8.3 filename

An 8.3 filename[1] (also called a short filename or SFN) is a filename convention used by old versions of DOS, versions of Microsoft Windows prior to Windows 95, and Windows NT 3.51. It is also used in modern Microsoft operating systems as an alternate filename to the long filename for compatibility with legacy programs. The filename convention is limited by the FAT file system. Similar 8.3 file naming schemes have also existed on earlier CP/M, TRS-80, Atari, and some Data General and Digital Equipment Corporation minicomputer operating systems.

Overview

8.3 filenames are limited to at most eight characters (after any directory specifier), followed optionally by a filename extension consisting of a period . and at most three further characters. For systems that only support 8.3 filenames, excess characters are ignored and if a file name has no extension, the ., if present, has no significance (that is, myfile and myfile. are equivalent). Furthermore, in these systems file and directory names are uppercase, although systems that use the 8.3 standard are usually case-insensitive (hence CamelCapitals.tpu will be equivalent to the name CAMELCAP.TPU). However, on non-8.3 Operating Systems (such as almost any modern Operating System) accessing 8.3 File Systems (including DOS-formatted diskettes, but also including some modern memory cards and networked file systems) the underlying system may alter filenames internally to preserve case and avoid truncating letters in the names, for example in the case of VFAT.

VFAT and Computer-generated 8.3 filenames

VFAT, a variant of FAT with an extended directory format, was introduced in Windows 95 and Windows NT 3.5. It allowed mixed-case Unicode long filenames (LFNs) in addition to classic 8.3 names by using multiple 32-byte directory entry records for long filenames (in such a way that only one will be recognised by old 8.3 system software as a valid directory entry).

To maintain backward-compatibility with legacy applications (on DOS and Windows 3.1), on FAT and VFAT filesystems an 8.3 filename is automatically generated for every LFN, through which the file can still be renamed, deleted or opened... although the generated name (e.g. OVI3KV~N) may show little similarity to the original. On NTFS filesystems the generation of 8.3 filenames can be turned off.[2] The 8.3 filename can be obtained using the Kernel32.dll function GetShortPathName.[3][4]

Although there is no compulsory algorithm for creating the 8.3 name from an LFN, Windows uses the following convention:[5]

  1. If the LFN is 8.3 uppercase, no LFN will be stored on disk at all.
    • Example: TEXTFILE.TXT
  2. If the LFN is 8.3 mixed case, the LFN will store the mixed-case name, while the 8.3 name will be an uppercased version of it.
    • Example: TextFile.Txt becomes TEXTFILE.TXT.
  3. If the filename contains characters not allowed in an 8.3 name (including space which was disallowed by convention though not by the APIs) or either part is too long, the name is stripped of invalid characters such as spaces and extra periods. If the name begins with periods . the leading periods are removed. Other characters such as + are changed to the underscore _, and uppercased. The stripped name is then truncated to the first 6 letters of its basename, followed by a tilde, followed by a single digit, followed by a period ., followed by the first 3 characters of the extension.
    • Example: TextFile.Mine.txt becomes TEXTFI~1.TXT (or TEXTFI~2.TXT, should TEXTFI~1.TXT already exist). ver +1.2.text becomes VER_12~1.TEX. .bashrc.swp becomes BASHRC~1.SWP
  4. Beginning with Windows 2000, if at least 4 files or folders already exist with the same initial 6 characters in their short names, the stripped LFN is instead truncated to the first 2 letters of the basename (or 1 if the basename has only 1 letter), followed by 4 hexadecimal digits derived from an undocumented hash of the filename, followed by a tilde, followed by a single digit, followed by a period ., followed by the first 3 characters of the extension.
    • Example: TextFile.Mine.txt becomes TE021F~1.TXT.

NTFS, a file system used by the Windows NT family, supports LFNs natively, but 8.3 names are still available for legacy applications. This can optionally be disabled to improve performance in situations where large numbers of similarly named files exist in the same folder.[2]

The ISO 9660 file system (mainly used on compact discs) has similar limitations at the most basic Level 1, with the additional restriction that directory names cannot contain extensions and that some characters (notably hyphens) are not allowed in filenames. Level 2 allows filenames of up to 31 characters, more compatible with Mac OS filenames.

During the Microsoft antitrust trials, the names MICROS~1 and MICROS~2 were humorously used to refer to the companies that might exist after a proposed split of Microsoft.[6]

Compatibility

This legacy technology is used in a wide range of products and devices, as a standard for interchanging information, such as compact flash cards used in cameras. VFAT LFN long filenames introduced by Windows 95/98/ME retained compatibility. But the VFAT LFN used on NT-based systems (Windows NT/2K/XP) uses a modified 8.3 shortname.

