How do acq and rel work?

Hi everybody,
Im just fiddling with critical regions in
assembly. From what i have seen from intel documentation you can make a critical region by enclosing it between .acq and .rel instructions.
But it looks like with these instructions
you have to set a flag as a side-effect and check for the flag status every time you want to enter the critical region. Is that how they work? or are there other ways of doing it.
And finally can somebody point out good links to implementing semaphores and like in itanium. -Ravi

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