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NOTES ON THE STRUCTURE OF THE VFAT FILESYSTEM
----------------------------------------------------------------------
(This documentation was provided by Galen C. Hunt <gchunt@cs.rochester.edu>
and lightly annotated by Gordon Chaffee).
This document presents a very rough, technical overview of my
knowledge of the extended FAT file system used in Windows NT 3.5 and
Windows 95. I don't guarantee that any of the following is correct,
but it appears to be so.
The extended FAT file system is almost identical to the FAT
file system used in DOS versions up to and including 6.223410239847
:-). The significant change has been the addition of long file names.
Theses names support up to 255 characters including spaces and lower
case characters as opposed to the traditional 8.3 short names.
Here is the description of the traditional FAT entry in the current
Windows 95 filesystem:
struct directory { // Short 8.3 names
unsigned char name[8]; // file name
unsigned char ext[3]; // file extension
unsigned char attr; // attribute byte
unsigned char lcase; // Case for base and extension
unsigned char ctime_ms; // Creation time, milliseconds
unsigned char ctime[2]; // Creation time
unsigned char cdate[2]; // Creation date
unsigned char adate[2]; // Last access date
unsigned char reserved[2]; // reserved values (ignored)
unsigned char time[2]; // time stamp
unsigned char date[2]; // date stamp
unsigned char start[2]; // starting cluster number
unsigned char size[4]; // size of the file
};
The lcase field specifies if the base and/or the extension of an 8.3
name should be capitalized. This field does not seem to be used by
Windows 95 but it is used by Windows NT. The case of filenames is not
completely compatible from Windows NT to Windows 95. It is not completely
compatible in the reverse direction, however. Filenames that fit in
the 8.3 namespace and are written on Windows NT to be lowercase will
show up as uppercase on Windows 95.
Note that the "start" and "size" values are actually little
endian integer values. The descriptions of the fields in this
structure are public knowledge and can be found elsewhere.
With the extended FAT system, Microsoft has inserted extra
directory entries for any files with extended names. (Any name which
legally fits within the old 8.3 encoding scheme does not have extra
entries.) I call these extra entries slots. Basically, a slot is a
specially formatted directory entry which holds up to 13 characters of
a files extended name. Think of slots as additional labeling for the
directory entry of the file to which they correspond. Microsoft
prefers to refer to the 8.3 entry for a file as its alias and the
extended slot directory entries as the file name.
The C structure for a slot directory entry follows:
struct slot { // Up to 13 characters of a long name
unsigned char id; // sequence number for slot
unsigned char name0_4[10]; // first 5 characters in name
unsigned char attr; // attribute byte
unsigned char reserved; // always 0
unsigned char alias_checksum; // checksum for 8.3 alias
unsigned char name5_10[12]; // 6 more characters in name
unsigned char start[2]; // starting cluster number
unsigned char name11_12[4]; // last 2 characters in name
};
If the layout of the slots looks a little odd, it's only
because of Microsoft's efforts to maintain compatibility with old
software. The slots must be disguised to prevent old software from
panicing. To this end, a number of measures are taken:
1) The attribute byte for a slot directory entry is always set
to 0x0f. This corresponds to an old directory entry with
attributes of "hidden", "system", "read-only", and "volume
label". Most old software will ignore any directory
entries with the "volume label" bit set. Real volume label
entries don't have the other three bits set.
2) The starting cluster is always set to 0, an impossible
value for a DOS file.
Because the extended FAT system is backward compatible, it is
possible for old software to modify directory entries. Measures must
be taken to insure the validity of slots. An extended FAT system can
verify that a slot does in fact belong to an 8.3 directory entry by
the following:
1) Positioning. Slots for a file always immediately proceed
their corresponding 8.3 directory entry. In addition, each
slot has an id which marks its order in the extended file
name. Here is a very abbreviated view of an 8.3 directory
entry and its corresponding long name slots for the file
"My Big File.Extension which is long":
<proceeding files...>
<slot #3, id = 0x43, characters = "h is long">
<slot #2, id = 0x02, characters = "xtension whic">
<slot #1, id = 0x01, characters = "My Big File.E">
<directory entry, name = "MYBIGFIL.EXT">
Note that the slots are stored from last to first. Slots
are numbered from 1 to N. The Nth slot is or'ed with 0x40
to mark it as the last one.
