The Xbox has a specific function: Non-booting or non-reading from CDs and DVD-Rs as a method of game copy protection. Also, the Xbox is said to use a different DVD file system (instead of UDF). It has been theorized that the discs have a second partition that is read from the outside in (opposite current standards thus making the second partition unreadable in PC DVD drives) which give the tracks the appearance that the disc was spun backwards during manufacture. The Xbox 360 copy protection functions by requesting the DVD drive compute the angular distance between specific data sectors on the disc. A duplicated DVD returns different values than does a pressed original.