I'm sure it's written somewhere and I'm not opposed to looking for it. However I have looked and found nothing so I'll ask. There are a host of "behind the scenes" library functions that perform such magic as right or left shifts of various data types, etc. Even the ubiquitous switch() statement compiles into a special library call (LCALL ?C?CCASE) to do the work. My (probably paranoid) concern is the degree to which these functions may be employed in an interrupt routine. I'm concluding that since none of my programs crash miserably that it's OK to use any "c" construct in an interrupt routine and that the behind the scenes library functions are designed for reentrancy. It would just be nice to find that in print somewhere. Thanks for reading & commenting. Steve C.