In computing, format, a command-line utility included in 86-DOS, MS-DOS, IBM PC DOS and OS/2 and Microsoft Windows operating systems, carries out disk formatting. The command is also available in the DEC RT-11 operating system.
The command performs the following actions by default on a floppy disk, hard disk drive, solid state (USB), or other magnetic medium (it will not perform these actions on optical media):
clearing the FAT entries by changing them to 0x00
clearing the FAT root directory by changing any values found to 0x00
checking each cluster to see if it is good or bad and marking it as good or bad in the FAT
Optionally (by adding the /S, for "system" switch), Format can also install a Volume Boot Record. With this option, Format writes bootstrap code to the first sector of the volume (and possibly elsewhere as well). Format always writes a BIOS Parameter Block to the first sector, with or without the /S option.
Another option (/Q) allows for what Microsoft calls "Quick Format". With this option the command will not perform steps 2 and 3 above. Format /Q does not alter data previously written to the media.
Typing "Format" with no parameters in MS-DOS 3.2 or earlier would automatically, without prompting the user, format the current drive; however in MS-DOS 3.3 and later it would simply produce the error: "required parameter missing".
Any storage device must have its medium structured to be useful. This process is referred to as "creating a filesystem" in Unix, Linux, or BSD. Under these systems the command "mkfs" exists. It creates many kinds of file systems, including those used by DOS, Windows, and OS/2.