Dismissing a defamation suit brought by the inventor of DOS against a British writer, a judge has left unchallenged computer industry lore that holds the operating system Microsoft licensed to IBM in the 1980s -- thereby launching Bill Gates' multibillion dollar software empire -- was a knock off.

In a book on American innovation, author Sir Harold Evans wrote that DOS inventor Tim Paterson relied heavily on an existing OS called CP/M (Control Program/Monitor) created by a programmer who has since died. Microsoft in 1980 struck a licensing deal with Paterson's company -- Seattle Computer Products -- to obtain access to DOS and resell it to IBM.

DOS in its various incarnations, including MS-DOS and PC-DOS, ultimately netted Microsoft billions in sales and paved the way for the Windows operating system.

In his book "They Made America", Evans writes that Paterson, in developing DOS, took "a ride on" CP/M, which was created by the late Gary Kildall. Evans also wrote that Paterson's DOS operating system appropriated the "look and feel" of CP/M, copied its user interface, and "ripped-off" CP/M.

Paterson sued Evans -- the husband of former New Yorker editor Tina Brown and a fixture in British literary circles -- after the book was published in 2004. The suit, which sought unspecified damages, also named publishers Little, Brown and Co. and Time Warner Book Group.

In dismissing the lawsuit in a 34-page ruling handed down last week, U.S. District Court Judge Thomas Zilly noted that DOS' true lineage already has been widely questioned in the computer industry. "Even before IBM unveiled the IBM Personal Computer, the industry began to note similarities between DOS and CP/M," Zilly wrote.

Zilly further remarked that, "In the years that followed ... commentary on Paterson's DOS would become increasingly critical, with regard to its similarities to CP/M."

As a result, Zilly ruled that Evans' characterization of DOS as a "rip off" of CP/M is legally protected opinion under the First Amendment, in part because it's based on some facts not generally in dispute. Ultimately, Zilly said that "Tim Paterson has failed to provide any evidence that statements in Sir Harold Evans' chapter on Gary Kildall are provably false or defamatory."

Zilly also ruled that Paterson is a public figure for the purpose of cases related to the computer industry, meaning he faces a higher evidentiary threshold for proving defamation.

Paterson, who now operates Seattle-based hardware and software developer Paterson Technology, said in an e-mail Monday to InformationWeek that he was "disappointed" in Zilly's decision.

The public figure ruling "turns attacks on my personal integrity into protected free speech," he wrote.

Paterson added that he has made a number of suggestions to Evans for revisions to the book. "If he was honest with me about setting the record straight, he has a chance to prove it in the next printing," Paterson said in his e-mail. "That's all I ever wanted."