From time to time I have seen messages on various bulletin boards complaining
about and questioning the volume serial numbers used in DOS for versions 4 and
5.  Here's the full scoop on diskette serial numbers:

The volume serial number was introduced with DOS 4.0 as part of an extended
boot record and is created through either FORMAT or DISKCOPY.  The serial
number is a function of the time/date of formatting.

The first part of the serial number is equal to the sum of the time (seconds
and hundredths of a second) and the date (month and day);   The second part of
the serial number is equal to the sum of the time (hours and minutes) and date
(year), where all numbers are in hex.  For example, a diskette formatted at
8:32:43.65 on 7/21/1991, will contain a serial number of 3256:0FE7, derived as
follows:

the first part of the serial number will be
  Time    43.65    2B41  (43 decimal = 2B hex; 65 decimal = 41 hex)
+ Date     7/21    0715  ( 7 decimal = 07 hex; 21 decimal = 15 hex)
                   3256

the second part of the serial number will be
  Time     8:32    0820  ( 8 decimal = 08 hex; 32 decimal = 20 hex)
+ Date     1991    07C7  ( 1991 decimal = 07C7 hex)
                   0FE7

There is nothing unique about the serial number.  It is possible to have
different diskettes, which were formatted at wildly different times contain
the same serial number.  However the probability of such an occurrence is very
small.

The following DOS INT 21h functions may be used to set or determine the serial
number (taken from Ralf Brown's INTERRUPT listing):

INT 21 - DOS 3.2+ - IOCTL - GENERIC BLOCK DEVICE REQUEST
        AX = 440Dh
        BL = drive number (00h=default,01h=A:,etc)
        CX = 0846h      - set volume serial number
        CX = 0866h      - get volume serial number

DS:DX
Offset  Size    Description
 00h    WORD    info level (00h)
 02h    DWORD   disk serial number (binary)
 06h 11 BYTEs   volume label or "NO NAME    "
 11h  8 BYTEs   file system type "FAT12   " or "FAT16   " (CL=66h only)

Return: CF set on error            AX = error code
        CF clear if successful

-------------------------------------------------------------------------
INT 21 - DOS 4.0 internal - GET/SET DISK SERIAL NUMBER
        AX = 6900h      - get serial number
        AX = 6901h      - set serial number
        BL = drive (0=default, 1=A, 2=B, etc)

DS:DX
Offset  Size    Description
 00h    WORD    info level (zero)
 02h    DWORD   disk serial number (binary)
 06h 11 BYTEs   volume label or "NO NAME    " if none present
 11h  8 BYTEs   (AL=00h only) filesystem type--string "FAT12" or "FAT16"

Return: CF set on error            AX = error code
        CF clear if successful
-------------------------------------------------------------------------
NOTE -- DON'T TRY THESE PATCHES UNLESS YOU HAVE EXPERIENCE WITH DEBUG AND
        KNOW EXACTLY WHAT YOU'RE DOING.

Patching FORMAT.COM (DOS 5.0) to create a serial number of 0000-0000 on
your diskette requires changing bytes at
DEBUG offset 4B91 from 03 C2 (ADD AX,DX) to 31 C0 (XOR AX,AX) and at
DEBUG offset 4B9C from 03 C1 (ADD AX,CX) to 31 C0 (XOR AX,AX).

The same patch can be applied to DISKCOPY.COM (DOS 5.0) using DEBUG byte
addresses at 0B2C and 0B36.

Byte 26h of the diskette boot record determines whether or not the volume
serial number will be displayed by the DOS DIR command.  A value of 29h in
this byte will display the volume serial number; any other value causes DIR to
ignore the volume serial number (my diskettes contain the value 2Ah).

If you want to eliminate the serial number from ever being displayed by the
DOS DIR command (regardless of byte 26h in the diskette boot record), and you
really want to live dangerously, you can patch the DOS 5.0 COMMAND.COM file by
replacing the byte at DEBUG offset 40A0 (currently 72h) to EBh.  This will
change the instruction from JB 40AD to JMP 40AD.

                                                        Les Moskowitz
                                                        11/23/91