Thread: Trying to follow C18 interrupt context code...
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Unread 21-12-2006, 09:01
dcbrown dcbrown is offline
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Re: Trying to follow C18 interrupt context code...
One of the other things that confused me is that the C18 stack calling convention is initially broken in the interrupt routine by using the PREINC to do stack pushes instead of the expected POSTINC I was expecting. The stack pointer should always be pointing to the next unused location on the stack.

It turns out this is true with one exception. Since there is no PREDEC instruction, the compiler to pop things off the stack must use two instructions, a MOVF POSTDEC followed by a MOVFF INDF, <reg>. Because these two instructions are non-atomic and an interrupt can catch the execution between these two instructions. Interrupt routines must assume that the stack location being currently pointed to may currently be in use and shouldn't be overwritten. Hence the interrupt stack protocol of apparently allocating an extra stack entry at the beginning of the stack manipulation.

Now on to the fun stuff, figuring out how to inject an event driven handler with user level code onto the stack - <insert evil laugh here>

Bud