bit mask
     
         a pattern of binary values which is combined
        with some value using bitwise and with the result that bits
        in the value in positions where the mask is zero are also set
        to zero.  for example, if, in c, we want to test if bits 0
        or 2 of x are set, we can write
     
        	int mask = 5;	/ binary 101 /
     
        	if x  mask ...
     
        a bit mask might also be used to set certain bits using
        bitwise or, or to invert them using bitwise exclusive or.
     
        1995-05-12
     
     
see also:
binary bitwise c exclusive or