The startup process of STM8 microcontroller

When I was in college, I studied STM8 microcontrollers for one semester. I put a lot of effort into it. I used a textbook from a teacher I admired very much, which was taught in assembly language. However, I admired it very much. Now I think about it, some basic things in this book are not very clear. For example, in the first chapter, the implementation principle of microcontrollers was explained by 51 microcontrollers, and the code of 51 microcontrollers was also used. But this book is about STM8, okay? I have to say that the 51 was used. It is obvious that the book was published in a hurry. There are also some typos in the book, which is a bit strange for someone like me with obsessive-compulsive disorder. Of course, the good thing is that there are many practical engineering things in it, and it also talks about examples of using STM8 assembly to implement multi-tasking, and digital filtering and other things that seemed very interesting to me at the time. I still admire him (Pan Yongxiong) very much. This is the truth. I have read this book many times and I have been reluctant to donate it. Other circuits, analog electronics, and digital electronics have all been donated to others. 

Okay, let's get to the point. Before talking about the boot process of STM8 microcontroller, we must first have a basic understanding of STM8 memory map, which can be found in the STM8 data sheet. The following is a screenshot (of course, this is the memory map of one series of STM8, and others may be different): 
 
You can see a storage area called "2 Kbyte boot Rom", and the reset vector of hardware reset (from shutdown to startup) is in this area. This is very special because there is a storage area called "32 interrupt vectors" below, and all other interrupt vectors (see the table below) are in this area, only the hardware reset vector is not. 
 
So after the hardware reset, that is, after the STM8 is turned off and on, the first code executed is the code on the boot Rom. The code on the boot Rom is called bootloader. The main function of the bootloader is to burn the microcontroller program into the microcontroller through the integrated peripherals (UART, SPI, CAN) of the microcontroller without using ST-Link. However, this can only be burned when the microcontroller is burned for the first time or the BL and NBL bytes in the Option bytes (this is also in the storage mapping diagram) are set to 55 and AA respectively and the read protection of the memory is not enabled. If the conditions are not met, you can use ST-Link to connect the microcontroller and set the Option bytes in STVP to meet them. The specific flow chart after starting from the boot Rom can be seen in the figure below (the figure comes from the official UM0560), or you can read the text directly. 
 
After meeting the conditions mentioned above, STM8 will detect whether there is a synchronization signal from the host (usually the computer end). If there is a synchronization signal, STM8 will hand over control to the host, and the host can download the program to STM8 (after downloading, the host specifies the address where the program starts to execute). If the STM8 does not detect the synchronization signal of the host, it will wait for 1 second and then time out. After the timeout, it will determine whether the microcontroller has not been programmed. If so, it will return to the hardware reset state and repeat the process from the hardware reset. If the program has been programmed before, find the entry of the reset vector in "32 interrupt vectors", enter the address pointed by the vector (usually what we call the main function) to start executing the program. 
What if the bootloader does not meet the conditions for programming when it starts executing? It also finds the entry of the reset vector in "32 interrupt vectors", enters the address pointed by the vector to start executing the program. 
The above is the startup process of the STM8 microcontroller.