Allocation Bitmap Directory Entry

Offset

Size

Description

Comments

0 (0x00)

1

EntryType

0x81

0x01

1

BitmapFlags

Bits

Size

Description

Comments

0 1 BitmapIdentifier 0 – 1st bitmap, 1 -2nd bitmap
1 7 Reserved  

Clusters 10-17

2 (0x02)

18

Reserved

 

20 (0x14)

4

First Cluster

 

24 (0x18)

8

Data Length

 

The number of bitmaps and therefore a number of Bitmap Allocation entries is equal to the number of FATs. In case of TexFAT two FATs are used and bit 0 of Flags indicates which bitmap and FAT are referred.

The First Allocation Bitmap shall be used in conjunction with the First FAT and the Second Allocation Bitmap shall be used with the Second FAT. ActiveFat field in Boot Sector defines which FAT and Allocation Bitmap are active.

Bitmap size in bytes must be a number of clusters in the volume divided by 8 and rounded up.

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