; signed multiply a * b
; save sign of result, make both positive
; multiply, set result sign, shift accuracy
; return high order byte of result
; basically this multiplies signed byte1 bits 0-7 
; by signed byte2 bits 0-7 and returns bits 7-14 
; of the 16 bit result preserving the sign bit.
;
mpy:	mov	r0,a		; save multiplier
	xrl	a,b		; xor signs
	mov	c,acc.7
	push	psw		; save result sign
	clr	c
	mov	a,r0		; get multiplier
	jnb	acc.7,mpy2
	cpl	a
	inc	a
mpy2:	xch	a,b
	jnb	acc.7,mpy3
	cpl	a
	inc	a
mpy3:	mul	ab		; unsigned math
	pop	psw		; get result sign
	jnc	mpy1		; result is positive
	cpl	a
	addc	a,#0		; make result negative
	xch	a,b
	cpl	a
	addc	a,#0
	xch	a,b
mpy1:	rlc	a		; low order bit 7 to carry
	xch	a,b		; get high order
	rlc	a		; shift in 1 bit more of accuracy
	mov	acc.7,c		; insure sign
	ret