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File Allocation Table (FAT)
12 Bit Meaning 16 Bit
000 free space 0000
FF1-FF7 bad track marking FFF1-FFF7
FF8-FFE may be used to mark end of a file chain FFF8-FFFE
FFF standard marker for end of a file chain FFFF
Notes: : The FAT is implemented as an array containing a linked list
for each file. The files directory entry has a pointer to the
first cluster which contains the cluster number of the next
cluster in the chain until the pointer contained is 0xFFF
(12 bit FAT) and 0xFFFF (16 bit FAT) marking end of file.
: DOS maintains two copies of the FAT, but does not use the
second copy for anything other than a mirror image of the
first. CHKDSK doesn't even read the second FAT.
: DOS 4.x did not change the size of the cluster number as some
suggest, but instead increased the size of the sector number
Calculating 12 bit FAT Entries
1. Get starting cluster from directory entry.
2. Multiply the cluster number just used by 1.5
3. The whole part of the product is the offset into the FAT,
of the entry that maps to the cluster in the directory.
This entry contains the number of the next cluster.
4. Move the word at the calculated FAT into a register.
5. If the last cluster used was an even number, keep the
low order 12 bits of the register, otherwise, keep the
high order 12 bits.
6. If the resultant 12 bits are (0FF8h-0FFFh) no more clusters
are in the file. Otherwise, the next 12 bits contain the
cluster number of the next cluster in the file.
Calculating 16 Bit FAT Entries
1. Get the starting cluster of the file from the directory.
2. Multiply the cluster number found by 2.
3. Load the word at the calculated FAT offset into a register.
4. If the 16 bits are (0FFF8h-0FFFFh) no more clusters are in
the file. Otherwise, the 16 bits contain the cluster number
of the next cluster in the file.
To convert the cluster to a logical sector number (relative
sector, similar to that used by DEBUG, int 25h and 26h):
1. Subtract 2 from the cluster number
2. Multiply the result by the number of sectors per cluster.
3. Add the logical sector number of the beginning of the data area. |