mult27.a

; mult27.a
; from https://sites.google.com/site/h2obsession/programming/6502
; tweaked slightly
;
; 8 bit x 8 bit unsigned multiply, 16 bit result
; Average cycles: 46.49
; 1312 bytes

temp = $02

* = $0200

; Tables must be aligned with page boundary
sqrLow
    !for i, 0, 511 {
        !byte <((i*i)/4)
    }

sqrHigh
    !for i, 0, 511 {
        !byte >((i*i)/4)
    }

idTab
    !for i, 0, 255 {
        !byte i
    }

; ***************************************************************************************
; On Entry:
;   A = multiplicand
;   X = multiplier
; On Exit:
;   temp: low byte of product
;   A: high byte of product
mult
   sta getLow+1
   sta getHigh+1
   sec
   sbc idTab,x
   bcs doMult
   sbc #0
   eor #255
doMult
   tay
getLow
   lda sqrLow,x
   sbc sqrLow,y
   sta temp
getHigh
   lda sqrHigh,x
   sbc sqrHigh,y
;   tax
;temp = *+1
;   lda #0
   rts