Describe the boot process in microcontrollers.

Boot sequence: 1) Power-on reset initializes hardware to known state, 2) Processor fetches reset vector from fixed address (e.g., 0x00000000), 3) Vector points to startup code that initializes stack pointer, 4) Startup code initializes memory (.data copied from flash, .bss zeroed), 5) C runtime initialization (static constructors in C++), 6) Main() called. Boot options may include: Boot from internal flash, external memory, serial bootloader (UART/USB), or debug interface (JTAG/SWD). Bootloaders enable field firmware updates without special tools.