The code is modified from it's original format to KickAssembler format by TWW

The routine alters $d000 & $d001. It shuldn't be too hard to modify it.

There are some enhancments possible as well. Self calibration and even limiting the returned alteration to a signed byte since the movements never exceeds 6 bits…

The rest of the routine should speak for itself…

.var iirq    = $0314
.var vic     = $d000
.var sid     = $d400
.var potx    = sid+$19
.var poty    = sid+$1a
.var vicdata = $d000         // basics copy of vic register image
.var xpos    = vicdata+$00   // low order xposition
.var ypos    = vicdata+$01   // y position
.var xposmsb = vicdata+$10   // bit 0 is high order x position

    lda potx                 // get delta values for x
    ldy opotx
    jsr movchk
    sty opotx

    clc                      // modify low order xposition
    adc xpos
    sta xpos
    txa
    adc #$00
    and #%00000001
    eor xposmsb
    sta xposmsb

    lda poty                 // get delta value for y
    ldy opoty
    jsr movchk
    sty opoty

    sec                      // modify y position ( decrease y for increase in pot )
    eor #$ff
    adc ypos
    sta ypos
    rts
//==================================================
// movchk
//       entry   y = old value of pot register
//               a = currrent value of pot register
//       exit    y = value to use for old value
//               x,a = delta value for position
//==================================================
movchk:
    sty oldvalue
    sta NewValue
    ldx #0

    sec
    sbc oldvalue
    and #%01111111	
    cmp #%01000000
    bcs !+
    lsr
    beq !Slutt+
    ldy NewValue
    rts

!:  ora #%11000000
    cmp #$ff
    beq !Slutt+
    sec
    ror
    ldx #$ff
    ldy NewValue
    rts
!Slutt:
    lda #0
    rts
opotx: .byte $00
opoty: .byte $00