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locked r15 in IAR AVR

Posted by Nobody/Anonymous on June 14, 2006
Regarding the locked register r15 in iar avr port,
can i use c code in the enter critical-exit critical macro instead of the asm? i do not like the idea of having a reg less in a whole project just for saving the interupt status
also the asm( "st -y, r15" ) of enter critical and asm( "ld r15, y+" ) in exit, can that cause troubles as the y index must be in the correct position when using exit critical?
I was thinking of using a static pointer to a location in ram(that will be excluded from compiler usage) and using c code in the macros thus removing the need for a locked register and tempering with stack, is that correct ?

RE: locked r15 in IAR AVR

Posted by Nobody/Anonymous on June 14, 2006
> Regarding the locked register r15 in iar avr port,
> can i use c code in the enter critical-exit critical macro instead of the asm?

This AVR port is one of the original ports and overdue for some improvements. It is based on the GCC port which has a tmp register that can always be scratched. Rather than IARing the port it was basically copied, hence the need for a locked register to mimic the GCC tmp register.

Newer ports keep a variable that has a critical nesting count (look for CriticalNesting in the port files). This is saved as part of the task context. Doing it this way is a little less efficient but can be used no matter what the addressing mode of the processor as it does not alter the stack (as the AVR port does).

Do you mean you want to write the enter and exit critical macros in C rather than assembly. Or that you want to place C code amid the assembly code? Provided you store the interrupt flag in a manner that allows calls to enter critical to be nested then I think you could do what you want.


> also the asm( "st -y, r15" ) of enter critical and asm( "ld r15, y+" ) in exit,
> can that cause troubles as the y index must be in the correct position when
> using exit critical?

The y reg is used by the compiler as the stack pointer, so this will always just push onto the stack.


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