Using C and reading CPU Register (Carry-Flag) via assembler

A little clue:

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
}

Assuming you are working with an ARM7 core, you are interested in CRSR. What is that according to the user manual...?

A little clue:

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
}

Assuming you are working with an ARM7 core, you are interested in CRSR. What is that according to the user manual...?

For the sake of completeness -

STMDB   R13!, {R1}

and

LDMDB   R13!, {R1}

above can be discarded.

you are interested in CRSR -> you are interested in CPSR

Hey Tamir,
thanks for your fast reply.

I am working on an ARM Cortex-M3. CPSR was unknown to the compiler, but APSR wasn't :)
Anyways, I placed your code (with CPSR changed to APSR) in a new .c file and it compiled without grumping :]

For evaluation I tried that:

int
main(void)
{
        unsigned long dummy;
        unsigned long a=1000;
        unsigned long b=10000;

        dummy = a - b;  //Carry-bit should be set
        UARTprintf("\n: %d \n", processor_in_user_mode);
        dummy = b - a;//Carry-bit shouldn't be set
        UARTprintf("\n: %d \n", processor_in_user_mode);

but both times it prints out '489' which is:

111101001

so nothing changed. But register 17 bit 31 is according to the datasheet the carry bit, which should have changed here...
I also tried PSR instead of APSR: no change

By the way: CPSR is printed bold in code, PSR or APSR isn't.

Hi Susan

Couple of options:

Use this standardized CMSIS function portable between popular toolchains

__get_APSR(void)

Or use MDK/RVCT specific named register variables

uint32_t apsr __asm("APSR");

if (apsr & APSR_C)
   // do this
else
   // do that

Hope this helps
Marcus

Note that you can manage quite well without ever knowing the content of the carry bit.

if (b >= a) {
    b -= a;
    // no borrow
} else {
    b -= a;
    // handle borrow
}

unsigned a,b,c;
c = a+b;
if (c < a || c < b) {
    // addition overflow
}

Marcus, I tried yours, too.

__get_APSR(void)

Do I need a special header or include file for this?
Cause µVision tells me:

undefined symbol __get_APSR();

identifier "APSR_C" is undefined, respectively (when trying the other one)

__get_APSR() is defined in CMSIS, so yes, you do need to include a header file, namely core_cmFunc.h.

The identifier APSR_C was just a made up example, perhaps defined as macro, to demonstrate that you need to compare with a numeric value. BTW, CMSIS also defines a structure APSR_Type so that you could perhaps write something like:

if ((APSR_Type)apsr.b.C)
  // do this
else
  // do that

But as others have said already: For your purposes, this is completely unnecessary.

Best regards
Marcus