A mask is a pattern of bits used as a filter to selectively accept or reject bits in a set of data. (A bit is the smallest piece of information used by modern electronic computers and consists uniquely of the binary digits 0 and 1.) For example, when defining the field of a database, a mask is assigned to indicate what type of value the field should contain (such as only alphabetic characters). Values that do not conform to the mask requirements cannot be entered into the database.

The binary image of a mask, commonly called a bit mask, is used to selectively reject or accept certain corresponding bits in a data value when the mask is used in an expression with a logical operator. When a mask is used in this way, the areas with bit values of "1" are accepted while the areas with bit values of "0" are rejected...