Step1: When you power on the MCU, all register are reset to default values. Step2: Then the MCU reads the BOOT_PINS to determine from where it has to boot-up, for example, internal memory, external memory, etc. Step3: Stack pointer is initialized with the stack starting address Step4: Program Counter is loaded with Reset handler address. Inside Reset handler, there will be a couple of Assembly instructions and also __main(), which takes MCU control to your main().