In its most direct assault on Microsoft yet, Google on Tuesday evening announced the forthcoming release of the Google Chrome Operating System, a new open source operating system designed primarily for netbooks but also for desktop computers.

Chrome OS is Google's second operating system, after Android, the mobile device operating system that the company introduced in late 2008. That's also about the time when Google introduced its Chrome Web browser. Chrome OS is an extension of Google's browser code.

Though there's some overlap between the two operating systems, Google says that choice drives innovation and benefits everyone. An apparent distinction between Chrome OS and Android is that the latter operating system is designed to work on devices like mobile phones and set-top boxes that deny users access to certain resources. Further distinctions between the two Google operating systems may emerge once the Chrome OS license is published.

Chrome OS will introduce a new windowing system, presumably along the lines of GNOME or KDE, atop the Linux kernel. In a blog post, Sundar Pichai, VP of product management and engineering director Linus Upson explained, "All Web-based applications will automatically work and new applications can be written using your favorite Web technologies."

Chrome OS will run on x86 and ARM chips. Google says that it is working with several hardware manufacturers to deliver netbooks running Chrome OS in the second half of 2010.

Along with the mobile phone market, the netbook market is growing rapidly, in contrast to the desktop PC market. Last month, iSuppli reported that PC shipments in the first quarter of 2009 fell 23% compared with the same period in 2008, while netbook shipments grew 10% during the same time frame. Last November, IDC predicted that 42.2 million netbooks would be sold in 2012, almost four times as many as in 2008.