Hi @Maxim, and welcome to Interrupt!
- startup_samd21.c file contains branch to main . when the call to main , and which main (there are 2 of them in app and bootloader) is happened? I understand this file contains functions which at compile and link stage are used in places they are called. But how it happens this file call main then?
Each of the programs we compile (i.e. the app and the bootloader) contains only a single instance of main. Otherwise, the linker would complain! Which main is compiled in a given program is specified in the Makefile.
when (from where) are they further called? is the syntax for the field names governed by some standard?
The functions in the vector table are called by the hardware! You won’t find a single call to them in software. The ARMv7m spec defines where the exception table should be found, and the hardware will jump to those addresses when an exception/interrupt happens.
- Why we remap NVIC for the app? Why don’t to use the original one?
When we boot, the exception table points to the bootloader’s exception handlers. When we start the app, we want to use the app’s exception handles instead, so we have to remap it using the VTOR register.
- Reset first happens at the start up of the device, and Reset_Handler should be called and transfer execution to app (because it remaps NVIC and re-write PC). How then we can load bootloader? Why at startup Reset_Handler from startup_samd21.c file is not called and bootloader is still loaded?
Both the app and the bootloader have an exception table and a Reset_Handler. We write the bootloader at start of flash, so by default its exception table is used (until we remap it). Which binary is found where is defined by the Makefile and the linker scripts.