When the microcontroller is powered on, it starts the execution from the reset handler. This is a special ISR which does not return, but it rather performs initialization of the .data and .bss sections, and then calls the entry point of the application. The initialization of the .data and .bss sections consists of copying the initial value of the variables in the .data section in flash onto the actual section in RAM where variables are accessed at runtime, and filling the .bss section in RAM with zeros, so that the initial value of static symbols is guaranteed to be zero as per C convention.
Source and destination addresses of the .data and .bss sections in RAM are computed by the linker when generating the binary image, and ...
