Advanced Micro Devices ( AMD) shares plunged early Tuesday after the chipmaker late Monday warned about its fourth-quarter results, citing a challenging environment. The Sunnyvale, Calif. based company said revenue in the fourth quarter ended Dec. 2 would be up slightly from the $1.24 billion it earned in the third quarter. Fourth-quarter operating income is expected to be positive, but "down significantly" from third-quarter operating income of $68.4 million. Analysts expect the company to earn 19 cents a share on revenue of $1.35 billion. Previously, the company said it anticipated overall sales to increase, driven by strong processor and Flash memory sales. AMD reports results after the market close on Jan. 18. Shares fell $2.93, 14.6%, to $17.20 in premarket trading.