Allocation Bitmap

Allocation Bitmap keeps track of the allocation status of clusters. FAT does not serve this purpose as in FAT16/FAT32 file system.Allocation Bitmap consists of a number of 8 bit bytes which can be treated as a sequence of bits. Each bit in bitmap corresponds to a data cluster.

If it has a value of 1, the cluster is occupied, if 0 - the cluster is free. The least significant bit of bitmap table refers to the first cluster, i.e. cluster 2.

Offset

Size

Description

Comments

0x00

1

1st byte

Clusters 2-9

0x01

1

2nd byte

Clusters 10-17

0x02

1

3rd byte

Clusters 18-25

...

...

...

...

Bitmap allocation table resides in cluster heap and referred by Bitmap Directory entry in root directory.

In TexFAT could be 2 Bitmap Allocation tables, otherwise there will be only one bitmap. The NumberOfFats field in Boot Sectors determines the number of valid Allocation Bitmap directory entries in the root directory and the number of Allocation Bitmaps.

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