DISK VOL-SERIAL NUMBER

7/13/2002

Disk Volume-Serial Number: The original IBM PC included a user provided disk ID on each disk. While useful for identifying disks, this 11 character label is not very satisfactory for many computer purposes. It is not required to be unique. It may well be blank if the user has never set it. It may be changed at any time.

Without a unique ID, inopportune changes of removable disks may not be detected. Many software vendors would like a unique ID that can be used to discourage piracy. At a less analytic level, the Macintosh used a unique disk ID and IBM-Microsoft didn't want to be the only company on the block without one.

In order to provide a unique disk ID, a second, machine generated, ID called a Volume-Serial Number was added to disks. This was done in MSDOS4.0 in 1987 during a short troubled marriage between IBM and Microsoft. The Vol-Ser Number is a 32 bit number generated from the date/time and presented as two four digit Hexadecimal characters e.g. ABAD-1DEA. In the IBM-Microsoft vision, disk copy programs are supposed to ensure that the Volume-Serial number is not replicated. A few programs -- notably DISKCOPY -- actually do assign a new Vol-Ser Number to copies. Many do not.

The Volume Serial number is physically stored in the boot record of Microsoft partitions. It is at byte 27 of FAT12 and FAT16 drives stored in reverse byte order from the presented value. (The disk label follows it). It is at byte 43 of FAT32 boot records. I haven't been able to determine the handling for NTFS where the label can be up to 32 characters, is case sensitive, and allows characters like ? that are illegal in FAT labels. Neither do I know the format for HPFS. Some published methods of altering the Volume-Serial Number are OS specific. They do not always document their limitations.

For those adept at programming, look up INT21 AX=6900 and INT21 AX=6901. These calls also read and set the Volume label

The Volume-Serial number is decidedly a mixed blessing. It is used by some software vendors as a form of copy protection -- the software is "branded" to a specific vol-ser number. This of course causes difficulties when a hard drive must be replaced. In addition, some disks have a vol-ser number of 0000-0000. This is not supposed to happen, but it does and frequently causes problems when encountered by software that is sensitive to the serial number. A more subtle problem is that that the Volume Serial number is a partition identification. It is only a disk identification when the disk has only a single partition.

Return To Index Copyright 1994-2002 by Donald Kenney.