MS-DOS 6.22 Volume Label Changes on Logical Drives

In MS-DOS 6.22, changing the volume label of an active logical drive is handled via the external LABEL utility, which modifies specific metadata structures within the FAT12 or FAT16 file systems without requiring an unmount or system reboot. The operating system executes these modifications by searching and updating a special directory entry with a volume attribute in the root directory while concurrently updating the volume label field within the partition's Extended BIOS Parameter Block (EBPB) in the boot sector. Because MS-DOS 6.22 operates as a single-tasking operating system, it writes these changes directly to disk and flushes associated disk caches to ensure volume integrity.

The Role of the LABEL Command

MS-DOS 6.22 does not allow native internal commands (such as DIR or VOL) to alter a volume label; it relies entirely on the external utility LABEL.EXE. When invoked using the syntax LABEL [drive:][new_label], the utility first verifies whether the target logical drive is a valid, writable local volume. If no label is specified in the command arguments, MS-DOS displays the current label, serial number, and prompts the user to enter an 11-character string or delete the existing one.

Dual-Location Metadata Updates

When modifying the label on an active FAT partition, MS-DOS 6.22 updates two separate locations to maintain backward and forward compatibility:

  1. The Root Directory Entry:
    The primary storage location for the volume label is an entry in the partition’s root directory table. MS-DOS searches the root directory for an entry where the file attribute byte is set to 0x08 (ATTR_VOLUME_ID).

    • If a volume entry already exists, MS-DOS updates the 11-byte name field with the new user-defined string.
    • If the user chooses to delete the label, the first byte of this entry is marked with 0xE5 to signify a deleted entry.
    • If no volume entry exists, MS-DOS allocates the first available entry in the root directory, writes the label string, and sets the attribute flag to 0x08.

  2. The Extended BIOS Parameter Block (EBPB):
    The boot sector (Logical Sector 0 of the partition) contains an EBPB structure introduced with DOS 4.0. At offset 0x2B, an 11-byte field stores the volume label string. When LABEL.EXE executes, MS-DOS issues low-level absolute disk read and write interrupts (typically INT 25h and INT 26h or DOS generic IOCTL INT 21h, Function 440Dh) to read the boot sector into memory, alter the 11 bytes at offset 0x2B to mirror the new root directory entry, and rewrite the boot sector to disk.

Single-Tasking State and Partition Activity

Because MS-DOS 6.22 is a single-tasking, non-preemptive environment, an "active" logical drive merely indicates that the partition is mapped to a drive letter and available for file input/output operations. There is no active filesystem locking mechanism by default unless SHARE.EXE is loaded.

  • Drive Access: Changing a volume label on an active drive does not disrupt active working directories. A user can reside inside a subdirectory on D:\ (for example, D:\GAMES) and execute LABEL D: MYDRIVE without causing errors or changing the current working directory path.
  • Cache Management: If the SMARTDrive disk-caching utility (SMARTDRV.EXE) is running, MS-DOS ensures that the modified root directory sectors and boot sector are marked dirty and committed directly to the physical storage device, preventing discrepancies between cache and disk.

Restrictions and Error Handling

MS-DOS 6.22 applies specific operational checks before committing volume label changes on logical partitions:

  • Character Restrictions: Labels are limited to 11 characters conforming to the FAT 8.3 character set rules (spaces are permitted, but control characters and symbols like *, ?, /, \, |, and . are rejected).
  • Write Protection: If the underlying physical media is write-protected or marked read-only, MS-DOS aborts the operation and returns a write-fault error.
  • Network and Substituted Drives: The LABEL command refuses to modify active drives created via SUBST or network shares redirected using the Microsoft Network Client, returning an error that the operation is unsupported on the specified drive.