Checksum correction using the math!!!

Hello to all, according to you ? can correct the checksum using only a mathematical calculation?
Let me explain, I took the file to a chroma edc16c39 (chk =89e5), a point on the map I added 1 to the new file I saved with the name "test 01 +1" (chk =89e6), on the same file in another point I removed 1, the new file I saved with the name "test 01 +1 -1" (chk =89e5) at this point, in theory, if riscrivessi the files in the ecu, not me should give no error, right?
If this ? true means that at the end of any editing just make the difference between the two chk and subtract (or add it) in the area of the chk to report the value of the chk as the original.
I keep the tests in the meantime, I'd like to hear your opinion on this topic

Pi? the tests I do more? I do not return to the accounts...someone can? explain to me what you refer to those values that I have marked red

Very interesting, could be very useful in the case you do not have the driver that will automatically calculate the checksum and then ? doable the manual correction. But the area relative to the correction of the chksum how it is identified?

That difficulty? you've found? Changing pi? bit the speech is pi? compplesso?

Exactly, also I still don't understand what are those values that I have highlighted in the picture

there are cks 8-bit and cks 16-bit and 16-bit are not easy to fix by hand , then you need to know c? The rsa and it gets more and more difficult, then there are various areas of cks, doesn't it ? that you can go to correct where you want, I say unto you, that in the end ? almost impossible in the ecu of the latest generation.

p.s. why you want to complicate things for your life when there are the sw that can help you.

Could be useful in case you do not have the driver..***** s, thank you for the info, cos'? the' RSA?

your assumption that with the +1 and -1 in different locations was valid on many units up until 1995, 2000, when they used the eprom or the like.
with the introduction of the flash have adopted different systems and your calculation ? pi? valid.

hi

can you expain it

I'm intressed for it please

i like to learn ;)

Hello all, I am quite ignorant on the subject, maybe you can enlighten me....

I was looking at a file .the gold of a FIAT Coup? (1996) with HxD (hex editor).
By selecting the various blocks of code, I found that the calculated checksum is equal to the one contained in the byte immediately after the block.
From what I've read the ecu, ignition, controls in any way the checksum.
Now, if I go to edit the map, of course, change the checksum; if you wish to leave the checksum intact, I suppose you could change a few bytes.
How do I know which is which ? the part of the map that I can change without changing the behavior of the car?
Thanks in advance.

first of all you need to know which unit it is.
however, you have found the point and the method.
the solutions are 2
1 change, we recalculate the checksum and write in the address that you have found
2 let the checksum inaterato and write something that makes up for it (to go to the accounts) in an area of 00 or FF just before the checksum

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first of all you need to know which unit it is.

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Bosch Motronic (I can't remember the initials) of a FIAT Coup? 20v turbo

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2 let the checksum inaterato and write something that makes up for it (to go to the accounts) in an area of 00 or FF just before the checksum

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Then I confirm that the block of code all oxFF does nothing with regard to the map?

O. T.
It's an old ball that I try to do for years: a double map.
In practice, I install 2 memory chips in place of the EPROM original, with a switch select the EPROM or the other.
The maps both work, only that they have obviously, the checksum is different, in fact if I move from one map to the other with the switch, the machine does not turn on, do I have to staccara and re-attach the control unit (or cos? I suppose I am).

a block of ff in the queue at the maps before the checksum, you can compensate for.
but considering that you can calculate and know where scrivrlo not waste time offset calculations, the new and the write

if you can even do the double eprom you can even have 2 checksum is different, each person counts for his map

who is doing the double map must not disconnect the ecu, just the switch should do it with the engine stopped. ( though not essential)

But the ecu, the ignition does not check the checksum?
In the sense: the attack on the ecu (I select map 1), I put in motion, everything is ok.
I turn off the engine, I select map 2, I put in motion.... should not be.
Ex-control unit (is select it in the map 2), piecing unit, I put in motion, the engine part.
So I assumed you must have the 2 checksum equal....