WASHINGTON (CNN/Money) - A computer expert testifying on behalf of Microsoft Corp. in the ongoing antitrust action stumbled several times while on the stand.

"I'm not trying to be evasive," Stuart E. Madnick, a computer and software expert and professor at MIT, said at one point during Wednesday's testimony. "I'm just trying to be precise."

Madnick was sometimes anything but precise, however. When government attorney Kevin Hodges asked him to name an operating system besides those made by Microsoft in which the Web browsing software could not be removed, Madnick immediately offered up KDE as an example. But KDE is a computer program designed to run on top of the Linux operating system, as Hodges pointed out. Madnick conceded that was true, and instead suggested GNOME as an example.

But GNOME performs the same function as KDE on a computer equipped with the Linux operating system. Hodges was never able to get an answer to his question.

One of the key issues in the antitrust case revolves around Microsoft's incorporation of the Internet Explorer Web browsing software into its Windows operating system in such a way that IE can't be removed without damaging Windows. The courts found that Microsoft has monopoly power in the market for desktop computer operating systems, and violated antitrust law to protect that power.

Madnick was expected to return to the stand Thursday. Microsoft executive Will Poole should take the stand as soon as Madnick completes his appearance.

The U.S. Department of Justice and half the states involved in the original antitrust action are ready to settle with Microsoft, but the rest of the states involved in the original case say the proposed settlement doesn't go far enough. The current hearings, before U.S. District Judge Colleen Kollar-Kotelly, are intended to decide what should be done.

Hodges and Madnick sparred over a number of issues, including the concept of interoperability. Critics, including the dissenting states, have argued that Microsoft deliberately designs its products to interfere with technology made by other companies, forcing people to use Microsoft products. The dissenting states are pressing for complete interoperability. But Madnick argued that being able to exchange and use data on any level -- even if the process is clumsy -- is enough to claim interoperability.

Madnick argued that perfect interoperability, which would allow products to be substituted for each other with no performance degradation, was a theoretical impossibility. "It would be surprising if two different products behaved exactly alike," he told the court Wednesday.

Madnick testified that Microsoft (MSFT: Research, Estimates) probably would not be able to develop and market a workable version of Windows under the terms proposed by the dissenting states. He believes the requirements -- such as building Windows in such a way that computer manufacturers could alter it -- are not technically feasible.

Hodges hammered away at Madnick throughout the day, leaving the weary academic floundering at times.

Asked to evaluate language in the proposed settlements, Madnick studied the documents, then shook his head and said, "I somehow think there's something I'm missing, but I can't spot it at the moment."

Wednesday morning, Kollar-Kotelly returned to the issue of whether to allow the states to introduce additional evidence into the case. She held open the possibility that the documents could be introduced during the rebuttal phase of the trial, after each side has made their case. But she was clear that it wouldn't happen while Microsoft was presenting its witnesses. "You can't get these in during their case," she told Howard Gutman, an attorney for the states. Kollar-Kotelly may rule on the issue Thursday.

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The issue arose Tuesday after Microsoft withdrew several witnesses Monday night, one of whom, Richard Fade, is senior vice president of the OEM division, which works with computer manufacturers. But the government lawyers were counting on his testimony to introduce documents as evidence that Microsoft has continued to use its monopoly power to squeeze computer manufacturers.

The government team apparently had every intention of introducing the documents as part of its own case -- as the courtroom rules require -- but botched that attempt several weeks ago by apparently misunderstanding the court's rules on the introduction of witnesses. The states tried to introduce deposition testimony from over a dozen witnesses near the end of their case, but Kollar-Kotelly, in response to an objection by Microsoft, ruled that those individuals should have been on the original witness list and denied the states an opportunity to bring whatever facts those witnesses might have offered into the trial.

Wednesday, Gutman brought the number of documents he'd like to see admitted down to 7, and Kollar-Kotelly said she'd take the request under advisement, though she seemed unenthusiastic.  

-- from CNNfn producer Dave Wilson