Accessing a register with a larger size from generic

[lutz@iks-jena.de (Lutz Donnerhacke)]

  ------------------------------
[...with, use, package (body), ...]
subtype long is Interfaces.C.long;

type call_code is (KILL, FORK, EXIT, ...);
for call_code use (KILL => 1, FORK => 2, ...);

generic
   call : call_code;
   type type1 is private;
function syscall1 (arg1 : type1) return long;

function syscall1 (arg1 : type1) return long is
   res : long;
begin
   Asm ("int $0x80",
        Outputs => long'Asm_Output ("=a", res),
        Inputs  => (call_code'Asm_Input ("0", call),
                    type1'Asm_Input ("b", arg1)),
        Volatile => True);
   return res;
end syscall1;
  ------------------------------

generates code (using GNAT 3.13p) like:
  movb $2, %al
  xorw %bx, %bx
  int  0x80
instead of
  movl $2, %eax
  xorl %ebx, %ebx
  int  0x80


How can I change the type of the generated RTL expression to SI independed
of the generic type?