* Free space determination

* This function may be called from Pascal.  It returns the distance
* between the stack and the heap as an unsigned 16-bit integer.
* IX contains the stack address, and the alternate HL pair contains
* the heap address.  It appears to Pascal as:

*	function space : integer;
*	    external;

	entry	space

space	push	ix
	pop	h		HL = stack address
	exx
	push	h
	exx
	pop	d		DE = heap address
	dsbc	d		HL := distance between stack and heap
	xchg			DE := HL (function return in Pascal)
	ret