2) Checksum. Each slot has an "alias_checksum" value. The
checksum is calculated from the 8.3 name using the
following algorithm:
for (sum = i = 0; i < 11; i++) {
sum = (((sum&1)<<7)|((sum&0xfe)>>1)) + name[i]
}
3) If there is in the final slot, a Unicode NULL (0x0000) is stored
after the final character. After that, all unused characters in
the final slot are set to Unicode 0xFFFF.
Finally, note that the extended name is stored in Unicode. Each Unicode
character takes two bytes.
NOTES ON UNICODE TRANSLATION IN VFAT FILESYSTEM
----------------------------------------------------------------------
(Information provided by Steve Searle <steve@mgu.bath.ac.uk>)
Char used as Char(s) used Char(s) used in Entries which have
filename in shortname longname slot been corrected
0x80 (128) 0x80 0xC7
0x81 (129) 0x9A 0xFC
0x82 (130) 0x90 0xE9 E
0x83 (131) 0xB6 0xE2 E
0x84 (132) 0x8E 0xE4 E
0x85 (133) 0xB7 0xE0 E
0x86 (134) 0x8F 0xE5 E
0x87 (135) 0x80 0xE7 E
0x88 (136) 0xD2 0xEA E
0x89 (137) 0xD3 0xEB E
0x8A (138) 0xD4 0xE8 E
0x8B (139) 0xD8 0xEF E
0x8C (140) 0xD7 0xEE E
0x8D (141) 0xDE 0xEC E
0x8E (142) 0x8E 0xC4 E
0x8F (143) 0x8F 0xC5 E
0x90 (144) 0x90 0xC9 E
0x91 (145) 0x92 0xE6 E
0x92 (146) 0x92 0xC6 E
0x93 (147) 0xE2 0xF4 E
0x94 (148) 0x99 0xF6
0x95 (149) 0xE3 0xF2
0x96 (150) 0xEA 0xFB
0x97 (151) 0xEB 0xF9
0x98 (152) "_~1" 0xFF
0x99 (153) 0x99 0xD6
0x9A (154) 0x9A 0xDC
0x9B (155) 0x9D 0xF8
0x9C (156) 0x9C 0xA3
0x9D (157) 0x9D 0xD8
0x9E (158) 0x9E 0xD7
0x9F (159) 0x9F 0x92
0xA0 (160) 0xB5 0xE1
0xA1 (161) 0xD6 0xE0
0xA2 (162) 0xE0 0xF3
0xA3 (163) 0xE9 0xFA
0xA4 (164) 0xA5 0xF1
0xA5 (165) 0xA5 0xD1
0xA6 (166) 0xA6 0xAA
0xA7 (167) 0xA7 0xBA
0xA8 (168) 0xA8 0xBF
0xA9 (169) 0xA9 0xAE
0xAA (170) 0xAA 0xAC
0xAB (171) 0xAB 0xBD
0xAC (172) 0xAC 0xBC
0xAD (173) 0xAD 0xA1
0xAE (174) 0xAE 0xAB
0xAF (175) 0xAF 0xBB
0xB0 (176) 0xB0 0x91 0x25
0xB1 (177) 0xB1 0x92 0x25
0xB2 (178) 0xB2 0x93 0x25
