Okay, a checksum on a single byte of FF yeilds an FF I found this to be interesting...... Byte 0 = F3 Sig = FE Byte 1 = 3E Sig = 1EE8 Byte 2 = 00 Sig = 1EE8 Byte 3 = ED Sig = 225A Byte 4 = 47 Sig = 56B6 It isn't using the last read byte as the divisor is it? Kev

"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)."

John :-#)#

At 12:14 AM 09/05/2002 +0100, Phillip Eaton wrote:

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Just for the fun of it I wrote a quick program that tried all possible feedback combinations for 16 bit CRCs (65536 combinations), none of them generated the proper signatures, so it's more than just a variation of the standard

CRC

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calculations.

Ah-ha! BUT!! Did you try for all possible combinations of /starting/ CRC? Try that 'just for fun' - it should only take 65536 times as long :-)

In my old serial comms days, we always started our CRC's at some bizarre number (3403 IIRC), for two reasons 1) it offered better data security of our protocols and 2) because it was safer to start there as some random numbers are better generators than others.

If you run a mathematical random number generators over a long period then you get less random bits that others (i.e. the numbers stay about the same for a while). This is the same for CRC's and thus the safety of the calculation in spotting single or double bit errors is dimished in certain areas.

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Anybody know of two different ROMs, or byte streams that result in the same signature?

There's loads in ROMIdent - just run with a bunch of EPROMS from your MAME directory, and you're bound to find a couple.

Cheers, Phillip Eaton