If a filename contains only lowercase letters, or is a combination of a lowercase basename with an uppercase extension, or vice versa; and has no special characters, and fits within the 8.3 limits, a VFAT entry is not created on Windows NT and later versions such as XP. Instead, two bits in byte 0x0c of the directory entry are used to indicate that the filename should be considered as entirely or partially lowercase. Specifically, bit 4 means lowercase extension and bit 3 lowercase basename, which allows for combinations such as example.TXT or HELLO.txt but not Mixed.txt. Few other operating systems support this. This creates a backward-compatibility problem with older Windows versions (95, 98, ME) that see all-uppercase filenames if this extension has been used, and therefore can change the capitalization of a file when it is transported, such as on a USB flash drive. This can cause problems for operating systems that do not exhibit the case-insensitive filename behavior as DOS and Windows do. Current 2.6.x versions of Linux will recognize this extension when reading (source: kernel 2.6.18 /fs/fat/dir.c and fs/vfat/namei.c); the mount option shortname determines whether this feature is used when writing.[7]

Directory table

Main article: FAT directory table

A directory table is a special type of file that represents a directory. Each file or directory stored within it is represented by a 32-byte entry in the table. Each entry records the name, extension, attributes (archive, directory, hidden, read-only, system and volume), the date and time of creation, the address of the first cluster of the file/directory's data and finally the size of the file/directory.

Legal characters for DOS filenames include the following:

This excludes the following ASCII characters:

The DOS filenames are in the OEM character set. Code 0xE5 as the first byte (see below) makes troubles when extra-ASCII characters are used.

Directory entries, both in the Root Directory Region and in subdirectories, are of the following format:

Further information: FAT directory entry
Byte Offset Length Description
0x00 8 DOS filename (padded with spaces)

The first byte can have the following special values:

0x00 Entry is available and no subsequent entry is in use
0x05 Initial character is actually 0xE5
0x2E Dot entry: either . or ..
0xE5 Entry has been previously erased. File undelete utilities must replace this character with a regular character as part of the undeletion process.
0x08 3 DOS file extension (padded with spaces, may be empty)
0x0b 1 File Attributes

The first byte can have the following special values:

Bit Mask Description
0 0x01 Read Only
1 0x02 Hidden
2 0x04 System
3 0x08 Volume Label
4 0x10 Subdirectory
5 0x20 Archive
6 0x40 Device (internal use only, never found on disk)
7 0x80 Unused

An attribute value of 0x0F is used to designate a long filename entry.

0x0c 1 Reserved; two bits are used by NT and later versions to encode case information
0x0d 1 Create time, fine resolution: 10 ms units, values from 0 to 199.
0x0e 2 Create time. The hour, minute and second are encoded according to the following bitmap:
Bits Description
15–11 Hours (0–23)
10–5 Minutes (0–59)
4–0 Seconds/2 (0–29)

Note that the seconds is recorded only to a 2 second resolution. Finer resolution for file creation is found at offset 0x0d.

0x10 2 Create date. The year, month and day are encoded according to the following bitmap:
Bits Description
15–9 Year (0 = 1980, 127 = 2107)
8–5 Month (1 = January, 12 = December)
4–0 Day (1–31)
0x12 2 Last access date; see offset 0x10 for description.
0x14 2 EA-Index (used by OS/2 and NT) in FAT12 and FAT16, High 2 bytes of first cluster number in FAT32
0x16 2 Last modified time; see offset 0x0e for description.
0x18 2 Last modified date; see offset 0x10 for description.
0x1a 2 First cluster in FAT12 and FAT16. Low 2 bytes of first cluster in FAT32.
0x1c 4 File size

How to convert a long filename to a short filename

Sometimes it may be desirable to convert a long filename to a short filename, for example when working with the command prompt. A few simple rules can be followed to attain the correct 8.3 filename.

1. A SFN filename can have at most 8 characters before the dot. If it has more than that, you should write the first 6, then put a tilde ~ as the seventh character and a number (usually 1) as the eighth. The number distinguishes it from other files with both the same first six letters and the same extension.

2. Dots are important and must be used even for folder names (if there is a dot in the folder name). If there are multiple dots in the long file/directory name, only the last one is used. The preceding dots should be ignored. If there are more characters than three after the final dot, only the first three are used.

3. Generally:

To find out for sure the SFN or 8.3 names of the files in a directory

use: dir /x shows the short names if there is one, and the long names.

or: dir /-n shows only the short names, in the original DIR listing format.

See also

References

  1. ↑ "Naming a File". Microsoft Developer Network.
  2. 1 2 "How to Disable the 8.3 Name Creation on NTFS Partitions". Microsoft.
  3. ↑ "GetShortPathName Function". MSDN.
  4. ↑ "How to Get a Short Filename from a Long Filename". Microsoft.
  5. ↑ "How Windows Generates 8.3 File Names from Long File Names". Microsoft.
  6. ↑ "Microsoft One-liner Digest".
  7. ↑ "mount(8): mount filesystem - Linux man page".
  8. ↑ Andries Brouwer (2007-12-26). "Directory Entry". The FAT filesystem. Retrieved 2013-07-30.
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