SAN FRANCISCO - Shares in Cadence Design Systems ( CDNS) got a boost Thursday from a pair of large premarket trades.Shares jumped 71 cents, or 21.5%, to $4.02 in heavy trading late in the regular session. Interest in the ticker picked up after two blocks of shares changed hands at $3.31 a share in premarket trading, according to sell-side analysts, who asked not to be named. The blocks reportedly totaled 12.5 million shares, or about 4.8% of outstanding shares. Analysts seemed perplexed by the subsequent rise in the stock, given the lack of clear indication that the trades were anything more than an exit from a large position. Revenue in Cadence, which develops software for the design of semiconductors, fell 18% year over year to $616.7 million during the first half of 2008, while competitors Synopsys ( SNPS) and Mentor Graphics ( MENT) have posted growth. Cadence shares lost a quarter of their value the day after the company delayed its third-quarter earnings report Oct. 22 and announced that it would restate earnings for the first half of the year. The one-year price target on Cadence is $5.50, according to Thomson Reuters.