volatile vs aliased

["REH" <spamjunk@stny.rr.com>]

I'm converting some code written for Gnat to compile with Apex.  The
software has bindings to C functions (OS calls).  Some of these take
System.Address as parameters.  Apex complains about the variables used
in these calls (via 'Address) because they not volatile or aliased (or
imported, exported, etc.).  My question is: which should I use?  Is
there any difference whether a variable is volatile or aliased?

I am assuming the compiler is concerned about the variable possibly
being changed in the call (which, of course, is true).  Is there a way
to satisfy the compiler without stopping optimizations of the variable
after the call, or do I need to use a second variable to do this?