(** 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. *)

Definition align_chunk (chunk: memory_chunk) : Z :=
  match chunk with
  | Mint8signed => 1
  | Mint8unsigned => 1
  | Mint16signed => 2
  | Mint16unsigned => 2
  | Mint32 => 4
  | Mint64 => 8
  | Mfloat32 => 4
  | Mfloat64 => 4
  | Many32 => 4
  | Many64 => 4
  end.

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