How Windows NT Generates MS-DOS Compatible Filenames

- Microsoft Windows NT operating system version 3.1

SUMMARY

Windows NT file system (NTFS) filenames do not have the same limitations that MS-DOS FAT style filenames have. MS-DOS style filenames are limited to eight and three character strings separated by a period. NTFS filenames can be much longer and may contain more types of characters. When Windows NT copies a file from an NTFS partition to a FAT partition, it needs to change the filename for compatibility. The following rules are applied when filenames are converted from NTFS to FAT:

• All spaces in the NTFS filename are removed.
• All periods except the last one preceding a character are removed.
• The first three characters following the last period are used for the three letter extension of the FAT filename.
• All characters that are illegal in FAT but legal in NTFS are replaced with underscore characters ("_").
• The first six characters of the NTFS filename, a tilde character (~), and a single digit are used for the eight letter prefix of the FAT filename.

  If this does not result in a unique filename, only the first five characters of the NTFS filename are used, followed by the tilde and two digits.