0xB3 (179) 0xB3 0x02 0x25
0xB4 (180) 0xB4 0x24 0x25
0xB5 (181) 0xB5 0xC1
0xB6 (182) 0xB6 0xC2
0xB7 (183) 0xB7 0xC0
0xB8 (184) 0xB8 0xA9
0xB9 (185) 0xB9 0x63 0x25
0xBA (186) 0xBA 0x51 0x25
0xBB (187) 0xBB 0x57 0x25
0xBC (188) 0xBC 0x5D 0x25
0xBD (189) 0xBD 0xA2
0xBE (190) 0xBE 0xA5
0xBF (191) 0xBF 0x10 0x25
0xC0 (192) 0xC0 0x14 0x25
0xC1 (193) 0xC1 0x34 0x25
0xC2 (194) 0xC2 0x2C 0x25
0xC3 (195) 0xC3 0x1C 0x25
0xC4 (196) 0xC4 0x00 0x25
0xC5 (197) 0xC5 0x3C 0x25
0xC6 (198) 0xC7 0xE3 E
0xC7 (199) 0xC7 0xC3
0xC8 (200) 0xC8 0x5A 0x25 E
0xC9 (201) 0xC9 0x54 0x25 E
0xCA (202) 0xCA 0x69 0x25 E
0xCB (203) 0xCB 0x66 0x25 E
0xCC (204) 0xCC 0x60 0x25 E
0xCD (205) 0xCD 0x50 0x25 E
0xCE (206) 0xCE 0x6C 0x25 E
0xCF (207) 0xCF 0xA4 E
0xD0 (208) 0xD1 0xF0
0xD1 (209) 0xD1 0xD0
0xD2 (210) 0xD2 0xCA
0xD3 (211) 0xD3 0xCB
0xD4 (212) 0xD4 0xC8
0xD5 (213) 0x49 0x31 0x01
0xD6 (214) 0xD6 0xCD
0xD7 (215) 0xD7 0xCE
0xD8 (216) 0xD8 0xCF
0xD9 (217) 0xD9 0x18 0x25
0xDA (218) 0xDA 0x0C 0x25
0xDB (219) 0xDB 0x88 0x25
0xDC (220) 0xDC 0x84 0x25
0xDD (221) 0xDD 0xA6
0xDE (222) 0xDE 0xCC
0xDF (223) 0xDF 0x80 0x25
0xE0 (224) 0xE0 0xD3
0xE1 (225) 0xE1 0xDF
0xE2 (226) 0xE2 0xD4
0xE3 (227) 0xE3 0xD2
0xE4 (228) 0x05 0xF5
0xE5 (229) 0x05 0xD5
0xE6 (230) 0xE6 0xB5
0xE7 (231) 0xE8 0xFE
0xE8 (232) 0xE8 0xDE
0xE9 (233) 0xE9 0xDA
0xEA (234) 0xEA 0xDB
0xEB (235) 0xEB 0xD9
0xEC (236) 0xED 0xFD
0xED (237) 0xED 0xDD
0xEE (238) 0xEE 0xAF
0xEF (239) 0xEF 0xB4
0xF0 (240) 0xF0 0xAD
0xF1 (241) 0xF1 0xB1
0xF2 (242) 0xF2 0x17 0x20
0xF3 (243) 0xF3 0xBE
0xF4 (244) 0xF4 0xB6
0xF5 (245) 0xF5 0xA7
0xF6 (246) 0xF6 0xF7
0xF7 (247) 0xF7 0xB8
0xF8 (248) 0xF8 0xB0
0xF9 (249) 0xF9 0xA8
0xFA (250) 0xFA 0xB7
0xFB (251) 0xFB 0xB9
0xFC (252) 0xFC 0xB3
0xFD (253) 0xFD 0xB2
0xFE (254) 0xFE 0xA0 0x25
0xFF (255) 0xFF 0xA0
Page 0
0x80 (128) 0x00
0x81 (129) 0x00
0x82 (130) 0x00
0x83 (131) 0x00
0x84 (132) 0x00
0x85 (133) 0x00
0x86 (134) 0x00
0x87 (135) 0x00
0x88 (136) 0x00
