FAT16 文件系统

The boot sector

A boot sector can be found in the first sector of every logical disk. On a floppy disk, the logical disk takes up all of the physical disk and the boot sector lives in the first physical sector. On a hard disk, a boot sector lives at the start of each partition.

An example boot sec

0000 EB 3E 90 4D 53 57 49 4E-34 2E 30 00 02 20 01 00 .>.MSWIN4.0.. ..
0010 02 40 03 00 00 F8 FF 00-3F 00 40 00 3F 00 00 00 .@......?.@.?...
0020 41 DE 1F 00 80 00 29 37-4B 84 32 46 55 4A 49 54 A…..)7K.2FUJIT
0030 53 55 31 32 32 34 46 41-54 31 36 20 20 20 F1 7D SU1224FAT16 .}
0040 FA 33 C9 8E D1 BC FC 7B-16 07 BD 78 00 C5 76 00 .3…..{…x..v.
0050 1E 56 16 55 BF 22 05 89-7E 00 89 4E 02 B1 0B FC .V.U."..~..N….
0060 F3 A4 06 1F BD 00 7C C6-45 FE 0F 8B 46 18 88 45 ……|.E…F..E
0070 F9 FB 38 66 24 7C 04 CD-13 72 3C 8A 46 10 98 F7 ..8f$|…r<.F…
0080 66 16 03 46 1C 13 56 1E-03 46 0E 13 D1 50 52 89 f..F..V..F…PR.
0090 46 FC 89 56 FE B8 20 00-8B 76 11 F7 E6 8B 5E 0B F..V.. ..v….^.
00A0 03 C3 48 F7 F3 01 46 FC-11 4E FE 5A 58 BB 00 07 ..H…F..N.ZX…
00B0 8B FB B1 01 E8 94 00 72-47 38 2D 74 19 B1 0B 56 …….rG8-t…V
00C0 8B 76 3E F3 A6 5E 74 4A-4E 74 0B 03 F9 83 C7 15 .v>..^tJNt……
00D0 3B FB 72 E5 EB D7 2B C9-B8 D8 7D 87 46 3E 3C D8 ;.r… …}.F><.
00E0 75 99 BE 80 7D AC 98 03-F0 AC 84 C0 74 17 3C FF u…}…….t.<.
00F0 74 09 B4 0E BB 07 00 CD-10 EB EE BE 83 7D EB E5 t…………}..
0100 BE 81 7D EB E0 33 C0 CD-16 5E 1F 8F 04 8F 44 02 ..}..3…^….D.
0110 CD 19 BE 82 7D 8B 7D 0F-83 FF 02 72 C8 8B C7 48 ….}.}….r…H
0120 48 8A 4E 0D F7 E1 03 46-FC 13 56 FE BB 00 07 53 H.N….F..V….S
0130 B1 04 E8 16 00 5B 72 C8-81 3F 4D 5A 75 A7 81 BF …..[r..?Mzu…
0140 00 02 42 4A 75 9F EA 00-02 70 00 50 52 51 91 92 ..Bju….p.PRQ..
0150 33 D2 F7 76 18 91 F7 76-18 42 87 CA F7 76 1A 8A 3..v…v.B…v..
0160 F2 8A 56 24 8A E8 D0 CC-D0 CC 0A CC B8 01 02 CD ..V$............
0170 13 59 5A 58 72 09 40 75-01 42 03 5E 0B E2 CC C3 .YZXr.@u.B.^….
0180 03 18 01 27 0D 0A 49 6E-76 61 6C 69 64 20 73 79 …'..Invalid sy
0190 73 74 65 6D 20 64 69 73-6B FF 0D 0A 44 69 73 6B stem disk…Disk
01A0 20 49 2F 4F 20 65 72 72-6F 72 FF 0D 0A 52 65 70 I/O error…Rep
01B0 6C 61 63 65 20 74 68 65-20 64 69 73 6B 2C 20 61 lace the disk, a
01C0 6E 64 20 74 68 65 6E 20-70 72 65 73 73 20 61 6E nd then press an
01D0 79 20 6B 65 79 0D 0A 00-49 4F 20 20 20 20 20 20 y key…IO
01E0 53 59 53 4D 53 44 4F 53-20 20 20 53 59 53 80 01 SYSMSDOS SYS..
01F0 00 57 49 4E 42 4F 4F 54-20 53 59 53 00 00 55 AA .WINBOOT SYS..U.

Structure of the boot sector

Structure of the BIOS paramter block (BPB)

Values for the example boot sector

Check the C source code to define the FAT boot sector.


Structure of a FAT disk

SectRootDir = RootDirSize / 16
BookKeep = PhysSectPerTrck 1 NumFATs * NumSectPerFAT SectRootDir

FAT

The FAT is an array of up to 65,536 16-bit unsigned integers. The first 2 and last 16 entries in the FAT are reserved. All other elements are of type: index into the FAT array. Each of the non-reserved FAT entries correspond to a cluster on the disk.

FAT entries 0x0000 and 0x0001 contain other information, usually 0xfff8 and 0xffff respectively.
The fragment below is from the start of a FAT
FFF8 FFFF 0000 0004 FFFF 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000
The fragment above shows a file using clusters 0x0003 & 0x0004.

Folders
A folder is a data structure stored on the disk. The root folder is stored before the data area. Other folders are stored in the data area in the same way as files; These folders may grow in size, shrink in size and be stored non-contiguously in the same way as files. The structure of all folders is as an array of folder entries. The end of the folder is marked by a folder entry starting with a 00h byte.

Folder entry structure

The date field is in DOS date format, and the time field is in DOS time.