Microsoft KB Archive/39893

Volume Serial Numbers in MS-DOS Version 4.00 ID Number: Q39893

4.00 4.01 MS-DOS

Summary:

MS-DOS Version 4.00 introduced volume serial numbers to allow programs to determine if disk switches have occurred. This serial number is assigned when a disk is formatted and is based on the time and date of the format. Using this method, no two disks from the same machine should ever have the same serial number unless the date and time are manually set to be the same for the two disks.

The only way for a program to obtain the serial number of an MS-DOS Version 4.00 volume is to read the boot sector of the volume and get the serial number from the information read in. The serial number is stored as a double word. For more information regarding the structure of the boot record, see below.

More Information:

The structure of the MS-DOS Version 4.00 boot record is as follows:

To get the serial number, a program needs to set up a buffer of 512 bytes and read in one sector from the disk using interrupt 25H, Absolute Disk Read.