Abstract: Method and apparatus are provided for encoding data types to identify the type of object being operated on during execution of a computer program on a data processing system including byte-addressed memory. The object is represented by at least one machine word having a plurality of bits. The method includes the steps of storing an object in the byte-addressed memory so that the lower three bits of the object are in a zero state and setting one of the lower three bits to a non-zero state to indicate a particular data type. Three different types of data can be verified by checking the state of a single bit. In one embodiment of the invention the method further includes the step of setting a second one of the lower three bits to a non-zero state to indicate that the object, such as NIL, is a member of two data types.