clc
lda c1a
adc c1b
bcs c1bc	c1bc:
		clc
adc c1c	        adc c1c
sta z1		sta z1
		lda #1
lda c2a	        adc c2a
		bcs c2ac;7	c2ac:
				clc
adc c2b	        adc c2b	        adc c2b

;y0 in X, y1 in Y, x0-1, x1-1 init to $02, all ram swapped in
x0=$fc;fb-fc is pointer
x1=$fe;fd-fe is pointer
jmp (x0-1)
mult-k:
;multiply by constant multiplier k
;and multiplicands X, Y
;then add
rts

;8x16=24 version by Strobe of Repose's original 16x16=32 "fastest multiplication"
;How to use: put numbers in x0/y0+y1 and result is Y reg (z2), A reg (z1), z0
;Clobbers Y, A but not X (in original form anyway)
umult8x16:
;set multiplier as x0
    lda x0              ;comment out and call with A=x0 -2b3c
    sta p_sqr_lo
    sta p_sqr_hi
    eor #$ff
    sta p_invsqr_lo
    sta p_invsqr_hi;17

    ldy y0              ;comment out and call with Y=y0 -2b3c
    sec
    lda (p_sqr_lo),y
    sbc (p_invsqr_lo),y;note these two lines taken as 11 total
    sta z0;x0*y0l       comment out if you don't care about z0, -2b3c, OR
    lda (p_sqr_hi),y    ; ..replace with TAX -1b1c (but destroys X obviously)
    sbc (p_invsqr_hi),y
    sta c1a+1;x0*y0h;33
;c1a means column 1, row a (partial product to be added later)

    ldy y1
    ;sec  ;notice that the high byte of sub above is always +ve
    lda (p_sqr_lo),y
    sbc (p_invsqr_lo),y
    sta c1b+1;x0*y1l
    lda (p_sqr_hi),y
    sbc (p_invsqr_hi),y
    tay ;Y=c2a;x0*y1h; 29 cycles

;17+33+29=79 cycles for main multiply part
do_adds:
;-add the first two numbers of column 1
        clc
c1a:	lda #0
c1b:	adc #0  ;A=z1  6 cycles

;-continue to column 2
        bcc fin   ;2+
        iny  ;add carry
        ;Y=z2, 4-5 cycles (avoiding page boundary cross)
;6+(4+)=10-11 cycles for adder
;total 89-90 cycles
fin:    rts

;Diagram of the additions for 16x8=24
;                y1    y0
;             x        x0
;                --------
;             x0y0h x0y0l
;+      x0y1h x0y1l
;------------------------
;          z2    z1    z0 

;Possible tweaks:
;1. call with A=x0 and comment out "lda x0" -2b3c  (*)
;2. call with Y=y0 and comment out "ldy y0" -2b3c
;3. if you don't need z0 (I didn't), comment out "sta z0" -2b3c  (*)
;   OR replace with TAX -1b1c (but destroys X register obviously which was safe)
;4. There's no point having do_adds in ZP with a JMP like the 16x16 version
;   suggests because you would lose the 2 cycles you gained with the JMP, BUT..
;   ..you could put the ENTIRE ROUTINE in zero page, -2b2c
;   AND if you do that you might as well replace "lda x0", "ldy y0" and "ldy y1"
;   with immediate versions and point x0,y0 & y1 to the appropriate ZP spot for
;   extra -3c, so -2b5c combined.
;5. OR forget the ZP stuff and just in-line it, saving the 12 cycle JSR/RTS
;   (it's only $36 bytes)
;6. mix 1 and/or 2 and/or 3 with 4 or 5
;(*) these also apply to Repose' original 16x16=32 routine.