The 7th entry of the Cortex-M3 exception vector is reserved for the 2's complement of the checksum of the exception vector. At run time the bootloader computes the checksum again and adds it to the one stored in the exception vector, if the result equals zero it starts executing the user code.
The checksum is usually computed by the software that flashes the binary like FashMagic or openocd, if you're using openocd like me,...
