Will windows automatically select the best allocation unit size when formatting a floppy disk?

Answer 1

Yes. Windows will select the correct allocation unit size according to the disk's capacity. A 1.44 MB disk will use one sector per cluster while a 2.88 MB disk will use 2 sectors per cluster. Short of using a disk sector editor there's no way to change this, but even if you did you'd render the disk unreadable.

Under Windows, all floppy disks are formatted according to the FAT12 specification. Hard disks and flash drives are typically formatted to the FAT32 or NTFS specifications. To support any other specifications besides FAT or NTFS you will need a suitable driver installed, however it's not recommended for normal use since non-standard formats are unreadable on other systems unless the appropriate driver is present on those systems.

FAT is an abbreviation of File allocation Table. The number (12, 16 or 32) simly determines how many bits are used to address clusters. FAT12 is therefore a 12-bit file allocation system, and is used by Windows to format floppy disks. The sectors on a floppy disk are fixed at 512 bytes and the allocation unit size simply determines how many sectors there are per cluster. A cluster is the minimum unit of storage. As such there can only be one file per cluster but a file may occupy one or more clusters. Any unused bytes in the final cluster of a file are wasted bytes, so the smaller the cluster size the better.

A 1.44 MB disk uses an allocation unit size of 1 sector per cluster which is the minimum possible. Any file that is not an exact multiple of 512 bytes will therefore waste anything from 1 to 511 bytes (inclusive) in the final cluster. A 2.88 MB disk uses 2 sectors per cluster, so files must be an exact multiple of 1024 bytes to minimise wastage. Hard disks are typically much larger than floppy disks so they not only use much larger clusters they require more bits to address them.

A 12-bit addressing system can address 4096 clusters in total. However, 33 sectors are reserved so the actual number of sectors for storage can be no more than 4063. This means that you can have no more than 4063 individual files or folders on a floppy disk. However, the more sectors per cluster, the fewer files you can physically store. In reality, the limit is much lower than this. A 1.44 MB disk has an overall capacity of 1,457,664 bytes so at 512 bytes per sector, that equates to just 2847 sectors in total, 33 of which are reserved, leaving 2814 sectors for actual storage.

The boot sector (sector 0) stores the basic disk geometry:

• Bytes 0 to 10 are ignored.
• Bytes 11 and 12 store the bytes per sector (512)
• Byte 13 stores the number of sectors per cluster (1 for 1.44 MB floppy, 2 for 2.88 MB floppy)
• Byte 14 and 15 store the number of reserved sectors (32)
• Byte 16 stores the number of FATs (2)
• Bytes 17 and 18 store the maximum number of root directory entries
• Bytes 19 and 20 store the total sector count
• Byte 21 is ignored
• Bytes 22 and 23 store the number of sectors per FAT
• Bytes 24 and 25 store the sectors per track
• Bytes 26 and 27 store the number of heads
• Bytes 28 to 31 are ignored
• Bytes 32 to 35 store the total sector count for FAT32 (0 for FAT12)
• Bytes 36 and 37 are ignored.
• Byte 38 stores the boot signature
• Byte 39 to 42 store volume identifier
• Byte 43 to 53 store the volume label
• Byte 54 to 61 store the file system type as a character array (e.g., "FAT12", information use only)
• Bytes 62 and beyond are ignored

Sectors 1 to 9 hold the primary FAT while 10 to 18 hold the secondary FAT. The secondary FAT provides a backup of the FAT.

Sectors 19 to 32 store the root directory. Each sector can hold 16 entries of 32 bytes each. The number of entries consumed is determined by bytes 17 and 18 in the boot sector.

Sector 33 marks the start of the actual data. Logically, this is cluster 0.

The FAT is simply a directory of clusters. FAT entries 0 and 1 are reserved so FAT entry 2 represents logical cluster 0 (starting at physical sector 33).

The root directory (which always exists) stores the root file entries, each of which holds a file's name, its timestamps (created, modified and accessed times), file size, file attributes (read-only, system, archive, hidden or directory) and the logical start cluster. Directories always have the directory attribute while files do not. Other than that both files and folders are treated exactly the same.

The logical start cluster determines where the file (or directory) physically starts. However, as files are created, modified and destroyed, the files can become fragmented, so the cluster that immediately follows the start cluster needn't necessarily be the next cluster for that file. To keep track of which files are using which clusters and in which order, the FAT entry for the start cluster will point to the next cluster, and it will point to the next, and so on, thus creating a daisy chain of clusters. By de-fragging drives, you not only ensure files occupy contiguous clusters but also that the chain of FAT entries is also contiguous. This reduces head movement and thus improves performance.

FAT values in the range 0xFF0 to 0xFFF are reserved. In particular, 0xFF7 is used to mark bad clusters. These are detected when the disk is formatted (unless you use quick format) or when you run a disk diagnostic program such as SCANDISK or CHKDISK. You can attempt to recover bad clusters to make them usable again but once a cluster is marked bad it's reliability is questionable at best.

FAT entries in the range 0xFF8 through 0xFFF are used to mark the final cluster of a file. As mentioned previously, if the file doesn't fully occupy the final cluster the unused bytes are wasted since no two files can share the same cluster.