1. In a computer system capable of maintaining in a first memory area a program data stack having a plurality of stack items, each of the plurality of stack items comprising data which is definable as either a reference or a primitive value, a method for determining whether data in a selected program stack item is a reference or a primitive value, the method comprising the steps of: A. maintaining in a second memory area a plurality of tag items, each of the plurality of tag items associated with a stack item, each tag item having a value indicating whether the associated stack item is a reference or a primitive value; B. retrieving one of the plurality of tag items which is associated with the selected stack item; and C. determining from the value of the retrieved tag item whether the data contained within the associated stack item is a reference or a primitive value.