NOTE: If anyone has any further questions about bootloaders on the S3, please note that I have moved on to another device. Sorry!
As you may or may not know, the latest builds of CyanogenMod 10.1 (and many other ROMs which use CM as a base) refuse to install without an updated bootloader on your device. Even if you bypass this requirement, the ROM won't boot. I haven't seen any clear-cut instructions on updating the bootloader except for a single post buried in the CM10.1 nightly thread, posted by Jiangyi, but that update also changes your modem, so I thought I'd throw together a thread dedicated to bootloaders only.
Step 1 - Identify your model
This subforum supports 2 different models; The I747 and the I747M. You can find your device's model in a few different ways:
While Android is running
In Settings, under About Phone, the Baseband version shows the currently installed modem firmware version. If your phone currently works on mobile networks, then you've probably got the right modem firmware for your model, and the Baseband version will start with either "I747" or "I747M". This is your model.
While the phone is off
Underneath your battery is a sticker showing your IMEI and other information, including the model number.
Step 2 - Download the correct update
You can install this updated bootloader in a couple different ways; Most people should be comfortable flashing ZIPs in their recovery (CWM or TWRP both work fine) while others may want to install the update via Odin. Choose your model and preferred method, and download the matching file from below.
I747UCUEMJB - Oct 11, 2013 - boot_CWM_I747UCUEMJB.zip - 1.14 MBWARNING: Do not attempt to downgrade from this bootloader. Doing so will brick your device. Use extra caution when flashing anything via Odin, as many Odin packages will attempt to flash the bootloader.
I747M (Canada and overseas) (Updating this list shortly...)
I747MVLUEMK5 - Nov 5, 2013 - boot_CWM_I747MVLUEMK5.zip - 1.14 MBWARNING: Do not attempt to downgrade from this bootloader. Doing so will brick your device. Use extra caution when flashing anything via Odin, as many Odin packages will attempt to flash the bootloader.
Put your phone into download mode. Some ROMs have this option in the Power>Reboot menu, otherwise just hold "Volume Down" and "Home" while powering on your device.
Launch Odin, then connect the USB cable to your device.
Click the "PDA" button and select the package you downloaded in step 2.
Click "Start" in Odin and wait for the process to complete. Your device should reboot automatically when the flash is complete, and you're done.
Flashing in recovery
If you've downloaded a flashable .ZIP from above, copy it to your SD card or internal storage. Reboot your phone into recovery mode (you can do this by holding "Volume Up" and "Home" while powering on your device; let go of the buttons when you see "Recovery booting..." in blue text in the top left of the screen) and install the .ZIP as an update. Reboot, and you're done.
Note: After flashing in recovery, you need to reboot before you (and ROM installers) actually see the new version number.
Did you brick your phone attempting to downgrade your bootloader?
There really is no reason to downgrade your bootloader; If you did it to be able to flash a ROM that doesn't recognize the newest bootloader version and therefore wouldn't let you flash, you should contact the ROM developer and tell them to add the newest bootloader version to their asserts. The latest bootloaders trigger eFUSEs within the device, no longer allowing it to boot from older bootloaders. These eFUSEs cannot be reset, but @-Mr. X- has come up with an unbricking method that may help you out. Check it out here.
If you're looking to update your modem as well, check my signature for a link to the modem thread.
So you created a thread for bootloader only updates, but there are no bootloader only updates available?
What files are available for download? Is that the full firmware or the files created by Jiangyi?
Currently I've linked to Jiangyi's files, I'm working on bootloader-only updates right now. I just wanted to get this thread up ASAP because I've had a few different people PM me asking for help. As a temporary option, you could just use the Odin update and then flash another modem on top of that.
Instead of posting links to someone else's work, you could have just directed them to Jiangyi's post in the CM10.1 thread.
The link in my first paragraph does just that, but since this is affecting a lot more ROMs than just CM, people are going to need this information and a single post on a random page of a massive thread usually gets overlooked by 99% of people. The point of this thread is to make information easy to find and ensure that people are able to update their bootloaders and run new ROMs.
Like I said, the links to his work are temporary while I get my own files uploaded. Not exactly on the fastest of connections right now.
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