By Rex Crum, MarketWatch

SAN FRANCISCO (MarketWatch) -- Tech stocks were mostly in the red in early trading Thursday, with Apple Inc. one of the rare exceptions following a letter from activist investor Carl Icahn to Apple Chief Executive Tim Cook.

Icahn, who owns about 53 million shares of Apple's stock, reiterated his support of Cook. However, Icahn also said Apple's stock is trading at about half of what it should be, and the company should return more of its $133 billion cash hoard to shareholders.

Apple (AAPL) was up by almost 1%, at $101.54 a share following the release of Icahn's letter to Cook.

Elsewhere, decliners ruled investors' sentiment, with Advanced Micro Devices Inc. (AMD) receiving some of the worst hits of the day.

AMD fell by 8%, to $3.02 a share, after the chipmaker late Wednesday surprised the market by naming Chief Operating Officer Lisa Su as CEO, replacing Rory Read. The timing of the executive change was viewed with concern, as AMD is set to report quarterly results on Oct. 16.

Losses also came from IBM Corp. (IBM), Microsoft Corp. (MSFT), eBay Inc. (EBAY) and Netflix Inc. (NFLX).

The tech-heavy Nasdaq Composite Index (RIXF) fell 32 points to 4,437. The Philadelphia Semiconductor Index (SOX) was also down sharply.

(Read more about the day's market action in Market Snapshot http://www.marketwatch.com/storyno-meta-for-guid.).

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