Manual: mkdosfs

MKFS.FAT(8) System Manager's Manual MKFS.FAT(8)

NAME
mkfs.fat - create an MS-DOS FAT filesystem

SYNOPSIS
mkfs.fat [OPTIONS] DEVICE [BLOCK-COUNT]

DESCRIPTION
mkfs.fat is used to create a FAT filesystem on a device or in an image file. DEVICE is the special file correspond‐
ing to the device (e.g. /dev/sdXX) or the image file (which does not need to exist when the option -C is given).
BLOCK-COUNT is the number of blocks on the device and size of one block is always 1024 bytes, independently of the
sector size or the cluster size. Therefore BLOCK-COUNT specifies size of filesystem in KiB unit and not in the num‐
ber of sectors (like for all other mkfs.fat options). If omitted, mkfs.fat automatically chooses a filesystem size
to fill the available space.

Two different variants of the FAT filesystem are supported. Standard is the FAT12, FAT16 and FAT32 filesystems as
defined by Microsoft and widely used on hard disks and removable media like USB sticks and SD cards. The other is
the legacy Atari variant used on Atari ST.

In Atari mode, if not directed otherwise by the user, mkfs.fat will always use 2 sectors per cluster, since GEMDOS
doesn't like other values very much. It will also obey the maximum number of sectors GEMDOS can handle. Larger
filesystems are managed by raising the logical sector size. An Atari-compatible serial number for the filesystem is
generated, and a 12 bit FAT is used only for filesystems that have one of the usual floppy sizes (720k, 1.2M, 1.44M,
2.88M), a 16 bit FAT otherwise. This can be overridden with the -F option. Some PC-specific boot sector fields
aren't written, and a boot message (option -m) is ignored.

OPTIONS
-a Normally, for any filesystem except very small ones, mkfs.fat will align all the data structures to cluster size,
to make sure that as long as the partition is properly aligned, so will all the data structures in the filesys‐
tem. This option disables alignment; this may provide a handful of additional clusters of storage at the expense
of a significant performance degradation on RAIDs, flash media or large-sector hard disks.

-A Select using the Atari variation of the FAT filesystem if that isn't active already, otherwise select standard
FAT filesystem. This is selected by default if mkfs.fat is run on 68k Atari Linux.

-b SECTOR-OF-BACKUP
Selects the location of the backup boot sector for FAT32. Default depends on number of reserved sectors, but
usually is sector 6. If there is a free space available after the backup boot sector then backup of the FAT32
info sector is put after the backup boot sector, usually at sector 7. The backup must be within the range of re‐
served sectors. Value 0 completely disables creating of backup boot and info FAT32 sectors.

-c Check the device for bad blocks before creating the filesystem.

-C Create the file given as DEVICE on the command line, and write the to-be-created filesystem to it. This can be
used to create the new filesystem in a file instead of on a real device, and to avoid using dd in advance to cre‐
ate a file of appropriate size. With this option, the BLOCK-COUNT must be given, because otherwise the intended
size of the filesystem wouldn't be known. The file created is a sparse file, which actually only contains the
meta-data areas (boot sector, FATs, and root directory). The data portions won't be stored on the disk, but the
file nevertheless will have the correct size. The resulting file can be copied later to a floppy disk or other
device, or mounted through a loop device.

-D DRIVE-NUMBER
Specify the BIOS drive number to be stored in the FAT boot sector. For hard disks and removable medias it is
usually 0x80–0xFF (0x80 is first hard disk C:, 0x81 is second hard disk D:, ...), for floppy devices or parti‐
tions to be used for floppy emulation it is 0x00–0x7F (0x00 is first floppy A:, 0x01 is second floppy B:).

-f NUMBER-OF-FATS
Specify the number of file allocation tables in the filesystem. The default is 2.

-F FAT-SIZE
Specifies the type of file allocation tables used (12, 16 or 32 bit). If nothing is specified, mkfs.fat will au‐
tomatically select between 12, 16 and 32 bit, whatever fits better for the filesystem size.

-g HEADS/SECTORS-PER-TRACK
Specify HEADS and SECTORS-PER-TRACK numbers which represents disk geometry of DEVICE. Both numbers are stored
into the FAT boot sector. Number SECTORS-PER-TRACK is used also for aligning the total count of FAT sectors. By
default disk geometry is read from DEVICE itself. If it is not available then LBA-Assist Translation and trans‐
lation table from the SD Card Part 2 File System Specification based on total number of disk sectors is used.

