; SUBROUTINE TO DO MULTIPLE-PRECISION SUBTRACTS
; ENTRY:	(IX) POINTS TO MOST SIGNIFICANT BYTE OF DESTINATION
;			(IY) POINTS TO MS BYTE OF SOURCE
;			(B) IS NUMBER OF BYTES IN OPERANDS
;			CALL MULADD
;			(RETURN)
; EXIT:		(IX) IS UNCHANGED
;			(IY) IS UNCHANGED
;			(A) IS DESTROYED
;			(B) IS 0
;
.ORG 0000h					; CHANGE ON REASSEMBLY.

			LD	IX,	0040h	; THESE LINES ADDED TO TEST THE PROGRAM.
			LD	IY,	0050h	; "
			LD	B,	4		; "
			LD	SP,	0070h	; "
			CALL MULADD		; "
			HALT			; "

MULADD:		PUSH	DE		; SAVE DE.
			LD	E,	B		; # BYTES TO E.
			LD	D,	0		; DE NOW HAS #.
			DEC DE			; DE NOW HAS # -1.
			ADD	IX,	DE		; POINT TO LS (LEAST SIGNIFICANT) BYTE.
			ADD	IY,	DE		; POINT TO LS BYTE.
			POP	DE			; RESTORE ORIGINAL.
			XOR	A			; RESET CARRY FLAG.
LOOP:		LD	A,	(IX +0)	; GET DESTINATION.
			SBC	A,	(IY +0)	; ADD SOURCE.
			LD	(IX +0), A	; STORE RESULT.
			DJNZ	LOOP1	; GO IT NOT DONE.
			RET				; RETURN.
LOOP1:		DEC	IX			; POINT TO NEXT HIGHER.
			DEC IY			; POINT TO NEXT HIGHER.
			JP	LOOP		; CONTINUE.
			
; THESE LINES ADDED TO TEST THE PROGRAM.

.org	0040h				; IX DESTINATION AREA. 00 05 3F F2.
		DEFB	00h			; +844,050.
		DEFB	05h
		DEFB	3Fh
		DEFB	0F2h

.org	0050h				; IY SOURCE AREA. 00 07 A1 20.
		DEFB	00h
		DEFB	07h
		DEFB	0A1h
		DEFB	20h
							; 00053FF2 + 0007A120 = +000C E112h = 844,050(DECIMAL).			
.END