Zero to main(): How to Write a Bootloader from Scratch | Interrupt

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Thanks for the comment, @simonhaines. It adds a ton to the conversation.

You make a great point that the bootloader may want to do something different based on the reset type. Some implementations also provide some RTC scratch registers (e.g. STM32) which can be used to communicate between the app & the bootloader without the linker complexity.

On the topic of setting the MSP, Keil makes this even nicer using named register variables. I’ll reproduce their example below:

register unsigned int _control __asm("control");
register unsigned int _msp     __asm("msp");
register unsigned int _psp     __asm("psp");

void init(void)
{
  _msp = 0x30000000;        // set up Main Stack Pointer
  _control = _control | 3;  // switch to User Mode with Process Stack
  _psp = 0x40000000;        // set up Process Stack Pointer
}

source: http://www.keil.com/support/man/docs/armcc/armcc_chr1359125006491.htm

Unfortunately this is not supported by GCC.

On the USB mass storage device: the nRF52 development kit does just this, and you are right to say it is a magical experience :slight_smile: