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The Bitmap keeps track of what blocks on disk are in use and which aren't.  The Bitmap is stored in special Bitmap blocks.  For this purpose the bitmap contains a single bit for each block on the disk.  The status of this bit determines whether a specific block on disk is in use or not.

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The number of bitmap blocks depends on how many blocks a disk consists of and how many bits can be fitted in a single bitmap block (minus space for the block header).  In the picture above a bitmap block holds 16 bits.  For a disk of 22 blocks you'd need 2 bitmap blocks.  As you can see the last bitmap block isn't used completely.   Note that this is a very downsized example.  A bitmap block of 512 bytes has enough room to store over 4000 bits.  Furthermore, a partition of 1 GB can easily have over 500 bitmap blocks.

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For comments, problems or questions regarding this page contact John Hendrikx.
Last updated: 17 oktober 1998.