How does a typical embedded boot loader end?

Question

What instruction or set of instructions would a typical embedded boot loader use to end boot loading? Please be tolerant about the question. I'm wondering if the boot loading actually ends or there is some sort of an infinite loop after transferring control to the operating system.

Answer 1

Once a bootloader has finished it's initialization tasks, it transfers control of the system to the operating program/system. The specific instruction is typically a jump or a branch depending on the specific bootloader or architecture.

Since you specifically mention an operating system, I will reference the Embedded Linux Primer:

Note that the bootm command is the death knell for U-Boot. This is an important concept. Unlike the BIOS in a desktop PC, most embedded systems are architected in such a way that when the Linux kernel takes control, the bootloader ceases to exist. The kernel claims any memory and system resources that the bootloader previously used. The only way to pass control back to the bootloader is to reboot the board.

And looking at the AT91 Assembler Code Startup Sequence for C Code Applications Software we can see that it uses bx which is the Branch and Exchange command of the THUMB Instruction Set:

;---------------------------------------------------------------------------
;- Branch on C code Main function (with interworking)
;---------------------------------------------------------------------------
    IMPORT __main

    ldr r0, =__main
    bx r0

    END