$Bitmap (6) - File - NTFS Documentation
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# File - $Bitmap (6)

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## Overview

This file lists which clusters are in use. Each bit in this file represents one LCN.

## Attributes

Type | Description | Name
--- | --- | ---
0x10 | $STANDARD_INFORMATION |
0x30 | $FILE_NAME | $Bitmap
0x80 | $DATA | [Unnamed]

## Layout of the File

### Unnamed Data Stream

The lowest bit represents the lowest numbered LCN. Thus:

Bit | LCN
--- | ---
000000012 | 0
000000102 | 1
000001002 | 2
... | etc

## Notes

### MFT Zone

To prevent the MFT becoming fragmented, Windows maintains a buffer around it. No new files will be created in this buffer region until the other disk space is used up. The buffer size is configurable and can be 12.5%, 25%, 37.5% or 50% of the disk. Each time the rest of the disk becomes full, the buffer size is halved.

### Other Information

The size of this file is always a multiple of 8 bytes (64 clusters). Because of this rounding-up, the $Bitmap will represent slightly more clusters than the disk has. These bit are always set to 1.

The backup copy of the boot sector lies in this no-mans-land the cluster is hence marked as in use.

In theory, on very small volume, this attribute could be resident. In practice, Windows crashes.

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