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In [[computer file system]]s, a '''block allocation map''' is a [[data structure]] used to track [[Block size (data storage and transmission)|disk blocks]] that are considered "in use". Blocks may also be referred to as allocation units or [[Data_cluster|clusters]].<ref>{{cite book|title=CP/M Plus Operating System System Guide|year=1983|publisher=Digital Research, Inc.|location=Monterey, CA|edition=Second|page=41}}</ref>

[[CP/M]] used a block allocation map in its [[Directory_(file_systems)|directory]]. Each directory entry could list 8 or 16 blocks (depending on disk format) that were allocated to a file. If a file used more blocks, additional directory entries would be needed. Thus, a single file could have multiple directory entries. A benefit of this method is the possibility to use [[sparse files]] by declaring a large file size but only allocating blocks that are actually used.<ref name="cpm_pg">{{cite book|title=CP/M Plus Operating System Programmer's Guide|year=1983|publisher=Digital Research, Inc.|location=Monterey, CA|edition=Second|page=2-12,3-16}}</ref> A detriment of this method is the disk may have free space (unallocated blocks) but data cannot be appended to a file because all directory entries are used.<ref name="cpm_pg" />

== Errata ==
The [[Commodore DOS]] used a similarly named but significantly different [[noting]].

== See also ==
* [[File Allocation Table]] (FAT)
* [[Design of the FAT file system]]

== References ==
{{Reflist}}
{{DEFAULTSORT:Block Allocation Map}}

[[Category:Computer file systems]]

{{Compu-storage-stub}}

Block allocation map - Revision history

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