The bootloader is a small, rarely changed program that lives at the start of flash memory. Its job is to find the application firmware, make sure it is valid and jump to it. In IoT devices it typically also manages update slots: a new image is downloaded into a second partition, the bootloader verifies it on the next reset, swaps it in, and rolls back if the new firmware fails to confirm it booted correctly.
That position makes the bootloader the foundation of secure boot. If it checks the firmware’s signature against a key it trusts, an attacker cannot run modified code; if it does not, every later protection can be bypassed. Because the bootloader itself is hard to update safely in the field, it is kept minimal and tested heavily.
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