Potential bug on Cortex-M due to used registers/interrupts.

[Kai Ruottu]

Vitalijus Jefišovas kirjoitti 16.11.2017 klo 18:54:
> On Cortex-M mcu’s, when interrupt happens, NVIC copies r0-r3 and couple
> other registers onto the psp stack, and then jumps to interrupt routine,
> when it finishes, NVIC restores these registers, and jumps back to user’s
> function.
> What is happening under the hood, NVIC only stacks 4 registers, r0, r1, r2,
> r3. The other ones r4-r12 is developer’s responsibility.
> I was looking at assembly code generated by GCC and there are plenty of
> instructions using r4-r12 registers.
>
>
>
> How does GCC handle scenario when execution is switched to unknown
> procedure, which changes all of these registers?

Cortex-M is an ARM-architecture processor or how?  So one uses things 
related to ARM
or how?

With ARM one uses function attributes available for ARM when the 
question is about a
special function like an ISR ("Interrupt Service Routine").  From the 
GCC 7.2.0 manual :

----------------- clip ------------
6.31.4 ARM Function Attributes

These function attributes are supported for ARM targets:

interrupt
 Â  Use this attribute to indicate that the specified function is an 
interrupt handler.
 Â  The compiler generates function entry and exit sequences suitable for 
use in an
 Â  interrupt handler when this attribute is present.
 Â  You can specify the kind of interrupt to be handled by adding an optional
 Â  parameter to the interrupt attribute like this:
 Â Â Â Â  void f () __attribute__ ((interrupt ("IRQ")));
 Â  Permissible values for this parameter are: IRQ, FIQ, SWI, ABORT and 
UNDEF.

 Â  On ARMv7-M the interrupt type is ignored, and the attribute means the 
function
 Â  may be called with a word-aligned stack pointer.

isr
 Â  Use this attribute on ARM to write Interrupt Service Routines. This 
is an alias
 Â  to the interrupt attribute above.
----------------- clip ------------

How this attribute works with the NVIC I don't know but using the aimed 
attribute
and seeing the result is one way to see what will happen...