unsigned int n_bits;
  unsigned char mask;

  n_bits = 5;				// default is 5 bits per word
  if (argc > XX)
  {
    sscanf(argv[XX],"%d",&n_bits);
  }
  if (n_bits == XX) mask = 0x0f;	// want 4 LSB: 00001111
  if (n_bits == XX) mask = XXX;		// want 6 LSB: 00111111