public	mpy16

; This routine multiplies the 16-bit values in DE and HL and returns the
; 32-bit result in HLBC (HL has high 16 bits; BC has low 16 bits).  Register
; pair AF is preserved.

mpy16:
	ex	af,af'		; Save AF
	ld	a,h		; Transfer factor in HL to A and C
	ld	c,l
	ld	hl,0		; Initialize product
	ld	b,16		; Set bit counter
	rra			; Shift AC right so first multiplier bit
	rr	c		; ..is in carry flag
mp161:
	jr	nc,mp162 	; If carry not set, skip the addition
	add	hl,de		; Add multiplicand
mp162:
	rr	h		; Rotate HL right, low bit into carry
	rr	l
	rra			; Continue rotating through AC, with
	rr	c		; ..next multiplier bit moving into carry
	djnz	mp161		; Loop through 16 bits

	ld	b,a		; Move A to B so result is in HLBC
	ex	af,af'		; Restore original AF registers
	ret

	end