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In `F33f`_`[computer file systems`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Computer_file_system]`_`f, a `!block availability map`! (`!BAM`!)`:cite-ref-1[`F5bf`_`[1`#cite-note-1]`_`f]`:cite-ref-2[`F5bf`_`[2`#cite-note-2]`_`f]`:cite-ref-3[`F5bf`_`[3`#cite-note-3]`_`f]`:cite-ref-4[`F5bf`_`[4`#cite-note-4]`_`f]`:cite-ref-5[`F5bf`_`[5`#cite-note-5]`_`f] is a `F33f`_`[data structure`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Data_structure]`_`f used to track `F33f`_`[disk blocks`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Block_size_(data_storage_and_transmission)]`_`f that are considered free (available for new data). It is used along with a `F33f`_`[directory`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Directory_(file_systems)]`_`f to manage files on a disk (originally only a `F33f`_`[floppy disk`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Floppy_disk]`_`f, and later also a `F33f`_`[hard disk`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Hard_disk_drive]`_`f).

In terms of `F33f`_`[Commodore DOS`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Commodore_DOS]`_`f (`F33f`_`[CBM`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Commodore_International]`_`f `F33f`_`[DOS`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Disk_operating_system]`_`f) compatible `F33f`_`[disk drives`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Disk_storage]`_`f, the BAM was a data structure stored in a reserved area of the disk (its size and location varied based on the physical characteristics of the disk). For each track, the BAM consisted of a `F33f`_`[bitmap`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Bitmap]`_`f of available `F33f`_`[blocks`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Block_(data_storage)]`_`f and (usually) a `F33f`_`[count`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Summation]`_`f of the available blocks. The count was held in a single byte, as all formats had 256 or fewer blocks per `F33f`_`[track`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Track_(disk_drive)]`_`f. The count byte was simply the sum of all 1-bits in the bitmap of bytes for the current track.

The following table illustrates the layout of `F33f`_`[Commodore 1541`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Commodore_1541]`_`f BAM. The table would be larger for higher-capacity disks (described below).

`t

| | Total byte | Bitmap byte 1 | Bitmap byte 2 | Bitmap byte 3 |

|---|---|---|---|---|

| Track 1 | blocks available | Blocks 0–7 | Blocks 8–15 | Blocks 16–23 |

| Track 2 | blocks available | Blocks 0–7 | Blocks 8–15 | Blocks 16–23 |

| ... | ... | ... | ... | ... |

| Track 35 | blocks available | Blocks 0–7 | Blocks 8–15 | Blocks 16–23 |

`t

The bitmap was contained in 3 bytes for Commodore 1541 format (`F33f`_`[single-sided`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Single-sided_disk]`_`f) disks because it had 17 to 20 `F33f`_`[sectors`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Disk_sector]`_`f per track (note 3 bytes can hold at least 20 bits).`:cite-ref-6[`F5bf`_`[6`#cite-note-6]`_`f] Similarly, the `F33f`_`[Commodore 1571`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Commodore_1571]`_`f used 3 bytes for the bitmap of each track, but the BAM was twice the size because there were twice as many tracks when formatted as `F33f`_`[double-sided`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Double-sided_disk]`_`f.`:cite-ref-7[`F5bf`_`[7`#cite-note-7]`_`f] In contrast, the `F33f`_`[Commodore 1581`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Commodore_1581]`_`f disk drive used 5 bytes for the bitmap because the disk format had 40 blocks per track (note 5 bytes can hold 40 bits).`:cite-ref-8[`F5bf`_`[8`#cite-note-8]`_`f]

In the bitmap of any format, a 1 `F33f`_`[bit`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Bit]`_`f indicated the block was available (free), while a 0 bit indicated the block was not available (used), and the bitmap data was stored `F33f`_`[low-byte first`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Little_endian]`_`f. So the first `F33f`_`[byte`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Byte]`_`f held a map for blocks 0 to 7, the second byte held a map for blocks 8 to 15, and so on. Within a byte, the bitmap was ordered `F33f`_`[low-bit`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Least_significant_bit]`_`f first. For example, the first byte would represent block 0 with the least significant bit and block 7 with the `F33f`_`[most significant bit`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Most_significant_bit]`_`f.

Storage devices by `F33f`_`[Creative Micro Designs`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Creative_Micro_Designs]`_`f, intended for use with CBM computers, also used a Block Availability Map which served the same purpose. However, these devices (`F33f`_`[FD-2000, FD-4000`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=CMD_FD_series]`_`f, and CMD-HD) did not include a count byte, and the bits in each byte were reversed (high-bit first). Although the bits were reversed (compared to CBM formats), the bytes were still stored in the same order (low-byte first).`:cite-ref-9[`F5bf`_`[9`#cite-note-9]`_`f]

`t

| | Bitmap byte 1 | Bitmap byte 2 | ... | Bitmap byte 32 |

|---|---|---|---|---|

| Track 1 | Blocks 0–7 | Blocks 8–15 | ... | Blocks 248–255 |

| Track 2 | Blocks 0–7 | Blocks 8–15 | ... | Blocks 248–255 |

| ... | ... | ... | ... | ... |

`t

>>Contents

• `F0af`_`[See also`#see-also]`_`f

• `F0af`_`[References`#references]`_`f

-─

>>See also

• `F33f`_`[File Allocation Table`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=File_Allocation_Table]`_`f (FAT)

• `F33f`_`[Design of the FAT file system`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Design_of_the_FAT_file_system]`_`f

• `F33f`_`[Free space bitmap`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Free_space_bitmap]`_`f

>>References

`:cite-note-1`!1.`! `F0af`_`[↑`#cite-ref-1]`_`f `:citerefenglisch1984`aEnglisch, Lothar (1984). `*The Anatomy of the 1541 Disk Drive`*. Grand Rapids, MI: Abacus Software. p. 89. `F33f`_`[ISBN`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=ISBN_(identifier)]`_`f 0-916439-01-1.

`:cite-note-2`!2.`! `F0af`_`[↑`#cite-ref-2]`_`f `*1541 User's Guide`*. Commodore Business Machines. 1982. p. 9.

`:cite-note-3`!3.`! `F0af`_`[↑`#cite-ref-3]`_`f `*1571 User's Guide`*. Commodore Business Machines. 1985. p. 23.

`:cite-note-4`!4.`! `F0af`_`[↑`#cite-ref-4]`_`f `*1581 User's Guide`*. Commodore Business Machines. 1987. p. 34.

`:cite-note-5`!5.`! `F0af`_`[↑`#cite-ref-5]`_`f `*FD Series User's Manual`*. Creative Micro Designs. 1992. p. 108.

`:cite-note-6`!6.`! `F0af`_`[↑`#cite-ref-6]`_`f `*1541 User's Guide`*. Commodore Business Machines. 1982. p. 65.

`:cite-note-7`!7.`! `F0af`_`[↑`#cite-ref-7]`_`f `*1571 User's Guide`*. Commodore Business Machines. 1985. pp. 108–109.

`:cite-note-8`!8.`! `F0af`_`[↑`#cite-ref-8]`_`f `*1581 User's Guide`*. Commodore Business Machines. 1987. pp. 119–120.

`:cite-note-9`!9.`! `F0af`_`[↑`#cite-ref-9]`_`f `*FD Series User's Manual`*. Creative Micro Designs. 1992. pp. 112–114.

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