0x89 (137) 0x00
0x8A (138) 0x00
0x8B (139) 0x00
0x8C (140) 0x00
0x8D (141) 0x00
0x8E (142) 0x00
0x8F (143) 0x00
0x90 (144) 0x00
0x91 (145) 0x00
0x92 (146) 0x00
0x93 (147) 0x00
0x94 (148) 0x00
0x95 (149) 0x00
0x96 (150) 0x00
0x97 (151) 0x00
0x98 (152) 0x00
0x99 (153) 0x00
0x9A (154) 0x00
0x9B (155) 0x00
0x9C (156) 0x00
0x9D (157) 0x00
0x9E (158) 0x00
0x9F (159) 0x92
0xA0 (160) 0xFF
0xA1 (161) 0xAD
0xA2 (162) 0xBD
0xA3 (163) 0x9C
0xA4 (164) 0xCF
0xA5 (165) 0xBE
0xA6 (166) 0xDD
0xA7 (167) 0xF5
0xA8 (168) 0xF9
0xA9 (169) 0xB8
0xAA (170) 0x00
0xAB (171) 0xAE
0xAC (172) 0xAA
0xAD (173) 0xF0
0xAE (174) 0x00
0xAF (175) 0xEE
0xB0 (176) 0xF8
0xB1 (177) 0xF1
0xB2 (178) 0xFD
0xB3 (179) 0xFC
0xB4 (180) 0xEF
0xB5 (181) 0xE6
0xB6 (182) 0xF4
0xB7 (183) 0xFA
0xB8 (184) 0xF7
0xB9 (185) 0xFB
0xBA (186) 0x00
0xBB (187) 0xAF
0xBC (188) 0xAC
0xBD (189) 0xAB
0xBE (190) 0xF3
0xBF (191) 0x00
0xC0 (192) 0xB7
0xC1 (193) 0xB5
0xC2 (194) 0xB6
0xC3 (195) 0xC7
0xC4 (196) 0x8E
0xC5 (197) 0x8F
0xC6 (198) 0x92
0xC7 (199) 0x80
0xC8 (200) 0xD4
0xC9 (201) 0x90
0xCA (202) 0xD2
0xCB (203) 0xD3
0xCC (204) 0xDE
0xCD (205) 0xD6
0xCE (206) 0xD7
0xCF (207) 0xD8
0xD0 (208) 0x00
0xD1 (209) 0xA5
0xD2 (210) 0xE3
0xD3 (211) 0xE0
0xD4 (212) 0xE2
0xD5 (213) 0xE5
0xD6 (214) 0x99
0xD7 (215) 0x9E
0xD8 (216) 0x9D
0xD9 (217) 0xEB
0xDA (218) 0xE9
0xDB (219) 0xEA
0xDC (220) 0x9A
0xDD (221) 0xED
0xDE (222) 0xE8
0xDF (223) 0xE1
0xE0 (224) 0x85, 0xA1
0xE1 (225) 0xA0
0xE2 (226) 0x83
0xE3 (227) 0xC6
0xE4 (228) 0x84
0xE5 (229) 0x86
0xE6 (230) 0x91
0xE7 (231) 0x87
0xE8 (232) 0x8A
0xE9 (233) 0x82
0xEA (234) 0x88
0xEB (235) 0x89
0xEC (236) 0x8D
0xED (237) 0x00
0xEE (238) 0x8C
0xEF (239) 0x8B
0xF0 (240) 0xD0
0xF1 (241) 0xA4
0xF2 (242) 0x95
0xF3 (243) 0xA2
0xF4 (244) 0x93
0xF5 (245) 0xE4
0xF6 (246) 0x94
0xF7 (247) 0xF6
0xF8 (248) 0x9B
0xF9 (249) 0x97
0xFA (250) 0xA3
0xFB (251) 0x96
0xFC (252) 0x81
0xFD (253) 0xEC
0xFE (254) 0xE7
0xFF (255) 0x98