segmented memory models

Not quite. There is no such thing as "a null address". Each pointer type has a distinguished value, termed a "null" pointer of that type, which does not point to any object, and which can be obtained by converting an integer constant expression with value 0 (of which a bare 0 is just one example), or such an expression cast to type "pointer to void", to the relevant pointer type. (This conversion may occur by casting, by assignment, by parameter passing....) Whether pointers are "addresses" - indeed, whether an "address" is a useful concept - is a matter for the implementation. I recall hearing of a C implementation for a Lisp Machine in which pointers were implemented as <array,index> pairs. Right. Indeed, the encoding of a null pointer may vary from type to type (within certain bounds; as a simple example, char * and void *, by special fiat, must use the same representation). /~\ The ASCII der Mouse \ / Ribbon Campaign X Against HTML mouse at rodents-montreal.org / \ Email! 7D C8 61 52 5D E7 2D 39 4E F1 31 3E E8 B3 27 4B