Date: Mon, 31 Oct 94 00:17:30 EST
From: cigna AT helios1 DOT phy DOT ohiou DOT edu (Mr. Salty)
To: djgpp AT sun DOT soe DOT clarkson DOT edu
Subject: volume serial number and time stamp

>> >                                                          Also, I
>> > think there are some time stamp in the boot record.  Semi-expensive
>> 
>> AFAIK, there is no time stamp in the boot record, but if you know
>> otherwise, please tell me where in the boot record it dwells.
>
>I've heard that the serial number itself is an encoding of the
>time the disk was formatted.

I've heard that too, but I couldn't find any info on it so I
disassembled format.com from dos 6.1 and found this little procedure:

sub_144   proc  near
    mov ah,2Ah
    int 21h             ; DOS Service  get date
                        ;  cx=year, dh=mon, dl=day
    push  cx
    push  dx
    mov ah,2Ch
    int 21h             ; DOS Service  get time
                        ;  ch=hour, cl=min, dh=sec, dl=hundredths of sec
    mov ax,dx
    pop dx
    add ax,dx
    mov data_780,ax
    mov data_781,ax
    mov ax,cx
    pop cx
    add ax,cx
    mov data_782,ax
    mov data_783,ax
    ret
sub_144   endp

It essentially adds the 32 bit date and 32 bit time ignoring overflow.
Presumably, data_780, etc are writtem to disk as the volume serial #.
Unfortunately, I don't think it's possible to extract 64 bits of info
from the 32 that we're given :) Besides, other formating programs might
use a different algorithm. All that seems to be advertised about the
serial number is that it's "random".

- Dave