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Date: Sat, 29 Jun 1996 18:59:41 -0500
From: Robert Vandervelde <RVand@SNOWHILL.COM>
Subject: Re: Long Filename Structure (Windows 95).
>Does anybody know how Windows 95 implemented long filenames?
>I used Norton Disk Editor to read the directory and found some encrypted
>form of the long filenames.
>I plan to write a utility that changes the long filenames only.
>
>Thank you,
>;-b Sintar Wirawan at Menur 30 Surabaya - Indonesia
>8-@ squid@sby.mega.net.id
>
>
How Windows 95 Stores Long Filenames
Copyright notice: Taken from PC Magazine, June 25, 1996 by Jeff Prosise
"Windows 95 stores short filenames the same way DOS and 16-bit windows so.
Every file on every disk is accompanied by a 32-byte directory entry that
records the name of the file as well as the file's attributes, a date and
time stamp, and other information."
The format of the short directory entry is as follows:
Offset Description Size
0 Filename 8 bytes (ASCII)
8 Filename extension 3 bytes (ASCII)
11 File attributes 1 byte (encoded)
12 reserved 10 bytes
22 Time stamp 2 bytes (encoded)
24 Date stamp 2 bytes (encoded)
26 Starting cluster 2 bytes
28 File size 4 bytes
File attributes byte
7: reserved 3: Volume label
6: reserved 2: System
5: archive 1: Hidden
4: subdirectory 0: Read-only
Time stamp byte
11-15: Hours (0-23)
5-10: Minutes (0-59)
0- 4: Seconds divided by 2 (0-29)
Date stamp byte
11-15: Year (relative to 1980)
5- 8: Month (1-12)
0- 4: Day of month (0-31)
"Because of compatibility issues, adding long filename support to an
operating system that uses 8.3 filenames isn't as expanding directory
entries to hold more than 11 characters. ...
Windows 95's designers devised a clever solution to the problem of
supporting long filenames while preserving compatability with previous
versions of DOS and Windows applications. ... Through testing, Microsoft
found that if a driectory entry is marked with an "impossible" combination
of read-only, hidden, system, and volume label attribute bits - that is,
if the directory entry's attribute byte holds the value 0Fh - the
enumeration functions built inot all existing versions of DOS and pre-95
versions of Windows will skip over that directory entry as if it weren't
there.
The solution for Windows 95, then, was to store two names for every file and
subdirectory: a short name that's visible to all applications and a long
name that's visible only to Windows 95 applications...Short filenames are
stored in 8.3 format in conventionl 32-byte directory entries. Windows
creates a short filename from a long one by truncating it to six uppercase
characters and adding "~1" to the end of the base filename. If there's
already another filename with the same first six characters, the number is
incremented. The extension is kept the same, and any character that was
illegal in earlier versions of Windows and DOS is replaced with an
underscore.
Long filenames are stored in specially formatted 32-byte long filename (LFN)
directory entries marked with attribute bytes set to 0Fh. For a given
file or subdirectory, a group of one or more LFN directory entries
immediately precedes the single 8.3 directory entry on the disk. Each LFN
directory entry contains up to 13 characters of the long filename, and the
OS strings together as many as needed to comprise an entire long filename.
Filenames are stored in Unicode format, which requires 2 bytes per character
as opposed to ASCII's 1 byte. Filename characters are spread among three
separate fields: the first 10 bytes (five characters) in length, the second
12 bytes (6 characters), and the third 4 bytes (two characters). The lowest
five bits of the directory entry's first byte hold a sequence number that
identifies the directory entry's position relative to other LFN directory
entries associated with the same file. If a long filename requires three
LFN directory entries, for example, the sequence number of the first will
be 1, that of the second will be 2, and the sequence of the third will be
3. Bit 6 of the third entry's first byte is set to 1 to indicate that it's
the last entry in the sequence.
The attribute field appears at the same location in LFN directory entries
as in 8.3 directory entries. ... The starting cluster number field also
appears at the same location, but in LFN directory entries its value is
always 0. The type indicator field also holds 0 in every long filename I've
examined, but Adrian King's Inside Windows 95 (Microsoft Press, 1994) says
it can also hold a nonzero value indicating that the directory entry
contains "class information" for the corresponding file. ... The LFN
directory entry's checksum byte holds an eight-bit checksum value computed
by adding certain fields of the 8.3 directory entry and performing a
modulo 256 operation on the result. Windows 95 uses this checksum to detect
orphaned or corrupted LFN directory entries.
Long filename directory entry
OFFSET DESCRIPTION Size
0 Sequence byte 1 byte
1 First five characters of LFN 10 bytes
11 File attributes 1 byte
12 Type indicator 1 byte (always 0??)
13 Checksum 1 byte
14 Next six characters of LFN 12 bytes
26 Starting cluster number 2 bytes (always 0)
28 Next two characters of LFN 4 bytes
*NOTE: The above structure may span up to 31 entries. The last entry will
be a standard 8.3 filename directory structure.
Sequence byte
7: apparently unused (always 0)
6: 1=final component of this LFN
5: apparently unused (always 0)
0-4: sequence number (1-31)
--------------------------------------------------------
Robert Vandervelde + ...that what we have learned and
RVand@snowhill.com + truly understood, we discovered
Enterprise, AL + ourselves.
The Wiregrass + - Richard C. Dorf
--------------------------------------------------------