(Updates from 11:39 a.m. ET with closing information.)

NEW YORK ( TheStreet) -- Apple ( AAPL ) shares rose 1.79% to $498.31 after Carl Icahn revealed he owns a "large stake" in the company.

In a tweet posted at 2:21 ET yesterday afternoon, the 77-year-old billionaire investor tweeted, "We currently have a large position in APPLE. We believe the company to be extremely undervalued. Spoke to Tim Cook today. More to come." Four minutes later, he added, "Had a nice conversation with Tim Cook today. Discussed my opinion that a larger buyback should be done now. We plan to speak again shortly." In response, Apple stock closed up 4.75% at $489.57 on Tuesday.

CEO Tim Cook has previously shown a willingness to comply with shareholder demands. Last month, he mollified a contingent of investors, including hedge fund manager David Einhorn, by buying back $16 billion in shares to return cash to shareholders.

Microsoft ( MSFT ) shares dropped 0.71% to $32.00 after announcing the release date of its Windows 8.1 operating system.

The company announced in a blog post that on October 17, "Windows 8.1 will begin rolling out worldwide as a free update for consumers on Windows 8 through the Windows Store," as well as at retail and on new devices.

Windows 8.1's predecessor, Windows 8, was widely criticized after its release in October 2012. Windows 8.1 is intended to update and improve on Windows 8; the new software's release comes only a year after Windows 8's.

Changes range from a new camera app to improved cloud integration.

Shares of Brocade Communications Systems ( BRCD ) jumped 13.99% to $7.86 after it reported strong third quarter earnings.

The San Jose-based company, which produces data and storage networking products, reported revenues of $536.6 million, a 3% year-over-year decline, yielding earnings per share of 19 cents. Analysts polled by Thomson Reuters expected 12 cents per share on $518.84 million in revenues.

"In Q3, Brocade exceeded our guidance for revenue, non-GAAP operating margin, non-GAAP EPS, and cash flow," said CEO Lloyd Carney in a statement. "The storage market is recovering more quickly than we had anticipated." He added, "We are making great progress toward our spending-reduction goal, and are already seeing the benefits in our financial results and cash flow."

Closing prices: AAPL rose 1.8% to $498.50, MSFT was flat at $32.35 but BRCD jumped 15.8% to $7.99.

-- Written by Laura Berman in New York