8.3 is a common shorthand for the limits on filename length imposed by the FATfilesystem used by CP/M, DOS and early versions of Microsoft Windows — at most eight characters, followed by a ".", followed by a filename extension of at most three characters. File and directory names are uppercase.
The FAT directory format has been extended when Microsoft introduced Windows 95 and Windows NT 3.5, to support much longer, mixed-case Unicode filenames in addition to classic 8.3 names.
Files have two different names as a result. Windows applications can see both names and use them alternatively, although they will usually ignore the 8.3 names completely, while DOS applications only see and manipulate 8.3 names. It is important to know that the 8.3 name associated with a file can change if a
file is moved around using the long name.
Technically, this is implemented by preceding each 8.3 directory entry on disk with one or more extra entries, of the Volume Label type. Such entries are ignored by MS-DOS, except in the root directory, for the purpose of naming the disk volume. Their presence does not prevent deleting a directory after the 8.3 entries has been removed. A slight problem can appear in the root directory: if it does not start with a real Volume Label, native MS-DOS will consider the first longname entry instead, which being in Unicode will display as short junk. This long name can also become corrupted if the LABEL command is used from MS-DOS to change the volume name. Windows can however detect this and
will ignore or fix corrupted or detached long names.
There is no compulsory algorithm for creating the 8.3 name from a long name. Windows uses the following convention:
if the original name is 8.3 uppercase (eg. "TEXTFILE.TXT"), no long name will be stored on disk at all
if the original name is 8.3 mixed case, the long name will store the mixed-case name, while the 8.3 name will be a converted uppercase version of it (eg. "TextFile.Txt" and "TEXTFILE.TXT")
otherwise the names will be different. If possible, Windows will use the first 6 letters of the basename, followed by a tilde, followed by a single digit, followed by the first 3 chars of the extension (whatever follows the last dot in file name, uppercased and stripped of invalid characters) in order to create a unique 8.3 name. For example, "TextFile1.Mine. txt" and "TextFile2.Yours. txt" could have "TEXTFI~1.TXT" and "TEXTFI~2.TXT" as 8.3 names.
if this still does not yield a unique name, Windows will use the first 2 letters of the base name (or 1 if the basename has only 1 letter), followed by 4 seemingly random hexadecimal digits, the rest remaining as above.
In every case, a 8.3 filename resulting from a truly long name will always contain a tilde followed by a digit.
The NTFS filesystem used by the Windows NT family supports long filenames natively, but 8.3 names are still generated and available for MS-DOS applications. This can be optionally disabled to increase performance.
