Upgrading and Repairing PCs

VFAT and Long Filenames

The original Windows 95 release uses what is essentially the same FAT file system as DOS, except for a few important enhancements. Like much of the rest of Windows 95, the operating system support for the FAT file system was rewritten using 32-bit code and called VFAT (virtual file allocation table). VFAT works in combination with the 32-bit protected mode VCACHE (which replaces the 16-bit real mode SMARTDrive cache used in DOS and Windows 3.1) to provide better file system performance. However, the most obvious improvement in VFAT is its support for long filenames. DOS and Windows 3.1 had been encumbered by the standard 8.3 filenaming convention for many years, and adding long filename support was a high priority in Windows 95—particularly in light of the fact that Macintosh and OS/2 users had long enjoyed this capability.

The problem for the Windows 95 designers, as is often the case in the PC industry, was backward compatibility. It is no great feat to make long filenames possible when you are designing a new file system from scratch, as Microsoft did years before with Windows NT's NTFS. However, the Windows 95 developers wanted to add long filenames to the existing FAT file system and still make it possible to store those names on existing DOS volumes and for previous versions of DOS and Windows to access the files.

VFAT provides the capability to assign file and directory names that are up to 255 characters in length (including the length of the path). The three-character extension is maintained because, like previous Windows versions, Windows 9x relies on the extensions to associate file types with specific applications. VFAT's long filenames can also include spaces, as well as the following characters, which standard DOS 8.3 names can't: +,;=[ ].

The first problem when implementing the long filenames was how to make them usable to previous versions of DOS and 16-bit Windows applications that support only 8.3 names. The resolution to this problem was to give each file two names: a long filename and an alias that uses the traditional 8.3 naming convention. When you create a file with a long filename in Windows 9x/Me, VFAT uses the following process to create an equivalent 8.3 alias name:

Aliasing in Windows NT/2000/XP Note that Windows NT/2000/XP creates aliases differently than Windows 9x/Me (as shown later). NT/2000/XP begins by taking the first six legal characters in the LFN and following them with a tilde and number. If the first six characters are unique, a number 1 follows the tilde. If the first six characters aren't unique, a number 2 is added. NT/2000/XP uses the first three legal characters following the last period in the LFN for a file extension. At the fifth iteration of this process, NT/2000/XP takes only the first two legal characters, performs a hash on the filename to produce four hexadecimal characters, places the four hex characters after the first two legal characters, and appends a ~5. The ~5 remains for all subsequent aliases; only the hex numbers change.

Tip You can modify the behavior of the VFAT filename truncation mechanism to make it use the first eight characters of the long filename instead of the first six characters plus ~1. To do this, you must add a new binary value to the HKEY_LOCAL_MACHINE\System\CurrentControlSet\control\FileSystem Registry key called NameNumericTail, with a value of 0. Changing the value to 1 returns the truncation process to its original state. Although this Registry change creates "friendly" alias names, it causes many programs working with alias names to fail and is not recommended.

VFAT stores this alias filename in the standard name field of the file's directory entry. Any version of DOS or 16-bit Windows can therefore access the file using the alias name. The big problem that still remains, however, is where to store the long filenames. Clearly, storing a 255-character filename in a 32-byte directory entry is impossible (because each character requires 1 byte). However, modifying the structure of the directory entry would make the files unuseable by previous DOS versions.

The developers of VFAT resolved this problem by using additional directory entries to store the long filenames. Each of the directory entries is still 32 bytes long, so up to 8 might be required for each long name, depending on its length. To ensure that these additional directory entries are not misinterpreted by earlier DOS versions, VFAT flags them with a combination of attributes that is not possible for a normal file: read-only, hidden, system, and volume label. These attributes cause DOS to ignore the long filename entries, while preventing them from being mistakenly overwritten.

Caution When using long filenames on a standard FAT12 or FAT16 partition, you should avoid storing them in the root directory. Files with long names that take up multiple directory entries can more easily use up the limited number of entries allotted to the root directory than files with 8.3 names. On FAT32 drives, this is not a problem because the root directory has an unlimited number of entries. In an experiment, I created a small (1KiB) text file on a floppy disk and gave it a 135-character long filename using Windows 98. I copied the file and pasted it repeatedly into the root directory of a floppy disk using Windows Explorer. Before I could make 20 copies of the file, the system displayed a File copying error. The disk could not accept any more files because the extremely long filename had used up all the root directory entries.

This solution for implementing backward-compatible long filenames in Windows 9x is ingenious, but it is not without its problems. Most of these problems stem from the use of applications that can access only the 8.3 alias names assigned to files. In some cases, if you open a file with a long name using one of these programs and save it again, the connection to the additional directory entries containing the long name is severed and the long name is lost.

This is especially true for older versions of disk utilities, such as Norton Disk Doctor for MS-DOS, that are not designed to support VFAT. Most older applications ignore the additional directory entries because of the combination of attributes assigned to them, but disk repair utilities usually are designed to detect and "correct" discrepancies of this type. The result is that running an old version of Norton Disk Doctor on a partition with long filenames results in the loss of all the long names. In the same way, backup utilities not designed for use with VFAT can strip off the long filenames from a partition.

Note When using VFAT's long filename capabilities, you definitely should use disk and backup utilities that are intended to support VFAT. Windows 9x includes VFAT-compatible disk repair, defragmentation, and backup programs. If, however, you are for some reason inclined to use an older program that does not support VFAT, Windows 9x includes a clumsy, but effective, solution. A program is included on the Windows 9x CD-ROM called LFNBK.EXE. It doesn't install with the operating system, but you can use it to strip the long filenames from a VFAT volume and store them in a text file called LFNBK.DAT. You can then work with the files on the volume as though they were standard 8.3 FAT files. Afterward, you can use LFNBK.EXE to restore the long filenames to their original places (assuming the file and directory structure has not changed). This is not a convenient solution, nor is it recommended for use in anything but extraordinary circumstances, but the capability is there if needed. Some backup programs designed for disaster recovery (which enable you to reconstruct the contents of the hard drive without reloading Windows first) have used this feature to enable restoration of a Windows drive with long filenames from a DOS prompt (where only 8.3 alias names usually are supported).

Another problem with VFAT's long filenames involves the process by which the file system creates the 8.3 alias names. VFAT creates a new alias every time you create or copy a file into a new directory; therefore, the alias can change. For example, you might have a file called Expenses-January98.doc stored in a directory with the alias EXPENS~1.DOC. If you use Windows 9x Explorer to copy this file to a directory that already contains a file called Expenses-December97.doc, you are likely to find that this existing file is already using the alias EXPENS~1.DOC. In this case, VFAT assigns EXPENS~2.DOC as the alias of the newly copied file, with no warning to the user. This is not a problem for applications that support VFAT because the long filenames are unchanged, but a user running an older application might open the EXPENS~1.DOC file expecting to find the list of January 1998 expenses and see the December 1997 expenses list instead.