Summary of FAT file system 3

Seven,  FAT table structure  

FAT table (File Alloacation Table) is the number of clusters in the data correspond to the list.File system allocates disk space allocated by cluster. Thus, when the file occupies disk space, but the basic unit is not byte clusters, even if only one byte of a file, the operating system will assign it a minimum unit: that a cluster. For orderly disk space can be allocated to the appropriate file, read the file when they can read files from the corresponding address, we can put the data area space into BPB_BytsPerSec * BPB_SecPerClus-byte clusters to manage, FAT table Type Size and FAT table entries related, FAT12 entries is 12bit, FAT16 is 16bit, and FAT32 was 32bit.For large files, you need to assign multiple clusters. Data is not necessarily the same file stored on disk in a complete one continuous area region, and often will be divided into several sections, the same as the chain store. This storage is called a chain store files. In order to achieve the chain store files, the file system must accurately record which has been occupied cluster file, you must also specify the storage space of the next successor for each cluster in the cluster has been occupied for the last cluster of the file, No successor will have to specify the cluster cluster. These are the FAT table to hold the corresponding entries in the FAT table records the information it represents clusters: such is empty, is not a bad cluster, if it is already the end of the cluster, such as a file. 
An example to FAT16 FAT is structured as follows: 

 

Entries
 
Sample Code
Description
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
FFF8
 
Disk identification word, must FFF8
 
 
 
1
 
FFFF
 
The first cluster is occupied
 
 
 
2
 
0003
 
0000h: Available Cluster
 
 
 
3
 
0004
 
0002h - FFFEF: file has been stored next cluster in with a cluster table entry
 
……
 
……
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
N
 
FFFF
 
FFFF0h - FFFF6: Reserved cluster
 
 
 
N +1
 
0000
 
FFFF7h: bad clusters
 
 
 
……
 
……
 
Finally, a cluster file: FFFFFh - FFFF8h
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Meaning FAT system records the value of each item (16 hex)
 
 
 
 
 
 
 
 
 
 
FAT12 records
FAT16 entries
The value of FAT32 entries
Corresponding cluster performance conditions
 
 
 
Value items
Values
 
 
 
 
 
 
 
 
0
0
0
Unallocated clusters
 
 
 
 
 
 
 
 
 
 
002 ~ FFF
0002 ~ FFEF
00000002 ~ FFFFFFEF
Assigned cluster
 
 
 
 
 
 
 
 
 
 
FF0~FF6
FFF0 ~ FFF6
FFFFFFF0 ~ FFFFFFF6
Reserved
 
 
 
 
 
 
 
 
 
 
 
 
 
FF7
 
FFF7
FFFFFFF7
Bad clusters
 
 
 
 
 
 
 
 
 
 
FF8~FFF
FFF8~FFFF
FFFFFFF8 ~ FFFFFFFF
End of file clusters
 

 
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