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how can I return a value within embedded assembler?

Note: This was originally posted on 8th May 2009 at http://forums.arm.com

Hi,

does anybody know how I can write an embedded ASM function that returns the actual PSR value? I prefered to go on like this:

int getCpsr( void )
{
int currentCPSR;
MRS currentCPSR, CPSR
return currentCPSR;
}

how can I return a value within embedded assembler?

Sebastian Gaertner over 12 years ago

Note: This was originally posted on 8th May 2009 at http://forums.arm.com

I cannot use inline assembler inside my function like this:
__asm { MRS currentCPSR, CPSR };

because I use a Cortex-M3 which only supports Thumb mode.
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Sebastian Gaertner over 12 years ago

Note: This was originally posted on 8th May 2009 at http://forums.arm.com

Thanks,

but this is also ARM machine language instruction. I cannot use this inline assembler
__asm".....
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zart zurt over 12 years ago

Note: This was originally posted on 8th May 2009 at http://forums.arm.com

Hi,

does anybody know how I can write an embedded ASM function that returns the actual PSR value? I prefered to go on like this:

int getCpsr( void )
{
int currentCPSR;
MRS currentCPSR, CPSR
return currentCPSR;
}

how can I return a value within embedded assembler?

[url="http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.kui0098a/armccref_cjahjdbg.htm"]http://infocenter.arm.com/help/index.jsp?t...ef_cjahjdbg.htm[/url]

ui32 read_cpsr()
{
register ui32 _cpsr __asm("cpsr");
// ...
return _cpsr;
}

I could not test this as I do not have a Cortex-M3 cpu...
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zart zurt over 12 years ago

Note: This was originally posted on 8th May 2009 at http://forums.arm.com

Normally ARM and Thumb-2 instruction sets are the same except the binary form. And in the link I provided , it said that CPSR named register was supported for all processors. Sorry , it did not work that way...
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Bhaveet Shah over 12 years ago

Note: This was originally posted on 8th May 2009 at http://forums.arm.com

Thanks,

but this is also ARM machine language instruction. I cannot use this inline assembler
__asm".....

In this context, AFAIK, it is not inline assembler. This is just telling the compiler to treat _cpsr as the if it were the APSR register in Cortex-M3 and it will generate the appropriate instructions for you when you use the _cpsr variable.
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guestposter guestposter over 12 years ago

Note: This was originally posted on 8th May 2009 at http://forums.arm.com

Which compiler are you using? In RVCT 4.0, you can refer to local variables as if they were registers (so called "virtual registers") so you are pretty close already. The code would look like this:

int currentCPSR;
__asm { MRS currentCPSR, CPSR };
return currentCPSR;
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guestposter guestposter over 12 years ago

Note: This was originally posted on 8th May 2009 at http://forums.arm.com

Try embedded Assembler

__asm unsigned getCpsr( void )
{
MRS R0, APSR
BX LR ; return
}

And call it as normal C function.
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