Worldwide personal computer microprocessor shipments were flat in the fourth quarter compared to a year ago, due in part to the popularity of the Apple iPad.

Market researcher IDC reported that PC microprocessors — the brains of computers — were down 0.04 percent from the third quarter and down 0.21 percent compared to a year ago. That shows the impact of non-PC microprocessors taking market share thanks to the popularity of Apple’s iPad, which sold more than 15 million units in 2010. (Pictured right, from Apple’s Mac vs. PC ads, are the boring PC guy on left and cool Apple Mac guy on the right).

For the full year, microprocessor shipments grew 17.1 percent and revenues were up 26.7 percent to $36.3 billion.

“The fourth quarter was weak and out of synch with normal seasonal patterns in terms of unit shipments,” said Shane Rau, an analyst at IDC. “The first half turned out to be the better half of the year. However, looking back at the whole year 2010, it’s clear that the ongoing shift to mobile processors, combined with a shift back towards high-performance mobile processors, drove a significant rise in overall processor average selling prices.”

Overall, average selling prices for microprocessors rose 8 percent, with prices approaching levels last seen in 2008. Laptop processor sales grew 26.2 percent in the year, while Intel-based (x86) server unit shipments grew 28.1 percent. But desktop processor unit shipments grew only 6.2 percent. Overall, mobile PC processors were 54.1 percent of the market in 2010, compared to 50.2 percent in 2009.

In the fourth quarter, Intel had 80.8 percent of PC microprocessor unit market share, up 0.4 percent, while Advanced Micro Devices had 18.9 percent, down 0.4 percent. Via Technologies had 0.3 percent market share. For the full year, Intel’s share was 80.7 percent, up 1.1 percent; AMD had 19 percent share, down 1.1 percent, and Via had 0.3 percent. Intel has the biggest share in the PC mobile processor market, with 86.1 percent in the fourth quarter, and in the server market, with 94.2 percent market share. In desktops, AMD is stronger with 27.3 percent of the market, compared to 72.5 percent for Intel.

IDC expects overall PC microprocessor sales to grow 10.1 percent in 2011. That’s slower growth than in 2010 and is consistent with what Hewlett-Packard, the world’s biggest PC maker, said today in its earnings call. Rau said the economic concerns around the world and the impact of tablet computers is leading IDC to be more conservative in 2011.