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A number of months ago I tossed the following into the mix, looks like it is time to repost it:<br><br>
The Operators Manual states:<br><br>
"Rom Signature is a four-digit HEXADECIMAL number that is a shorthand representation of the data obtained in an area of ROM memory. The ROM signature is obtained by successively dividing the data in ROM by a binary number (they DON't say what the @!$%#$@% number is! - JR). The resulting signature identifies the data from which it is obtained, and provides a convenient way of" (....blah blah, no other description of the process)."<br><br>
John :-#)#<br><br>
At 12:14 AM 09/05/2002 +0100, Phillip Eaton wrote:<br><br>
<br>
<blockquote type=cite class=cite cite>><br>
> Just for the fun of it I wrote a quick program that tried all<br>
> possible feedback<br>
> combinations for 16 bit CRCs (65536 combinations), none of them<br>
> generated the<br>
> proper signatures, so it's more than just a variation of the standard CRC<br>
> calculations.<br><br>
Ah-ha! BUT!! Did you try for all possible combinations of /starting/ CRC?<br>
Try that 'just for fun' - it should only take 65536 times as long :-)<br><br>
In my old serial comms days, we always started our CRC's at some bizarre<br>
number (3403 IIRC), for two reasons 1) it offered better data security of<br>
our protocols and 2) because it was safer to start there as some random<br>
numbers are better generators than others.<br><br>
If you run a mathematical random number generators over a long period then<br>
you get less random bits that others (i.e. the numbers stay about the same<br>
for a while). This is the same for CRC's and thus the safety of the<br>
calculation in spotting single or double bit errors is dimished in certain<br>
areas.<br><br>
> Anybody know of two different ROMs, or byte streams that result<br>
> in the same<br>
> signature?<br><br>
There's loads in ROMIdent - just run with a bunch of EPROMS from your MAME<br>
directory, and you're bound to find a couple.<br><br>
Cheers,<br>
Phillip Eaton</blockquote></html>
Received on Wed May 08 17:00:39 2002
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