How to determine where function was called from

Something like this:

int32u __asm processor_in_user_mode(void)
{
        STMDB   R13!, {R1}

        MRS     R0, CPSR
        AND     R0, R0, #0x1F   // the lowest 5 bits of CPSR respresent the processor mode
        MOV             R1, #0x10               // user mode = 0x10
        CMP             R0, R1
        BEQ             not_interrupt_context

        LDMIA   R13!, {R1} // restore the file name and the line number.

        MOV             R0,     #0x0 // indicate called while not in user mode

        BX              LR

not_interrupt_context

        // restore original values of R1
        LDMIA   R13!, {R1}

        MOV             R0,     #0x1 // indicate called while in user mode

        BX              LR
}

Something like this:

int32u __asm processor_in_user_mode(void)
{
        STMDB   R13!, {R1}

        MRS     R0, CPSR
        AND     R0, R0, #0x1F   // the lowest 5 bits of CPSR respresent the processor mode
        MOV             R1, #0x10               // user mode = 0x10
        CMP             R0, R1
        BEQ             not_interrupt_context

        LDMIA   R13!, {R1} // restore the file name and the line number.

        MOV             R0,     #0x0 // indicate called while not in user mode

        BX              LR

not_interrupt_context

        // restore original values of R1
        LDMIA   R13!, {R1}

        MOV             R0,     #0x1 // indicate called while in user mode

        BX              LR
}

I think

STMDB   R13!, {R1}

and

LDMIA   R13!, {R1}

above are redundant.

Are you sure that I can run this in user non-privileged mode? I need to check it because it doesn't match my understanding.

Sounds like you plan to do this on a Cortex-M3. I don't know - this code works well on an ARM7 chip. You may need to implement this in a SVC.

Tapir,

Why do you say "I think"?

Are you still unsure of the rules for register preservation.

Let's give it a few more years.

SVC from the user space does trigger hard fault, so your plan doesn't work.

Eric, take my request as given please. Explaining it will spark more issues then bring answers.

It is possible to perform read/write operations directly to the MSP and PSP provided that you are in privileged level, you can
access MSP and PSP using the MRS and MSR instructions:
MRS R0, MSP ; Read Main Stack Pointer to R0
MSR MSP, R0 ; Write R0 to Main Stack Pointer
MRS R0, PSP ; Read Process Stack Pointer to R0
MSR PSP, R0 ; Write R0 to Process Stack Pointer

And I'm not!!! Actually not always to be exact.