"Cortex-M0 Authoritative Guide" Architecture --- Program Image and Boot Process

　　Let's take a look at the program image first.

　　Typically, the program image of the Cortex-M0 processor starts at address 0x00000000.

　　The program image starts with a vector table, which contains the actual addresses (vectors) of the exceptions. The address of each interrupt vector is equal to "exception number*4". For example, the exception type of external IRQ0 is 16, so its vector address is 16*4=0x40. The lowest bit of these vectors is set to 1 to indicate the use of thumb instructions. The size of the vector table is determined by the number of interrupts actually used.

　　The vector table contains the initial value of the MSP, which is stored in the first four bytes of the vector table.

　　When reset, the processor first reads the first two words (8 bytes) of the vector table. The first word is the initial value of MSP, and the second word is the reset vector, which indicates the starting address of program execution.

　　For example, if the startup code is located at address 0x000000C0, this address needs to be written at the reset vector, and the lowest bit of the address is set to 1 to indicate that it is currently thumb code. Therefore, the value at address 0x00000004 is 0x000000C1. After obtaining the reset vector value, the processor starts executing program code from this address.

　　The reset process will also initialize the MSP. Assuming that the memory is located at 0x20000000~0x20007FFF, 0x20008000 can be written at address 0x00000000, thus placing the main stack pointer at the top of the memory.

　　If you want to use the PSP, you must first initialize it through software code before configuring the CONTROL register to switch the stack pointer, because the reset process only initializes the MSP, not the PSP.