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Driveline, Engine & DME Tuning
Talk about driveline improvements, NA tuning and DME tuning your E46 BMW here. This includes diffs, intakes, exhausts, chips, software and OBD tuning.

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Old 12-10-2012, 04:09 PM   #1
TerraPhantm
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Join Date: Feb 2008
Location: Mountain Top
Posts: 5,977
My Ride: 2005 M3 Coupe
MSS54 Checksum Calculation

I and a few other members at M3Forum have been researching the MSS54/HP and solving the checksum algorithm is part of that effort.

We recently solved the MSS54HP algorithm. It's actually quite simple. The MSS54HP ROM is a 64k file that consists of two 32k segments destined for separate EEPROMs. For simplicity sake, we'll consider these two 32k segments as entirely separate files. Checksum algorithm is the same for both halves.

What you do is take the first 0x4000 bytes and move them to the end (so what used to be byte 0 is now byte 4000, etc...).
Then select the first 7FFC bytes and calculate the CRC16 for those values (using the common polynomial - 8005). The result of this calculation is the checksum.
Rearrange the file back to the original order, and place the checksum at offset 3FFC.

Now the MSS54 non-HP ECUs are giving me a bit of trouble. These are 32k ROMs which can be treated two separate 16k ROMs.
In either one of these 16k pieces, calculate the CRC16 for the first 3FFC bytes.
If you compare it to a known checksum, you'll notice that it's not the same. Instead, I have found that the difference between the CRC16 and the actual checksum is always either C00 or 1400

I have found that if the first digit of the CRC16 is odd, you subtract C00 or 1400 from the checksum. If it's even, you add the value.

Problem is, I can't figure out a rhyme or a reason as to why sometimes the shift is C00 and why it's sometimes 1400. When I correct checksum via WinOLS, some changes cause the correction to be C00, others cause an addition/subtraction of 1400. So presumably there is a rule that determines the change that I am not able to see.
There may also be additions/subtractions of different values, though I have not come across any such examples yet (and I've looked at a lot of files)

Does anyone have any experience with these kind of things? Any hints you can give me?
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Last edited by TerraPhantm; 12-10-2012 at 06:05 PM.
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Old 12-11-2012, 10:50 AM   #2
smooth1
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Read the thread. PM'd.
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