The US Patent and Trademark Office (USPTO) has upheld Microsoft's patents on the FAT file system.

The final decision has come after nearly two years of investigation into the technology that controls how files are stored in Windows, and following their initial rejection in October.

Microsoft was informed yesterday that the USPTO was ending its re-examination of patents 5,579,517 and 5,758,352, and issuing updated patent certificates.

FAT is the technology in Windows that allows files to be stored under certain file names, but it is not exclusive to Windows. FAT also is widely used in removable media such as USB memory sticks and cameras.

Microsoft claims it developed FAT in 1976 and was granted a patent on the file system in 1996. But some in the industry, particularly those with interests in developing and promoting open-source software, have disputed the FAT patents Microsoft holds. Microsoft licenses FAT to third parties, who must pay the software company to use it in their technology, Payer said.

The Public Patent Foundation and California man David L. Hoffman requested separately that the patents covering FAT be re-examined in April 2004 and January 2005 respectively.

The USPTO initially rejected patent 5,579,517 in September 2004 after re-examination, but Microsoft submitted more materials to support its claim on the patent.

The decision unveiled Tuesday is the USPTO's final decision, according to USPTO documents. The upholding of the patents means that Microsoft can continue to license FAT to third parties for a fee.