为什么在为 Cortex-M 中断处理程序生成的程序集中存在死循环？

Question

I am learning the Cortex-M with the MDK uVision IDE. I wrote a simple SysTick_Handler() to replace the WEAK default SysTick_Handler() which is a simple dead loop.

My SysTick_Handler() :

The disassembly:

I am confused by the the highlighted assembly line. It is simply a dead loop.

Why is it there? Why the toolchain still generated it despite that I already overwrite the WEAK default implementation with my own SysTick_Handler ?

I can still place a breakpoint at that line and it can be hit . And in that case, my code will never be executed.

But strange thing is, if I removed the breakpoint at that line, my code can then be reached. How is that possible?

Answer 1

(Thanks to all the hints the community provided. I think I can explain it now.)

The dead loop is part of my main() function, which is like below. The main() function is just above my SysTick_Handler in the same C file.

int main (void)
{
    LED_Initialize();
    SysTick->VAL = 0x9000;                                                                                   
    //Start value for the sys Tick counter
    SysTick->LOAD = 0x9000;                                                                                  
    //Reload value 
    SysTick->CTRL = SYSTICK_INTERRUPT_ENABLE|SYSTICK_COUNT_ENABLE;  //Start and enable interrupt
    while(1)
    {
        ;  // <========= This is the dead loop I saw!
    }
}

To double confirm, I modified the while loop to below:

int main (void)
{
    volatile int32_t jj = 0;
    LED_Initialize();
    SysTick->VAL = 0x9000;                                                                                  //Start value for the sys Tick counter
    SysTick->LOAD = 0x9000;                                                                                 //Reload value 
    SysTick->CTRL = SYSTICK_INTERRUPT_ENABLE|SYSTICK_COUNT_ENABLE;  //Start and enable interrupt
    while(1)
    {
        ;
        jj+=0x12345; // <====== add some landmark value
    }
}

The generated code is like this now:

Though it is still placed under the SysTick_Handler . I place a break point there to check what's really going on:

The R1 is the constant 0x12345 . The R0 is the local variable jj . We can see the R1 does contain the landmark value 0x12345 , which is added to R0 ( jj ). So it must be part of my while(1) loop in the main() .

So, the disassembly is correct. Only that the debugger failed to provide a correct interleaving between the source and the disassembly.

And btw, remember to rebuild the target after modifying the code otherwise the uVision IDE debugger will not reflect the latest change....