By Benjamin Pimentel, MarketWatch

SAN FRANCISCO (MarketWatch) -- Shares of Apple Inc. reversed course, slipping in afternoon trades as the new iPhones went on sale, as the tech sector slipped into negative territory.

Sliding shares of Microsoft Corp also weighed on the group, even as shares of two tech companies roughly doubled on their first day of trading following initial public offerings.

Apple (AAPL) shed 0.3% as the iPhone 5S and 5C went on sale with reports of long lines at stores.

On the other hand, shares of some chip makers jumped on reports based on at least one tear-down analysis showed that Apple used their microprocessors in the new Apple products.

Broadcom Corp. (BRCM) were among the gainers, with its stock rising nearly 1%.

Benchmark analyst Gary Mobley said in a note to clients that a tear-down analysis conducted by iFixit of iPhone 5C and 5S models showed that "most component suppliers featured in the iPhone 5 are also included in the 5s/5c including Broadcom as the combo connectivity chip supplier and supplier for the touch screen controller IC."

"Broadcom's inclusion should, at least for the moment, alleviate investor fears that Apple is developing its own connectivity silicon or possibly using another merchant supplier," Mobley wrote.

The iFixit report also mentioned iPhone components from Avago Technologies, Skyworks Solutions and TriQuint Semiconductor. Avago (AVGO) climbed 3.5%, while Skyworks (SWKS) firmed 1% and TriQuint (TQNT) gained 2%.

Facebook Inc. (FB) also advanced 0.3% after Cowen analysts upgraded the stock to outperform from market perform, and raised their price target to $53 from $29.

Two tech firms added some fuel on the Nasdaq.

Shares of Rocket Fuel (FUEL) more that doubled as the automated ad buying platform company began trading under the ticker symbol "FUEL."

Rocket Fuel, which is based in Redwood City, Calif., and whose technology is geared to helping marketers buy Web ads, sold 4 million shares at $29 a share, the high end of a revised range of $27 to $29.

The stock was last up 91% at $55.43.

Shares in FireEye inc. (FEYE), a network security systems company, soared more than 100% on its debut.

The Milpitas, Calif.-based firm sold 15.2 million shares at $20 a share, above its proposed range of $15 to $17. The stock was last trading up 92% at $38.38.

The Nasdaq Composite Index (RIXF) traded down 0.2% to 3,782. The benchmark was poised to end the week up 1.6%.

The Philadelphia Semiconductor Index (SOX) was also down a fraction, as shares of Intel Corp.(INTC), Texas Instruments (TXN) and Nvidia Corp. (NVDA) slipped.

A notable decliner was Microsoft Corp. (MSFT), shares of which shed more than 2% a day after the company's meeting with analysts.

Microsoft was the top decliner on the Dow Jones Industrial Average (DJI) which was down 104 points.

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