MS-DOS 6.22 MSBACKUP Utility File Formats

The MS-DOS 6.22 MSBACKUP utility generates backups using a proprietary binary container format licensed from Symantec's Norton Backup. Rather than creating standard file system images or generic compressed archives like ZIP, the utility splits archived data across sequential volume files accompanied by dedicated catalog files. This article explains the internal naming conventions, file extensions, and structural components of the backups generated by the MS-DOS 6.22 MSBACKUP program.

The Underlying Format: Norton Backup Architecture

Microsoft licensed the backup engine for MS-DOS 6.x directly from Symantec. Because of this, MSBACKUP.EXE utilizes the proprietary Norton Backup stream format. Data stored inside these backups is compressed and structured sequentially with proprietary headers, meaning the files cannot be unpacked using standard archive managers without specialized conversion tools or the original software running in an MS-DOS environment.

Backup Data Files (.001, .002, etc.)

The primary backup data is stored across sequentially numbered volume files. These files follow a strict eight-character naming convention that encodes metadata directly into the filename:

• Character 1: The primary drive backed up (e.g., C).
• Character 2: The last drive included in the backup set (e.g., C if only backing up C:, or D if spanning across multiple drives).
• Character 3: The last digit of the year (e.g., 4 for 1994).
• Characters 4–5: The month of the backup (01 through 12).
• Characters 6–7: The day of the month (01 through 31).
• Character 8: An alphabetical sequence letter (A through Z) indicating the backup run of that specific day.

The file extension denotes the volume number in the backup sequence. The first diskette or file chunk receives the .001 extension, followed by .002, .003, and so forth as the backup spans across storage media. For example, the first volume of a full backup of drive C completed on October 15, 1994, would be named CC41015A.001.

Catalog Files (.CAT, .FUL, .INC, .DIF)

Alongside the raw backup volumes, MSBACKUP creates catalog files. The catalog acts as an index containing directory trees, file attributes, compression flags, file sizes, and pointers to the specific sequence volume containing each file.

During the backup process, the catalog is created with an extension that denotes the backup type:

• .FUL — Full backup catalog
• .INC — Incremental backup catalog
• .DIF — Differential backup catalog

Once the backup completes, a master catalog file with the .CAT extension is generated and stored both on the destination media and in the MS-DOS directory on the hard drive. Without the catalog file, MSBACKUP must rebuild the file index manually by reading every individual .001, .002, and subsequent volume file sequentially.