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newfs_msdos - construct a new MS-DOS (FAT) file system (newfs_msdos.rta)
INtime SDK v7.1 > INtime Utilities > DRTOS Utilities > newfs_msdos - construct a new MS-DOS (FAT) file system (newfs_msdos.rta)

NEWFS_MSDOS(8)   BSD System Manager's Manual (INtime Distributed RTOS only).


newfs_msdos | fmtfat  -- construct a new MS-DOS (FAT) file system

Command line

newfs_msdos | fmtfat [-N] [-@ offset] [-B boot] [-C create-size] [-F FAT-type] [-I VolumeId]
[-L label] [-O OEM] [-S sector-size] [-a FAT-size] [-b block-size] [-c cluster-size]
[-e DirEnts] [-f format] [-h heads] [-i info] [-k backup] [-m media] [-n FATs]
[-o hidden] [-r reserved] [-s total] [-u track-size] special [disktype]


The newfs_msdos utility creates a FAT12, FAT16, or FAT32 file system on device or file named special, using disktab(5) entry disktype to determine geometry, if required.

If special does not contain a / and -C is not used, it is assumed to be a device name and /dev/ is prepended to the name to construct the actual device name. To work a file in the current directory use ./filename

The fmtfat utility is the same as newfs_msdos with an 8.3 filename. 


Do not create a file system: just print out parameters
-@ offset
Build the filesystem at the specified offset in bytes in the device or file. A suffix s, k, m, g (lower or upper case) appended to the offset specifies that the number is in sectors, kilobytes, megabytes or gigabytes, respectively.
-B boot
Get bootstrap from file.
-C create-size
Create the image file with the specified size. A suffix character appended to the size is interpreted as for the -@ option. The file is created by truncating any existing file with the same name and resizing it to the requested size. If the file system supports sparse files, the space occupied on disk may be smaller than the size specified as parameter.
-F FAT-type
FAT type (one of 12, 16, or 32).
-I VolumeID
Volume ID, a 32 bit number in decimal or hexadecimal (0x...) format.
-L label
Volume label (up to 11 characters). The label should consist of only those characters permitted in regular DOS (8+3) filenames.
OEM string (up to 8 characters). The default is "BSD4.4 ".
-S sector-size
Number of bytes per sector. Acceptable values are powers of 2 in the range 512 through 32768, inclusive.
-a FAT-size
Number of sectors per FAT.
-b block-size
File system block size (bytes per cluster). This should resolve to an acceptable number of sectors per cluster (see below).
-c cluster-size
Sectors per cluster. Acceptable values are powers of 2 in the range 1 through 128. If the block or cluster size are not specified, the code uses a cluster between 512 bytes and 32K depending on the filesystem size.
-e DirEnts
Number of root directory entries (FAT12 and FAT16 only).
-f format
Specify a standard (floppy disk) format. The standard formats are (capacities in kilobytes): 160, 180, 320, 360, 640, 720, 1200, 1232, 1440, 2880.
-h heads
Number of drive heads.
-i info
Location of the file system info sector (FAT32 only). A value of 0xffff signifies no info sector.
-k backup
Location of the backup boot sector (FAT32 only). A value of 0xffff signifies no backup sector.
-m media
Media descriptor (acceptable range 0xf0 to 0xff)..
-n FATs
Number of FATs. Acceptable values are 1 to 16 inclusive. The default is 2.
-o hidden
Number of hidden sectors.
-r reserved
Number of reserved sectors.
-s total
File system size.
-u track-size
Number of sectors per track.


If some parameters (e.g. size, number of sectors, etc.) are not specified through options or disktype, the program tries to generate them automatically. In particular, the size is determined as the device or file size minus the offset specified with the -@ option. When the geometry is not available, it is assumed to be 63 sectors, 255 heads. The size is then rounded to become a multiple of the track size and avoid complaints by some filesystem code.

FAT file system parameters occupy a "Boot Sector BPB (BIOS Parameter Block)" in the first of the "reserved" sectors which precede the actual file system. For reference purposes, this structure is presented below.

struct bsbpb {
uint16_t bpbBytesPerSec; /* [-S] bytes per sector */
uint8_t bpbSecPerClust; /* [-c] sectors per cluster */
uint16_t bpbResSectors; /* [-r] reserved sectors */
uint8_t bpbFATs; /* [-n] number of FATs */
uint16_t bpbRootDirEnts; /* [-e] root directory entries */
uint16_t bpbSectors; /* [-s] total sectors */
uint8_t bpbMedia; /* [-m] media descriptor */
uint16_t bpbFATsecs; /* [-a] sectors per FAT */
uint16_t bpbSecPerTrack; /* [-u] sectors per track */
uint16_t bpbHeads; /* [-h] drive heads */
uint32_t bpbHiddenSecs; /* [-o] hidden sectors */
uint32_t bpbHugeSectors; /* [-s] big total sectors */
/* FAT32 extensions */
struct bsxbpb {
uint32_t bpbBigFATsecs; /* [-a] big sectors per FAT */
uint16_t bpbExtFlags; /* control flags */
uint16_t bpbFSVers; /* file system version */
uint32_t bpbRootClust; /* root directory start cluster */
uint16_t bpbFSInfo; /* [-i] file system info sector */
uint16_t bpbBackup; /* [-k] backup boot sector */


The maximum file size is 4GB, even if the file system itself is bigger.

Exit Status

Exit status is 0 on success and 1 on error.


Create a file system, using default parameters, on /dev/ada0s1:

newfs_msdos /dev/ada0s1

Create a standard 1.44M file system, with volume label foo, on /dev/fd0:

newfs_msdos -f 1440 -L foo fd0

Create a 30MB image file, with the FAT partition starting 63 sectors within the image file:

newfs_msdos -C 30M -@63s ./somefile


The newfs_msdos utility first appeared in FreeBSD 3.0.


Robert Nordier <>

See Also