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Does a custom ROM like CyanogenMod come with their own bootloader or does it just overwrite the /boot kernel and /system? Does the bootloader take responsibility to lock your SIM card on a carrier?

Basically, I'm interested in SIM unlocking my Huawei U8180, and would like to know if CyanogenMod will unlock the device.

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No, CyanogenMOD and other custom ROMs rely on your device's bootloader. In general you don't want to mess with the bootloader since doing so is an easy way to brick your device.

The bootloader doesn't really have anything to do with the SIM lock usually. I don't know of an Android-specific way to unlock this phone, which means you'll probably need to use a paid service, which are generally pretty sketchy. A quick Google shows that some people have had success with this tool.

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CyanogenMod - in a vanilla or official build - will not typically contain a bootloader. On many devices there is really no good way to even overwrite the bootloader because it is on protected memory (the bootloader is "locked") and cannot be overwritten. It installs a boot image and a system image (i.e. writes to the /boot and /system partitions). I've heard that some builds will also come with a recovery image, but I have never seen one that did this.

Unlocking your SIM is unrelated to your firmware, and CM will not perform a SIM unlock for you. CyanogenMod's wiki itself states:

CyanogenMod, however, does not "unlock" the device. Most carriers "lock" their handsets to prevent customers from buying a handset and moving to a different carrier. Carriers depend on these "exclusivity" agreements to bolster revenue. For example: if you buy an iPhone in the US, you are stuck with AT&T or Verizon, whichever you bought from. To use the handset on another carrier's network it would be necessary to "unlock" the handset. This is done with a code based on the IMEI of the handset that can be provided by your carrier or firms on the internet that are slightly more reliable than a Nigerian Prince.

Unlocking cannot be done by installing CyanogenMod, or any other firmware for that matter.

I would further assume that the bootloader on your device does not have any kind of bearing on the SIM lock. HTC allows bootloaders to be unlocked on certain devices, and their website specifically states:

Please note that unlocking your bootloader does not mean that you will be able to unlock the SIM lock. Unlocking your SIM lock is at the discretion of your operator/carrier and is not part of the bootloader unlocking scope.

I would guess this is likely similar on other devices, though I'm not specifically familiar with Huawei's hardware or software.

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Usually the easiest way is to call the carrier and ask for the Unlock Code. If you are a customer for a length of time (say 6 months+ or so) and tell them that you need it unlocked (maybe to travel outside the country) they will usually give it to you. –  Ryan Conrad Feb 15 '12 at 15:00
    
i'm pretty much interested to know what's bearing the SIM lock ?!... if it's not the bootloader so what would it be ?! .. i've already unlocked the bootloader of the phone, downloaded CM 7.2 but that didn't work out, i have no direction to go unless i specifically know how the phone is locked to a certain carrier !!! –  Muhammad Ghandour Feb 16 '12 at 9:44
    
@MuhammadGhandour: As Ryan noted, the easiest thing to do is probably to contact your carrier. I don't know exactly how SIM lock works on a technical level, but my understanding is that's it's built into the modem or SIM reader's firmware at some level, which is not modified by custom ROMs. –  eldarerathis Feb 16 '12 at 14:43
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