BACKUP (MS-DOS)

Latest revision as of 12:44, 23 August 2025

Some versions of MS-DOS have a BACKUP command, whose main use is to back up some or all of the contents of a hard disk to a set of floppy disks. The RESTORE command is used to restore files. The backup format was undocumented.

There are at least two quite different formats:

• The format used by MS-DOS 2.0(?) through 3.2. We'll call it 2.0 format.
• The format used by MS-DOS 3.3 through 5.x. We'll call it 3.3 format.

For the format used by MS-DOS 6.x's MSBACKUP.EXE utility, see Norton Backup.

[edit] Format details

[edit] Format details - 2.0

Backed up files usually keep their original filenames. The data is modified, at least to add a header. In case of a duplicate filename (because the same filename appeared in different directories), a replacement extension like .@01 or .@02 is evidently used. A file may be split into multiple backup files, each containing a fragment of the original data.

An extra BACKUPID.@@@ file is written to each disk, containing general information about the backup. It doesn't contain any critical information.

[edit] Format details - 3.3

Just two files are written to each floppy disk: CONTROL.001 (or .002, etc.), and BACKUP.001 (.002, etc.). The CONTROL file contains information about the file data in the corresponding BACKUP file.

[edit] Identification

[edit] Identification - 2.0

BACKUPID.@@@: The first byte of the file is either 0xff or 0x00. The file is exactly 128 bytes in size. (TODO: More info)

Data files: It's possible to heuristically identify data files from their contents, but they don't really have any distinctive markings. The first byte of a file is either 0xff or 0x00. Non-fragmented archives start with bytes ff 01 00. (TODO: More info)

[edit] Identification - 3.3

A CONTROL.* file starts with byte 0x8b, followed by ASCII "BACKUP  " (which ends with two spaces).

The BACKUP.* files can't be identified from their contents, as they contain nothing other than the contents of the backed up files.

[edit] Specifications

• Tech docs from FreeDOS
• One could presumably learn about at least one of the formats from the MS-DOS 4.0 source code, listed below.

[edit] Software

• MS-DOS, at WinWorld
• MS-DOS 4.0 source code: BACKUP, RESTORE
• Deark

[edit] Sample files

2.0 format:

• dexvert samples — archive/dosBackupFile
• [1] → *.ddi (DiskDupe format) → ...
  • Examples of split files are LNALLMS.BAT from parts 1 and 2, and BTPARSE.EXE from parts 2 and 3.

3.3 format:

• {VOL583.ZIP, VOL584.ZIP} → ...
• [2] → *.imz → ...

[edit] Links

• Backup command help