What is a bootloader in an embedded device, and why would a product need one?
What the interviewer wants to hear
- First code to run after reset, in its own flash region
- Validates the application, for example with a CRC or signature
- Can receive and write new firmware over UART, USB, CAN or wireless
- Jumps to the application by setting the stack pointer and vector table
- Enables field updates and recovery without a debugger
Model answer
A bootloader is a small program in its own region of flash that runs first after every reset. Its job is to decide what to run: it checks that the application image is valid, often with a CRC or, for secure products, a cryptographic signature, and then jumps to it by loading the application's stack pointer and reset vector and moving the vector table offset. If an update is requested or the application is broken, the bootloader stays active and receives a new image over UART, USB, CAN or a wireless link, writes it to flash and verifies it. Products need one because firmware will have bugs and new features after shipping, and you cannot attach a debugger to thousands of devices in the field. A good bootloader is small, rarely changed and can always recover the device.
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