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Boot Sequence & Startup Code

Everything that runs before main(): the reset vector, copying .data and zeroing .bss, the vector table, and how a bootloader hands off via VTOR.

22 min read

In this lesson:

what runs between power-on and main(), and who wrote it
the Cortex-M reset sequence: vector table → reset handler
how .data gets copied and .bss gets zeroed (so globals work)
the vector table and relocation (VTOR)
bootloaders, boot modes, and the gotchas that brick a board

This ties together the memory map (.data/.bss), the linker (sections/symbols), and vector tables (function pointers) from earlier lessons.

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5 interview questions on this topic

Each one opens a full worked answer.

What happens between reset and the first line of main()?How does a global variable get its initial value?What is the vector table and what's special about its first entry?Why must main() never return on a bare-metal system?A bootloader jumps to the application but the app's interrupts don't work. What's the likely cause?

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Boot Sequence & Startup Code | EmbeddedPrep.io