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  DOS version 4 also records a disk's volume label and volume serial number
  in a data structure that immediately follows the BPB in the boot sector.
  (See Figure D-8.) The volume label is the same 11-byte name that appears
  in a disk's volume-label entry in the root directory. You specify the
  volume label with the FORMAT or LABEL command, or with a call to interrupt
  21H,
  function 16H. The volume serial number, however, is computed by DOS
  itself.

  Offset in      Length         Description
  Boot Sector    (bytes)
  --------------------------------------------------------------------------
  24H            1              Physical drive number
  25H            1              (Reserved)
  26H            1              Signature byte (29H)
  27H            4              Volume serial number
  2BH            11             Volume label
  36H            8              (Reserved)
  --------------------------------------------------------------------------

  Figure D-8.  Boot sector extensions in DOS 4.0.

  When it formats a disk, DOS version 4 uses the current date and time to
  compute the disk's volume serial number. This means that volume serial
  numbers are almost always different among different disk volumes, so DOS
  version 4 can use the serial numbers to distinguish different disks with
  the same physical format. You can illustrate this in DOS version 4 by
  installing SHARE, opening a diskette file for input, and then changing
  diskettes. If you subsequently attempt to read from the file, DOS version
  4 generates an "invalid disk change" critical error. In a program, you can
  use your own critical-error handler to detect this error. (See Chapter
  15.) It's easier, of course, to rely on the DOS version 4 default
  critical-error handler, which displays the volume label and serial number
  of the diskette that contains the open file.