man > mkfs.msdos(8)

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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 option -C is given). BLOCK-COUNT is the number of blocks on the de‐
vice 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 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 filesystem. This option disables align‐
ment; 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, oth‐
erwise 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 filesys‐
tem 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 create 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 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 sector. Number SECTORS-PER-TRACK is used also
for aligning the total count of FAT sectors. By default disk geometry is read from DE‐
VICE 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. Nor‐
mally 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 par‐
tition), a quirk in the MS-DOS implementation of FAT requires it to be relative to the
partition's immediate containing Extended Boot Record. Boot code and other software han‐
dling FAT volumes may also rely on this field being set up correctly; most modern FAT im‐
plementations 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 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 prop‐
erly.

-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 emulation.

--mbr[=y|yes|n|no|a|auto]
Fill (fake) MBR table with disk signature one partition which starts at sector 0 (in‐
cludes 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. De‐
fault is auto mode in which mkfs.fat put MBR table only for non-removable disks when for‐
matting whole unpartitioned disk.

-n VOLUME-NAME
Sets the volume name (label) of the filesystem. The volume name can be up to 11 charac‐
ters 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 de‐
fault 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 default is 32. Otherwise the default is 1 (only the boot sec‐
tor). Note that this is minimal number and it may be increased by mkfs.fat due to align‐
ment 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 cre‐
ating 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 ;)