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Block allocation map

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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. This method also allowed sparse files to be created by declaring a large file size but only allocating blocks that are actually used.

Errata

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

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