Pretrial
Since the allegedly infringing software had been released to the public by the UC Berkeley, most of the case would hinge on events there. Because UC Berkeley was not originally a party to the suit, the University made its arguments against an injunction in a series of amicus briefs.
The University submitted the licenses UC Berkeley had from AT&T for UNIX, which specifically stated that the copyright on the Berkeley software built upon the 32V version of UNIX belonged to the University. They went on to claim that AT&T had confirmed this by allowing the free redistribution of NET-1, as well as allowing the distribution of the later BSDs, in which specific source code files were marked as containing no AT&T code, and were freely redistributable. Under US law, allowing this distribution was considered abandoning the copyright, so that code could not be considered a copyrighted part of UNIX.
Assuming this was true, the University still needed to show that NET-2 did not contain any validly copyrighted AT&T UNIX code. One claim the University made was that USL's copyright in the 32V version of UNIX that NET/2 was based on was invalid. At the time 32V was released, US copyright law did not automatically presume that a released work was copyrighted. In order to claim copyright, it was necessary to include copyright notices in the work—which the 32V source code did not have—or to register the work with the government. AT&T did not register the copyright on 32V UNIX until 1992, and so the grace period for registering an already-released work had expired.
The University also claimed that similar lines of source code (which were presented during discovery) did not infringe on USL's copyright because they had become public domain by the actions of AT&T: AT&T had promoted UNIX as a standard, licensing it to universities and allowing UNIX source code to be published in textbooks. The University submitted briefs from the UC Berkeley students and staff, explaining how they had audited the code, looking for freely available copies of the source code and methods. When they could find one, they said, they removed the code and rewrote it using publicly known techniques—and so any remaining similarities existed because AT&T had effectively abandoned the copyright to them.
The University also argued that the source code did not infringe because it was necessary for program compatibility—that certain code could be written only one way and still be compatible with the standards set by organizations like POSIX (which AT&T supported), and so was no longer a "creative" work that could receive copyright protection under US law.
Even if the code were validly copyrighted AT&T UNIX code, the University claimed, that would not be a copyright violation because it made up such a small fraction of the whole of NET-2 that it was not legally a derived work.
Read more about this topic: USL V. BSDi