 In order to support Java applications, the FlexOS-based IBM 4690 OS version 2 introduced its own virtual file system (VFS) architecture to store long filenames in the FAT file system in a backwards-compatible fashion.If enabled, the virtual filenames (VFN) are available under separate logical drive letters, whereas the real filenames (RFN) remain available under the original drive letters.The FAT file system itself is not designed for supporting Alternate Data Streams (ADS), but some operating systems that heavily depend on them have devised various methods for handling them on FAT volumes.