[Tutor] HD/DVD/CD

[Kent Johnson]

Terry Carroll wrote:
> I'm assuming you want the same serial # that shows up when you do a 
> DIR on the CD, e.g.:
> 
>   >dir d:
>    Volume in drive D is 050512_1752
>    Volume Serial Number is 8A73-780D
> 
> Here's some code:
> 
> 
>>>>import win32api
>>>>CD_Info = win32api.GetVolumeInformation("D:/")
>>>>serno = CD_Info[1]
>>>>serno
> 
> -1972144115
> 
>>>>"%X" % serno
> 
> '-758C87F3'
> 
>>>>"%X" % -serno
> 
> '758C87F3'
> 
> This is as far as I can get.  the hex string, in this case 758C87F3 is the 
> two's complement of the serial number that shows up when I do a "dir d:":
> 
>   >dir d:
>    Volume in drive D is 050512_1752
>    Volume Serial Number is 8A73-780D
> 
> Note that:
>     758C 87F3
>   + 8A73 780D
>     =========
>   1 0000 0000
> 
> Put another way, if you flip every bit in the 758C87F3 string and add one, 
> you'll get the 8A73780D serial no.
> 
> I hope someone more artful than I can show a quick and easy way to convert 
> either '758C87F3' or -1972144115 to the '8A73780D'  that is your goal.

What is happening here is that Python is treating the bit pattern 
'8A73780D' as a signed integer. A common way to represent signed 
integers is with two's complement notation, so Python interprets 
'8A73780D' as the number -1972144115. When you convert this to hex using 
%x formatting, the sign is preserved and you see the representation 
'-758C87F3'.

To get the desired representation, you have to find a way to make Python 
interpret the number as an unsigned integer. One way to do this is to 
convert it to a long integer, then mask off the sign bits:
  >>> i = long(-1972144115) & 0xFFFFFFFF
  >>> i
2322823181L
  >>> '%x' % i
'8a73780d'

As you noted, John Fouhy's number doesn't have the high bit set. So it 
is interpreted as a positive number. Masking the sign bits has no effect 
because they are already zeros:
  >>> i=0x49BC31DB
  >>> i
1237070299
  >>> i = long(i) & 0xFFFFFFFF
  >>> i
1237070299L
  >>> '%x' % i
'49bc31db'

More about two's complement notation here:
http://en.wikipedia.org/wiki/Two%27s_complement

Kent