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  1. #11
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    Original FW have checksum FE, but when i changed to version 3.11 then angelo have 0 or 65535 instedad of version 3.11 also checksume changed.
    I try and try but can't figure out how to calculate checksum to be after modifie again FE.

    Maybe kunix, Giomen or some others know how to calculate checksum to be again FE, but then maybe GTM wont update older FW anymore.

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  3. #12
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    All original FW that distributed as GCD have checksum FE. Actually checksum for GCD is 0 if you exclude last 4 bytes: "FF FF 00 00"

    For example, for 006B090100.GCD with version 3.10 we have "67 FF FF 00 00". "67" is checksum sign. Let's start the block from begining and finish on "67" with excluding last 4 bites "FF FF 00 00" - if we calculate the checksum it will be zero (0). But for whole files we will have the checksum equal "FE" due to adding "FF FF 00 00" at the end of files ("FE" is checksum for this 4 bytes).

    So you need to change byte that is the fifth from the end! It is offset x9067 for 006B090100.GCD with version 3.10.
    If you check for whole file - you need to adjust the value for obtain the checksum is equal "FE". If not and you select only block without last 4 bytes of file - you need to adjust the value for obtain the checksum of this block is equal "00".

    P.S. I am not completely honest! The 006B090100.GCD have more than the one section like all of GCD. All section have the checksum is equal "00". But if you change only main section for version number so you do not need correct checksum for header (it is equal "00").
    P.S.1 Section in GCD is ending usually with "01 00 01 00 XX" where "XX" is checksum sign. For some of GCD the RGNTool is suitable. For other you need find section yourself. For example, for 006B090100.GCD with version 3.10 we have header section on x00-x1004 and FW section on x1005-x9067 and "FF FF 00 00" at the end on x9068-x906B.
    P.S.2 I think what Kunix give more professional explanation. I make only reverse engineering for myself.
    Last edited by Giomen; 26th March 2013 at 23:35.
    Love your wife? Buy yourself a GPS for answering to her favorite question: "Dear, where are you?".

  4. #13
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    It is my script for WINHEX that allow to change checksum for block to 0. You must have already selected block and get focus of cursor on checksum sign (address for change) before running script:

    http://www2.zippyshare.com/v/75964282/file.html
    Love your wife? Buy yourself a GPS for answering to her favorite question: "Dear, where are you?".

  5. #14
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    I know that i must change byte that is the fifth from the end, becouse i compare to older FW and this byte change.
    I try with different number, but no luck.

  6. #15
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    3.1 also not good for me, then just try to find old backup and make a modification at x1027 to 3701 and it's work.
    Still need to take out this modified file and put back original backup else it will update the GTM receiver every power up.

    Was not aware on checksum but it's back working condition.

    Now realized my backup is 3.00 not 2.90 and this is not in update list.
    Last edited by bpb; 28th March 2013 at 04:40. Reason: typo

  7. #16
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    @bpb
    Can you share your file, please?

    Also Angelo use FW v3.00 from nuvi 3xxx for working condition
    This FW was on new nuvi, but never on garmin server.
    Last edited by lolypop000; 27th March 2013 at 11:34.

  8. #17
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    here you are

    [Only registered and activated users can see links. ]
    Attached Files Attached Files

  9. #18
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    Quote Originally Posted by bpb View Post
    3.1 also not good for me, then just try to find old backup and make a modification at x1027 to 3701 and it's work.
    Still need to take out this modified file and put back original backup else it will update the GTM receiver every power up.

    Was not aware on checksum but it's back working condition.

    Now realized my backup is 3.00 not 2.90 and this is not in update list.
    @bpb, as the Loly mentioned, we arrived at the same result, with the difference that we had more work, because we tried to change not one, but several indicative positions of the firmware version and struggling to hit the checkusum, but with the same result of yours, as I mentioned earlier in the post 10, and with a lot of instability of modified version (looping in installation) and subsequent stability and operation of traffic with version 3.00. Although we are unable to resolve the issue of the checksum, it was good to know that gives the same change 1 or more positions in the firmware, because without the checkum, even unstable correction allow the installation of a stable version (checksum FE).
    As in your case, my traffic went back to work with firmware v3.00, but only until I install another subscription, and there began my problems. With firmware 2.80 and 2.90 beginning the installation of the signature, but at the end the gps gives warning that exceeded the maximum number of subscriptions allowed. Already the v3.00 fw supports the installation of new subscriptions, showing them in the firmware, but when to turn off the gps or GTM cable, it disappears. So, I assume that it was in the volatile memory of GTM or GPS cable and disappears when reboot. This should be a specific problem of my cable GTM25, becouse I've done countless tests of signatures installation including the Jokers of Europe, Australia, etc.

    So my big problem now is delete the subscriptions, remembering that apply a key with xxxx, y, y, 0 only hidden and disables the subscription, prevents the operation of Traffic and not of fact erase it. If anyone has any bright idea, please tell me. I am also very interested in having older versions of gtm25 firmware (G3 ID 901). If anyone has and can post, I am thankful. I have the original versions of firmware 2.80 2.90 3.00, which, I'm posting attached to anyone interested. It is useful to remember the friends who want to downgrade from version 3.10: recommend change, for example, the 2.80 version, unstable, that even allows to upgrade to all higher version (2.90, 3.00, 3.10,). Other information version of parts of firmware that has not changed, continue to be read by the system. So, it is not possible to downgrade to version 3.00 modifying it to 3.11 (unstable), because when installing the v 3.00 original, nothing will happen because the other positions of the fw from the previous version will be read as 3.00, ie, equal and therefore will not install. So, best to use the first version of older installation, i.e., the modified 2.80 to 3.11, to then install the original 3.00.
    Attached Files Attached Files
    Last edited by angelozip; 30th March 2013 at 00:50.
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  10. #19
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    I'm using the GTM 35 then may not be same.

    Anyway the subscription I feel it's must be a clue inside the GTM firmware if we could gt it our or clear with a command,.

  11. #20
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    Quote Originally Posted by angelozip View Post
    it was good to know that gives the same change 1 or more positions in the firmware, because without the checkum, even unstable correction allow the installation of a stable version (checksum FE).
    Checksums is needed for assembling of software. Adrenalin and other often said what last software not check the checksum at the all. We have many examples with modified software but nobody correct checksum. Logo is it. No problem!
    What about changes in SW. If you compare a gcd with minor changes you find more differences then you would expected. In any SW we have both a program code and references in addressing. If you change code for code with equivalent length it is easy. We do this trick with logo name or text other text contest. But if we will increase length of data and by that reason shift the other data we will need renew all addressing scheme of file!
    Love your wife? Buy yourself a GPS for answering to her favorite question: "Dear, where are you?".

 

 
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