-h NUMBER-OF-HIDDEN-SECTORS
Specify the number of so-called hidden sectors, as stored in the FAT boot sector: this number represents the be‐
ginning sector of the partition containing the file system. Normally this is an offset (in sectors) relative to
the start of the disk, although for MBR logical volumes contained in an extended partition of type 0x05 (a non-
LBA extended partition), a quirk in the MS-DOS implementation of FAT requires it to be relative to the parti‐
tion's immediate containing Extended Boot Record. Boot code and other software handling FAT volumes may also
rely on this field being set up correctly; most modern FAT implementations will ignore it. By default, if the
DEVICE is a partition block device, mkfs.fat uses the partition offset relative to disk start. Otherwise,
mkfs.fat assumes zero. Use this option to override this behaviour.

-i VOLUME-ID
Sets the volume ID of the newly created filesystem; VOLUME-ID is a 32-bit hexadecimal number (for example,
2e24ec82). The default is a number which depends on the filesystem creation time.

-I Ignore and disable safety checks. By default mkfs.fat refuses to create a filesystem on a device with partitions
or virtual mapping. mkfs.fat will complain and tell you that it refuses to work. This is different when using
MO disks. One doesn't always need partitions on MO disks. The filesystem can go directly to the whole disk.
Under other OSes this is known as the superfloppy format. This switch will force mkfs.fat to work properly.

-l FILENAME
Read the bad blocks list from FILENAME.

-m MESSAGE-FILE
Sets the message the user receives on attempts to boot this filesystem without having properly installed an oper‐
ating system. The message file must not exceed 418 bytes once line feeds have been converted to carriage return-
line feed combinations, and tabs have been expanded. If the filename is a hyphen (-), the text is taken from
standard input.

-M FAT-MEDIA-TYPE
Specify the media type to be stored in the FAT boot sector. This value is usually 0xF8 for hard disks and is
0xF0 or a value from 0xF9 to 0xFF for floppies or partitions to be used for floppy emulation.

--mbr[=y|yes|n|no|a|auto]
Fill (fake) MBR table with disk signature one partition which starts at sector 0 (includes MBR itself) and spans
whole disk device. It is needed only for non-removable disks used on Microsoft Windows systems and only when
formatting whole unpartitioned disk. Location of the disk signature and partition table overlaps with the end of
the first FAT sector (boot code location), therefore there is no additional space usage. Default is auto mode in
which mkfs.fat put MBR table only for non-removable disks when formatting whole unpartitioned disk.

-n VOLUME-NAME
Sets the volume name (label) of the filesystem. The volume name can be up to 11 characters long. Supplying an
empty string, a string consisting only of white space or the string "NO NAME" as VOLUME-NAME has the same effect
as not giving the -n option. The default is no label.

--codepage=PAGE
Use DOS codepage PAGE to encode label. By default codepage 850 is used.

-r ROOT-DIR-ENTRIES
Select the minimal number of entries available in the root directory. The default is 112 or 224 for floppies and
512 for hard disks. Note that this is minimal number and it may be increased by mkfs.fat due to alignment of
structures. See also mkfs.fat option -a.

-R NUMBER-OF-RESERVED-SECTORS
Select the minimal number of reserved sectors. With FAT32 format at least 2 reserved sectors are needed, the de‐
fault is 32. Otherwise the default is 1 (only the boot sector). Note that this is minimal number and it may be
increased by mkfs.fat due to alignment of structures. See also mkfs.fat option -a.

-s SECTORS-PER-CLUSTER
Specify the number of disk sectors per cluster. Must be a power of 2, i.e. 1, 2, 4, 8, ... 128.

-S LOGICAL-SECTOR-SIZE
Specify the number of bytes per logical sector. Must be a power of 2 and greater than or equal to 512, i.e. 512,
1024, 2048, 4096, 8192, 16384, or 32768. Values larger than 4096 are not conforming to the FAT file system spec‐
ification and may not work everywhere.

-v Verbose execution.

--offset SECTOR
Write the filesystem at a specific sector into the device file. This is useful for creating a filesystem in a
partitioned disk image without having to set up a loop device.

--variant TYPE
Create a filesystem of variant TYPE. Acceptable values are standard and atari (in any combination of upper/lower
case). See above under DESCRIPTION for the differences.

--help
Display option summary and exit.

--invariant
Use constants for normally randomly generated or time based data such as volume ID and creation time. Multiple
runs of mkfs.fat on the same device create identical results with this option. Its main purpose is testing
mkfs.fat.

BUGS
mkfs.fat can not create boot-able filesystems. This isn't as easy as you might think at first glance for various
reasons and has been discussed a lot already. mkfs.fat simply will not support it ;)

Manual: mkdosfs

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