FROM: Les Moskowitz                 Nov. 23, 1991  8:04 A.M

Here's the full scoop on diskette serial numbers:
 
The volume serial 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, if the diskette is formatted at 8:32:43.65 on 7/21/1991, the
first part of the serial number will be 2B41+0715, or 3256;  the second
part of the serial number will be 0820+07C7, or 0FE7.
 
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   filesystem 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
-------------------------------------------------------------------------
 
Patching FORMAT.COM (DOS 5.0) to eliminate the serial number from 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.
 
Finally, 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).