Google Inc is opening its much-anticipated Chrome software to external developers, as the search giant prepares to expand its reach into operating systems.

The company hosted a media event on Thursday to show off the free cloud-based software, which it hopes to have on the market in netbooks ahead of the 2010 holiday season. Google executives said they designed Chrome with an emphasis on speed, security and simplicity.

The software currently starts up on a PC in seven seconds, and Google said it is hoping to cut the boot time further.

But the company said Chrome at least initially will not be able to run on just any PC, and it plans to specify certain hardware requirements to run the software, such as solid-state hard drives.

With Chrome, Google is seeking to challenge the dominance of Microsoft Corp's Windows, which runs on nine out of 10 personal computers.

By opening up Chrome's source code, Google hopes to encourage outside developers to innovate on top of its operating system and design new applications.

Google said all data in Chrome will automatically be housed in the so-called cloud, or on external servers, but also cached on the computer's internal hardware to boost performance.

It also said Chrome applications, which are used for such every day tasks such as word processing and email, will be Web-based.

Shares of Mountain View, California-based Google fell 1 percent to $570.44 in afternoon trading on the Nasdaq.

(Reporting by Alexei Oreskovic, writing by Gabriel Madway; Editing Bernard Orr)

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