The BACKUP command in MS-DOS backed up the contents of a hard disk to files of a proprietary, undocumented format which could be returned to their original file structure using the RESTORE command. A number of different formats have been used in different versions of DOS. Files generally had extensions consisting of three digits (incremented consecutively across the files of the backup set), with a control file called BACKUPID.@@@. The aim was to store the entire file and directory structure of a disk onto a set of files that each fit on a floppy disk.
The developers of FreeDOS have attempted to reverse-engineer these formats to produce backup and restore commands in that system that are compatible with Microsoft's.