how dose the PC run to startup.s when the mcu reset

Hi，dear enginner：

    i want to know how the PC run to startup.s when mcu reset？

   The Definitive Guide to the ARM Cortex-M3 says address 0x00000000 is MSP‘，address 0x00000004 is ResetVector.

   But when I look at STM32F767, I find that 0x00000000 and 0x4 addresses are other instructions.

   Is there any reference material that can solve my confusion?

 Thanks.

Regards.