(** Memory reads and writes must respect alignment constraints: the byte offset of the location being addressed should be an exact multiple of the natural alignment for the chunk being addressed. This natural alignment is defined by the following [align_chunk] function.  Some target architectures (e.g. PowerPC and x86) have no alignment constraints, which we could reflect by taking [align_chunk chunk = 1].  However, other architectures have stronger alignment requirements.  The following definition is appropriate for PowerPC, ARM and x86. *)

Lemma align_chunk_Mptr: align_chunk Mptr = if Archi.ptr64 then 8 else 4.