Nvidia Corp's forecast for first-quarter revenue fell short of Wall Street expectations as the chipmaker wrestles with a tepid personal computer market and more competition in smartphones and tablets, sending its shares down sharply after hours.

Anxious to move beyond its traditional business of designing graphics chips for PCs, Nvidia has jumped into mobile devices and late last year unveiled its new Tegra 3 processor.

Nvidia has said it expects Tegra revenues to hit $1 billion in 2012, with most growth in the second half of the year.

But with competitors Texas Instruments and Qualcomm turning up the heat with their own new mobile processors, many investors are waiting to see if the Tegra 3 chip finds its way into a top-selling device.

The bottom line is the stock has moved a lot on anticipation of Tegra growth and they haven't seen it yet, and they're certainly not guiding on it, said Evercore Partners analyst Patrick Wang.

Nvidia had some successes early in 2011 with its Tegra 2 chips appearing in tablets made by Samsung Electronics and LG Electronics, although sales have grown less quickly than many investors had expected.

Along with the rest of the PC supply chain, Nvidia has also been hurt by a shortage of hard drives after flooding in Thailand last year damaged factories.

Nvidia said revenue in its fiscal first quarter would be between $900 million and $930 million compared with analysts' average forecast of $944.63 million.

Nvidia also guided for an April-quarter gross margin forecast of 49.5 percent, plus or minus 1 percentage point, less than the 52.1 percent expected by analysts.

The company's shares were down 4.2 percent in extended trading after closing down 0.4 percent at $16.17 on Wednesday.

Nvidia said it had fourth-quarter net income of $116 million, or 19 cents per share, compared with $172 million, or 29 cents a share, a year ago. Analysts expected 19 cents per share.

After cutting its forecast in January due to the hard drive shortage and slower mobile chip sales, Nvidia said revenue for the quarter that ended January 29 was $953 million, up from $886 million a year ago and slightly higher than analysts' average estimate of $950 million, according to Thomson Reuters I/B/E/S.

(Reporting By Noel Randewich; Editing by Tim Dobbyn)