if (VM.VerifyAssertions) VM.assert(VM_Memory.roundDownPage(size) == size);
if (VM.VerifyAssertions) VM._assert(VM_Memory.roundDownPage(size) == size);
  public final void zero() throws VM_PragmaUninterruptible {    if (VM.VerifyAssertions) VM.assert(VM_Memory.roundDownPage(size) == size);    VM_Memory.zeroPages(start, size);  }