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 corresponding to the device (e.g.
/dev/sdXX) or the image file (which does not need to exist when the op-
tion -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 number 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 al-
ways 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 gener-
ated, 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 other-
wise. 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 filesystem. This option disables alignment; this
may provide a handful of additional clusters of storage at the ex-
pense of a significant performance degradation on RAIDs, flash me-
dia 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 sec-
tor, usually at sector 7. The backup must be within the range of
reserved 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 us-
ing dd in advance to create a file of appropriate size. With this
option, the BLOCK-COUNT must be given, because otherwise the in-
tended 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 cor-
rect 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 partitions 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 automatically 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 sec-
tor. 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
translation 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 beginning 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 partition's immediate con-
taining Extended Boot Record. Boot code and other software han-
dling FAT volumes may also rely on this field being set up cor-
rectly; most modern FAT implementations will ignore it. By de-
fault, if the DEVICE is a partition block device, mkfs.fat uses the
partition offset relative to disk start. Otherwise, mkfs.fat as-
sumes 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 parti-
tions 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 operating 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 emu-
lation.
--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 de-
vice. 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 direc-
tory. 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 default is 32. Other-
wise 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 specification 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 im-
age without having to set up a loop device.
--variant TYPE
Create a filesystem of variant TYPE. Acceptable values are stan-
dard 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 dis-
cussed a lot already. mkfs.fat simply will not support it ;)
SEE ALSO
fatlabel(8), fsck.fat(8)
HOMEPAGE
The home for the dosfstools project is its GitHub project page
⟨https://github.com/dosfstools/dosfstools⟩.
AUTHORS
dosfstools were written by Werner Almesberger ⟨werner.almesberger@
lrc.di.epfl.ch⟩, Roman Hodek ⟨Roman.Hodek@informatik.uni-erlangen.de⟩,
and others. Current maintainers are Andreas Bombe ⟨aeb@debian.org⟩ and
Pali Rohár ⟨pali.rohar@gmail.com⟩.
dosfstools 4.2 2021-01-31 MKFS.FAT(8)
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