Startup code: what happens before main

Reset sequence

1. The CPU reads the initial stack pointer value from entry 0 of the vector table and puts it in sp
2. It reads the address of Reset_Handler from entry 1 and jumps there
3. Reset_Handler does the work below
4. SystemInit(): basic settings such as clocks and Flash wait states
5. main() is called

What Reset_Handler does

• Copy .data: a global variable with an initial value, such as int count = 5;, has that initial value stored in Flash. It is copied to the variable's location in RAM.
• Initialise .bss: the area for global variables without an initial value, such as int total;, is filled with 0, because the C standard guarantees that globals start at 0.
• C++ constructors / library initialisation (__libc_init_array)

Problems you meet in practice

• If variables have strange values after jumping from a bootloader to the application, check that the application's startup code ran properly.
• Values that must survive a reset (a record of the reset cause, for example) go in a .noinit section to avoid .bss initialisation.
• On chips with a lot of RAM, the time to zero .bss can affect the boot time.