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He was there to make the case for porting Microsoft's Office suite to other operating systems such as Linux, explain the need for mandatory distribution of a compliant version of Sun Microsystems' Java, and justify the call that Internet Explorer code be made open source.
"Office is the albatross around Red Hat's neck," Tiemann had written in his prepared testimony.
"The first question that prospective customers ask when I approach them about switching to Linux is whether they could still run Office if they made the switch."
Tiemann reiterated that position on the witness stand, saying Office for Linux would do a "tremendous" amount to expand Linux's desktop adoption. At present, Red Hat Linux has about 2% desktop market share, he said.
However, Microsoft attorney Stephanie Wheeler argued that the success of vendors of operating systems was linked to their investment in research and development in applications.
Wheeler said Apple spent $431m (£303m) on research and development in 2001, Sun spent more than $2bn (£1.4bn) and Microsoft spent $4.3bn (£3bn). Red Hat spent $18.8m (£13.2m) last year, she added.
Tiemann said Red Hat relies on third parties to develop applications that run on Linux. The witness and attorney sparred over whether Sun's Office product, StarOffice, is interoperable with MS Office.
Wheeler cited material from Sun's Web site promising interoperability, but Tiemann argued that "the fact that they [Sun] promise that doesn't make it true".
Under the state's remedy proposal, Microsoft would be required to auction Office licences to independent software developers to create versions that would run on operating systems other than Windows.
Tiemann said he's "fairly certain" that Red Hat would bid on one of these licences to port Office to Linux.
During the antitrust trial that began in 1998, Microsoft attorneys pushed to convince the court that Linux was a threat. Since then, Linux has done well in the server market, but it has made no headway in denting Microsoft's desktop monopoly because PC makers will not install it, said Tiemann.
But the US states participating in the trial have evidence about Microsoft's concern about Dell's desktop Linux offering.
Gateway counsel Anthony Fama, who was expected to argue that Microsoft's new uniform contract licensing terms would increase the software giant's influence over Gateway, was expected to follow Tiemann on to the stand.