* Unsigned multiplication
* A*X -> A:Y (A-high byte, Y-low)
* b. Fox/Tqa

/*     
     jsr fast_mul.init

     lda #123
     ldx #22
     jsr fast_mul
     rts

*/

.proc fast_mul

sq1l equ $a000
sq1h equ $a200
sq2l equ $a400
sq2h equ $a600

     sta l1+1
     sta h1+1
     eor #$ff
     sta l2+1
     sta h2+1
     sec
l1   lda sq1l,x
l2   sbc sq2l,x
     tay
h1   lda sq1h,x
h2   sbc sq2h,x
     rts

init ldx #0
     stx sq1l
     stx sq1h
     stx sq2l+$ff
     stx sq2h+$ff
     ldy #$ff
msq1 txa
     lsr @
     adc sq1l,x
     sta sq1l+1,x
     sta sq2l-1,y
     sta sq2l+$100,x
     lda #0
     adc sq1h,x
     sta sq1h+1,x
     sta sq2h-1,y
     sta sq2h+$100,x
     inx
     dey
     bne msq1
msq2 tya
     sbc #0 -
     ror @
     adc sq1l+$ff,y
     sta sq1l+$100,y
     lda #0
     adc sq1h+$ff,y
     sta sq1h+$100,y
     iny
     bne msq2
     rts

.endp