Q: setting an address with pure?

[Simon Wright <simon@pushface.org>]

I need to specify the address at which a hardware object appears.

This works:

   PIOA : PIO
     with
       Import,
       Convention => Ada,
       Address => System.Storage_Elements.To_Address (16#400E0E00#);

but means that I can't declare the package Pure (or even Preelaborate).

Is anyone aware of any GNAT feature that would allow such a package to
be Pure? There are similar things in their package Standard, for example
Standard'Address_Size.

I tried

   PIOG : PIO with Import, Convention => Ada;
   for PIOG use at 16#400E1800#;

but, besides warning me that 'use at' is obsolescent, GNAT still expects
System.Address not universal integer.