lda #6
sta $0 ;xpos
lda #6
sta $1 ;ypos
lda #0
sta $5 ;previous address
lda #0
sta $6

;$2 and $3 are the two addresses of the dot.
jmp lewp

duplicateBallAddress:
lda $2
sta $4
lda $3
sta $5
rts

drawBall:
lda #0
sta ($4), y   ;erase previous ball

lda $1        ;y position
adc $1        ;double to get memory location
tax
lda ypos, x
sta $2        ;lsb
inx
lda ypos, x   ;msb
sta $3
lda $0        ;load x position
clc
adc $2        ;add x position
sta $2
lda #1
sta ($2),y    ;draw pixel
rts

lewp:
  jsr drawBall
  jsr duplicateBallAddress
  inc $0
  inc $1
  jmp lewp

ypos:
 dcb $00,$02,$20,$02,$40,$02,$60,$02
 dcb $80,$02,$a0,$02,$c0,$02,$e0,$02
 dcb $00,$03,$20,$03,$40,$03,$60,$03
 dcb $80,$03,$a0,$03,$c0,$03,$e0,$03
 dcb $00,$04,$20,$04,$40,$04,$60,$04
 dcb $80,$04,$a0,$04,$c0,$04,$e0,$04
 dcb $00,$05,$20,$05,$40,$05,$60,$05
 dcb $80,$05,$a0,$05,$c0,$05,$e0,$05