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[Q] Bootloaders, JTAG and Serial Ports

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Default [Q] Bootloaders, JTAG and Serial Ports

I have a general curiosity question. If the bootloader on a phone, such as a Galaxy S or S4, is not functioning properly, and the JTAG port nonfunctional, is there any other way to reprogram the bootloader, such as through a serial port? If not, can the chip be de-soldered and replaced? Is it a single chip that contains the CPU and the flash that contains the bootloader and android operating system, or are these on separate chips in general? Thanks.
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I have limited knowledge and can't generalize, but here's what I do know about one specific device.
That's the Nook Touch with a TI OMAP3621 processor.

The OMAP has ROM directly on chip for a very simple bootloader.
Depending on how the boot_sys pins are configured it can load from NAND, SD card, UART or USB.
It will load a primary bootloader (mlo, x-loader) which loads the secondary bootloader (u-boot.bin) which loads the kernel and ramdisk image(s).

Some devices do have a locked ROM bootloader which will only load signed primary bootloaders.

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