Header Word Format

Type of Object	Header word format	Comments
Forwarded/Empty		Assumes pointers are word aligned. So the bitmask represents a valid address if/when we forward objects. We use an address of zero to represent an empty header word.
Small Record		Bitmasks identify pointers in the object by 1's in the bitmask. Small objects can have at most 22 words.
Large Record(Begin)		The beginning header word occurs adjacent to the object. Immediately preceding it is the first bitmask word, whose bit 0 tells us whether the first word of the object is a pointer or not.
Large Record(End)		This header word marks the end of a sequence of bitmask words for a large record. The top bits store the number of header words to facilitate finding the beginning header word.
Array		We support arrays that have either one word or two word elements, since these are the only kind that occur in ML.
Conservative Object		Conservative Objects are those for which no exact pointer information is provided. However we allow the user to tell us whether the object has pointers or not.