Microsoft is throught to have already prepared the code that will let users download the first service pack for its new operating system, Windows 7.

Prominent blogger Rafael Rivera, who writes the Within Windows blog, sniffed out several keys in Windows 7's registry that add an eligibility check for Service Pack 1 (SP1).

A first service pack - which includes already-issued security patches as well as new bug fixes, and in some cases, new features - is important because many corporations won't widely deploy a new edition of Windows until that milestone has been reached.

Service packs are also relatively rare: Windows XP, which was launched in October 2001, has had only three.

Rivera took the appearance of the eligibility check to mean that SP1 testing is imminent. "External Windows 7 SP1 testing should commence soon, if it hasn't already," he said.

When can we expect Win 7 SP1?

But even if Rivera's hunch is correct, it could be months before Microsoft launches a public preview of SP1. For example, although Microsoft seeded an invite-only group of testers with Vista SP1 in September 2007, it didn't make the beta available to the general public until December 2007.

Microsoft has embedded similar keys in Vista, as well as Windows XP, to determine who is eligible, and when, to receive a service pack beta via Windows Update.

In the cases of both Vista and Windows XP, Microsoft offered small downloads when it was ready to expand the preview to the public; those downloads modified the registry so users' PCs would "see" the service pack on Windows Update.

If Microsoft adheres to the timeline it used for Vista, Windows 7's SP1 should be available to a small group of testers in June, and to the public in September.

Code detectives

This isn't the first time that Rivera has rooted out details of Microsoft's software.

Last November, Rivera accused Microsoft of using open-source code in a Windows 7 installation tool without acknowledging where it got the code. Microsoft quickly pulled the Windows 7 USB/DVD Download Tool, then later re-released it as open-source.

Microsoft did not immediately respond to questions about its Windows 7 SP1 plans, or comment on Rivera's blog post.

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See also: Windows 7: How happy are early adopters?