Intel's power-efficient 3D transistor couldn't have come any sooner. IDC on Thursday, in its quarterly forecast, is listing not just Intel-compatible PC chips but other categories of processors, namely ARM--the power-frugal silicon of choice in the tablet and smartphone world.

"For the first time, IDC is forecasting PC microprocessor units by processor architecture, including those based on x86 (Intel and Advanced Micro Devices) and those based on ARM," IDC's Shane Rau wrote in a research note dated Thursday. ARM chips are made by Texas Instruments, Qualcomm, Apple (via Samsung), and Nvidia, to name a few.

"By 2015, IDC expects that over 13 percent of PC processors will be based on the ARM architecture." That bears repeating. That's not smartphones but PCs.

Though not included in Rau's report, there are a few things to consider. Windows 8 will run on ARM--which is expected to happen in the 2012-2013 timeframe. So, conceivably there will be newfangled ARM-based PCs running Windows.

Tablets based on ARM chips such as HP&39's TouchPad and Apple&39's iPad are expected to make gains at the expense of PCs.

Before that happens, however, there is the tablet. For now, that means Apple's iPad, which can be categorized as a PC.

Shipments of tablets are forecast to fall in the 40 million to 50 million range in the U.S. in 2011. Earlier this year, Gartner, another market research firm, said "growing consumer enthusiasm for mobile PC alternatives, such as the iPad and other media tablets [is expected to] dramatically slow home mobile PC sales."

In the more immediate future, IDC said global PC microprocessor unit shipments were up 7.4 percent year over year the first quarter. For the full year, IDC raised its forecast for year-over-year growth in PC (mobile, desktop, x86 server) microprocessor unit shipments in 2011 from 10.1 percent to 10.3 percent.

"Generally, the demand environment for the second half of this year looks decent. The earthquake and tsunami had minor effects on the PC supply chain," said Rau. "However, the real near-to-mid term concern there is the effect on Japanese demand for PCs and so microprocessors," he added.

Market revenue for the year is expected to grow 17.6 percent to nearly $43 billion, IDC said.

In the first quarter, PC processor vendor shares "overall were remarkably stable," Rau said. Intel had a 80.8 percent unit market share and AMD maintained a 18.9 percent share, both flat compared to the fourth quarter of 2010.

"The first quarter, which is usually weak, was strong in terms of unit shipments but surely benefited from an extra calendar week," wrote Rau. "Both Intel and AMD grew unit shipments sequentially, which indicates some decent strength in their new platforms."