The prefix of this attribute must be a general access-to-array type (or subtype); the attribute takes a System.Address argument and possibly some additional arguments (described below) and yields a value of the given access type that designates an array object located at the given address. In the case of a non-null array this means that the given address is the address of the first element of the array object (not the address of any sort of bounds descriptor). This allows associating bounds with an address that is, for example, passed in from C code.
If the designated array subtype is unconstrained (which is the usual case), then for each dimension (in order) the attribute takes either one or two additional arguments of the corresponding index type - one if the index subtype is a fixed lower bound subtype, two (low bound first) otherwise. In this case, the access type shall be an extended access type (see the description of the Extended_Access aspect). These additional arguments specify the bounds of the designated array object.
If the designated array subtype is constrained then no additional arguments are provided and the bounds of the designated object are those of the designated subtype.
Roughly speaking, My_Access_Type’From_Address (Addr, Lo, Hi) is equivalent to a declare expression:
(declare
Obj : aliased Designated_Array_Type (Lo .. Hi)
with Import, Address => Addr;
begin
My_Access_Type'(Obj'Unchecked_Access)
end)