STM32 BOOT Overview

1) User Flash = Flash built into the chip.
2) SRAM = RAM area built into the chip, which is the memory.
3) System Memory = A specific area inside the chip. A Bootloader is pre-installed in this area when the chip leaves the factory, which is usually called the ISP program.
The content of this area cannot be modified or erased after the chip leaves the factory, that is, it is a ROM area.

There are two pins BOOT0 and BOOT1 on each STM32 chip. The level status of these two pins when the chip is reset determines which area the
program starts from after the chip is reset. See the table below:

BOOT1=x BOOT0=0 Boot from user flash memory, which is the normal working mode.

BOOT1=0 BOOT0=1 Boot from system memory. The program function started in this mode is set by the manufacturer.
       BOOT1=1 BOOT0=1 Boot from built-in SRAM. This mode can be used for debugging.

When the system is powered on, the CPU first determines which mode to start based on these two pins, then maps the starting address of the corresponding mode to address 0, and starts execution from address 0.
 
When the chip leaves the factory, ST burns a bootloader into ROM, which is system memory. The main task of this bootloader is to download the program to the built-in flash through uart1. The workflow is as follows:

 
The system memory boot mode must be exited after completing its task. This exit method is achieved through a hardware reset. During reset, the two pins BOOT[1:0] must be configured to enable the CPU to enter the appropriate mode after restarting.
 

        It should be noted that the built-in SRAM is generally not used for booting (BOOT1=1 BOOT0=1), because the data in the SRAM will be lost after the power is off. In most cases, SRAM is only used for debugging, and it can also be used for other purposes. For example, to do local diagnosis of faults, write a small program to load it into the SRAM to diagnose other circuits on the board, or use this method to read and write the Flash or EEPROM on the board. This method can also be used to remove the read and write protection of the internal Flash. Of course, when removing the read and write protection, the content of the Flash is also automatically cleared to prevent malicious software from copying.

Generally, the BOOT0 and BOOT1 jumpers are set to 0.