Is __bis_SR_register() necessary?

Raju Purohit

Other Parts Discussed in Thread: MSP430G2553

Hey everyone,

I have always seen people using __bis_SR_register() to set interrupts and/or go to LPM. In users guide, I observed that this may also be done by setting Status Register (SR). For eg to set to LPM0 with interrupts on, I can write,

SR = LPM0 + GIE;

Is this acceptable? And where is the definition of __bis_SR_register() function? (I couldn't find it.)

Thanks

over 13 years ago

0 Raju Purohit over 13 years ago

Prodigy 110 points

I just tried that on my Launchpad (MSP430G2553) but it failed to recognise SR. But after debugging (using _bis_SR_register() function of course), in view->Registers, I can see SR register. Can I not set this register like any other register?

0 Tony Janke over 13 years ago

TI__Intellectual 1100 points

Why is the __bis_SR_register() command not acceptable? Does your system behave as expected when you use it?

0 Raju Purohit over 13 years ago in reply to Tony Janke

Prodigy 110 points

No, I meant SR = LPM0 + GIE is not acceptable according to CCS (__bis_SR_register() works fine).

0 Tony Janke over 13 years ago in reply to Raju Purohit

TI__Intellectual 1100 points

So why can't you use that method to configure the SR?

0 Raju Purohit over 13 years ago in reply to Tony Janke

Prodigy 110 points

I am just curious. When (almost) every other register can directly be assigned, why can't SR? How is it different?

0 Jens-Michael Gross over 13 years ago

Guru 227245 points

Raju Purohit said:
I observed that this may also be done by setting Status Register

Yes, that is exactly what this intrinsic does. The C language does not know about processor registers. You can only assign values to memory locations. There is no C instruciton that would change the value of the status register (or any other processor register).

To do so, the compile adds 'intrinsics' called psedo functiosn which aren't compiled according to the C standard, but are recognized by the compiler as an order to perform a specific operation that is outisde the C language scope. Like changing the status register, reading the stack pointer, or waste processor cycles.
In case of __bis_sr_register, this is simply translated into a single assembly instruciton that is inserted into the code at the position of the intrinsic.
You could use inline assembly code as well for this job.

However, for changing the stack-saved SR content from inside an ISR, inline assembly won't help you as only the compiler knows how much he has already put onto stack after start of the ISR. So there is no simple iline assembly replacement for the *on_exit* intrinsics.

Raju Purohit said:
And where is the definition of __bis_SR_register() function? (I couldn't find it.)

In the source code of the compiler. And only there. It is, however, documented in the compiler manual.

0 Jens-Michael Gross over 13 years ago in reply to Raju Purohit

Guru 227245 points

Raju Purohit said:
I am just curious. When (almost) every other register can directly be assigned, why can't SR? How is it different?

The hardware registers are memory-mapped. for processor (and compiler/linker) they are just memory locations like ram or flash. The processor registers (status register, stack pointer, program counter and R4..R15) are internal to the processor and totally unknown to the C programming language.

0 Samuel Wantman over 11 years ago in reply to Jens-Michael Gross

Intellectual 370 points

Thanks for this useful explanation. It might help anyone else that finds this discussion for a link to the documentation that explains all these pseudo code and how they work. That is what I am currently looking for, and how I ended up here...

0 mike newsome over 10 years ago in reply to Samuel Wantman

Prodigy 10 points

I was also unclear about the intrinsic functions. I found the documentation on pages 131 and 132 of SLA132J - "MSP430 Optimizing C/C++ Compiler v 4.4"

0 Harry over 8 years ago in reply to Raju Purohit

Expert 2920 points

thanks for this question! I was gonna ask this question on the forum and found yours!

0 Harry over 8 years ago in reply to Jens-Michael Gross

Expert 2920 points

Thank you! Saved my life.

0 Harry over 8 years ago in reply to Samuel Wantman

Expert 2920 points

you predicted it right!

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Is __bis_SR_register() necessary?