Block allocation map

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In computer file systems, a block allocation map is a data structure used to track disk blocks that are considered "in use". Blocks may also be referred to as allocation units or clusters.[1]

CP/M used a block allocation map in its 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.[2] 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.[2]


The Commodore DOS used a similarly named but significantly different Block availability map.

See also[edit]


  1. ^ CP/M Plus Operating System System Guide (Second ed.). Monterey, CA: Digital Research, Inc. 1983. p. 41. 
  2. ^ a b CP/M Plus Operating System Programmer's Guide (Second ed.). Monterey, CA: Digital Research, Inc. 1983. p. 2-12,3-16.