Instruction Access Violation for forcing to Process Thread mode on Cortex M3

Alexander Madill said:

I checked the Fault Status Registers and found it is an instruction access violation.

An Instruction Access Violation the processor attempted an instruction fetch from a location that does not permit execution - any access to a Execute Never (XN) region.

Alexander Madill said:

My thumb bit was set and at the "BX LR" command my process stack pointer pointed to the proces's r0.

Alexander Madill said:

13131313(lr)

The stacked LR value of 13131313 looks to be invalid, and could explain why the BX LR instruction generates an Instruction Access Violation. Can you check that the value of the LR points to a valid address (in flash) when the BX LR instruction is executed.

I force the current LR register value to be 0xFFFFFFFD by:

		__asm("	movw 	LR,#0xFFFD");  /* Lower 16 [and clear top 16] */
		__asm("	movt 	LR,#0xFFFF");  /* Upper 16 only */
		__asm("	bx 	LR");          /* Force return to PSP */

The LR value of 13131313 is what the process's LR value will become after the BX LR

Alexander Madill said:

I force the current LR register value to be 0xFFFFFFFD by

Can you clarify which device you are using, as am not sure of the memory map.

ah sorry, I am using a Cortex M3 little endian, Stellaris LM3S9D92

Alexander Madill said:

I force the current LR register value to be 0xFFFFFFFD by:

		__asm("	movw 	LR,#0xFFFD");  /* Lower 16 [and clear top 16] */
		__asm("	movt 	LR,#0xFFFF");  /* Upper 16 only */
		__asm("	bx 	LR");          /* Force return to PSP */

Is the processor is in Handler Mode when that is executed, to return to Thread Mode.

[I now understanding the significance of 0xFFFFFFFD after reading about the Cortex-M3 Exception Return]

It is in thread mode but using the Main Stack Pointer and I want to flip it over to use the process stack pointer

Alexander Madill said:

It is in thread mode but using the Main Stack Pointer and I want to flip it over to use the process stack pointer

Section 2.5.7.2 Exception Return of the LM3S9D92 datasheet is:

I think this means that if the processor is already in Thread mode, a branch to 0xFFFFFFFD is invalid.

If the processor is already in Thread mode, to switch from the Main Stack Pointer to Process Stack Pointer try writing to the ASP bit in the Control Register, as in the following from the LM3S9D92 datasheet:

that fixed it.
However I wanted to the program to also jump to other function (and switch to using the process pointer) which was the purpose of setting up a PC for the process stack pointer to grab. So I made the function which executed the BX LR an interrupt and signaled the interrupt.
Thanks a lot!!

Alexander Madill said:

So I made the function which executed the BX LR an interrupt and signaled the interrupt.

Maybe a Supervisor Calls (SVC) would be useful in that context. e.g. see http://coactionos.com/embedded%20design%20tips/2013/10/12/Tips-Effective-Use-of-ARM-Cortex-M3-SVCall/