aPDP-11 installed in 1974, running Version 4 Unix, and the computer science department used it for extensive research thereafter. In 1975, Ken Thompson took a sabbatical from Bell Labs and went to his alma mater, UC Berkeley. There, Thompson helped to install Version 6 Unix on a PDP-11/70.
Other universities became interested in the software at Berkeley, and so in 1977 Bill Joy, then a graduate student at Berkeley, started compiling the first Berkeley Software Distribution (1BSD), which was released on March 9, 1978. 1BSD was an add-on to Version 6 Unix rather than a complete operating system in its own right; its main components were a Pascalcompiler and Joy's exline editor.