VM_Address result = VM_Memory.mmap(start, VM_Extent.fromInt(size),
VM_Address result = VM_Memory.mmap(start, VM_Extent.fromIntZeroExtend(size),
  public static int mmap(VM_Address start, int size) {    VM_Address result = VM_Memory.mmap(start, VM_Extent.fromInt(size),                                       VM_Memory.PROT_READ | VM_Memory.PROT_WRITE | VM_Memory.PROT_EXEC,                                        VM_Memory.MAP_PRIVATE | VM_Memory.MAP_FIXED | VM_Memory.MAP_ANONYMOUS);    if (result.EQ(start)) return 0;    if (result.GT(VM_Address.fromIntZeroExtend(127))) {      VM.sysWrite("mmap with MAP_FIXED on ", start);      VM.sysWriteln(" returned some other address", result);      VM.sysFail("mmap with MAP_FIXED has unexpected behavior");    }    return result.toInt();  }