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Inside Macintosh: Files /

Chapter 2 - File Manager / Data Organization on Volumes

Volume Bitmaps

The File Manager uses a volume bitmap to keep track of whether each block in a volume is currently allocated to some file or not. The bitmap contains one bit for each allocation block in the volume. If a bit is set, the corresponding allocation block is currently in use by some file. If a bit is clear, the corresponding allocation block is not currently in use by any file and is available for allocation.

Note

The volume bitmap indicates which blocks on a volume are currently in use, but it does not indicate which files occupy which blocks. The File Manager maintains file-mapping information in two locations: in each file's catalog entry and in the extents overflow file.

The size of the volume bitmap depends on the number of allocation blocks in the volume, which in turn depends both on the number of physical blocks in the volume and on the size of the volume's allocation blocks (the number of physical blocks per allocation block). For example, a floppy disk that can hold 800 KB of data and has an allocation block size of one physical block has a volume bitmap size of 1600 bits (200 bytes). A volume containing 32 MB of data and having an allocation block size of one physical block has a volume bitmap size of 65,536 bits (8192 bytes). However, the size of the volume bitmap is rounded up, if necessary, so that the volume bitmap occupies an integral number of physical blocks.

Because the drNmAlBlks field in the MDB occupies only 2 bytes, the File Manager can address at most 65,535 allocation blocks. Thus, the volume bitmap is never larger than 8192 bytes (or 16 physical blocks). For volumes containing more than 32 MB of space, the allocation block size must be increased. For example, a volume containing 40 MB of space must have an allocation block size that is at least 2 physical blocks; a volume containing 80 MB of space must have an allocation block size that is at least 3 physical blocks; and so forth.

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