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What is a bootloader unlocked phone? How we can make use of it while creating or flashing ROMs? What are advantages of unlocked bootloaders over locked bootloaders? Why does not any OEM generally make bootloader unlocked phones?

I am pretty naive in this topic. Thanks in advance.

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Check out Fairphone - that is pure unlocked handset, there's another IIRC, NeoRunner, and also, as per Google IO 2013, Samsung Galaxy S4 will be unbranded as Google nexus (?) that will be unlocked. –  t0mm13b May 27 '13 at 15:32
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Every Android phone has a bootloader that instructs the operating system kernel to boot normally. But you need to understand one thing here that as Android OS is an open source OS and is available on a variety of different hardware, every manufacturer has their own version of bootloader specific for the hardware present in its environment. (...) A bootloader is usually locked on an Android device because although it’s an open source OS, still the manufacturers want you to stick to their Android OS version specifically designed for the device. In order to apply this concept, manufacturers lock the bootloader.

(Source)

Essentially, the advantages of having an unlocked bootloader for your device is that the manufacturer of that device has granted you the right to modify the operating system (Android) present on the device.

It's interesting to note that the term "unlocked" bootloader would seem absurd if we were talking about a PC; the freedom to boot any OS you want has always been taken for granted on that platform, but because locked bootloaders are so common in the mobile world for many different reasons, it has become something odd for the bootloader to be "unlocked".

The reason that bootloaders are locked on some devices are fairly obvious: the manufacturer of the device wants to be in control of what software is running on it. Since devices are often locked and tied to a contract, having control of what code runs on the device is essential. It's also a question of support; if you were the manufacturer, you wouldn't want to handle support calls from people all over the world who had accidentally bricked their phones by flashing corrupt software onto their devices. In a sense, locked bootloaders protect (some) users from themselves. Depending on your point of view, this can be good or bad. For a corporate customer buying devices for employees, access to the bootloader probably would not be high on the wish list.

Please let me know if there's anything in this answer that is unclear and I will try to elaborate.

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I also want to know that when a new phone launches, some developers try to root it by finding some flaw in hardware or drivers something. When they succeed, its like a big news. How its related to unlocking the bootloader? In PC if they have unlocked bootloaders by default why cant we easily install Macintosh easily (need to use Hackintosh), OS is locked here? –  Akshay Kulkarni May 27 '13 at 14:46
    
Yes, sometimes reverse engineering is used to find ways to open bootloaders that were not created to be unlockable. Jailbreaking iPhones is a good example of this. It's big news because it usually takes a lot of work and some pretty smart people to do it. The reason you cannot run OSX on your PC is different; OSX is made for macs, the only reason it doesn't run (or runs badly) on your PC is that Apple never bothered to make it work there. It does work on some PC:s obviously (the one you call Hackintoshes), and technically Macs are PCs ;) –  bigbadonk420 May 27 '13 at 14:53
    
I am sorry the comment thread is turning to discussion but its helping clearing my doubts.So while rooting phones or unlocking bootloader how is it possible that hardware breaks(bricks) its just a software process right? Ideally any software operation should not break hardware permanently! –  Akshay Kulkarni May 27 '13 at 14:59
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I think the best explanation would be this: Flashing your BIOS with a bunch of junk (say, only zeroes) is a "software process". However, you just erased the software that helps the most basic functions of your PC start and function! Your OS will not even start. Technically, the hardware isn't "broken", you just need another way to write a working BIOS to wherever it's stored, which isn't easy. The same way a USB isn't "broken" if everything is deleted from it; it's just that this special "USB stick" isn't exactly easy to read and write from when you messed it up. –  bigbadonk420 May 27 '13 at 15:09
    
If you "brick" your device, its hardware is still fine -- it's the software part that's broken then. A real "hard brick" (which means there's no way to recover from) is quite unusual -- but imagine you flash your PC's BIOS with something completely broken. You didn't damage any hardware -- but still the PC wouldn't boot up as it no longer knows how to address its hardware. That's about what "bricking" means. Even to flash a new ROM, you will need a basic system running to do the job; break that, and you have a "brick". –  Izzy May 27 '13 at 15:14
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